C++ PATCH for c++/78244 - narrowing conversion in template not detected, part 2

This patch ought to fix the rest of 78244, a missing narrowing warning in
decltype.

As I explained in Bugzilla, there can be three scenarios:

1) decltype is in a template and it has no dependent expressions, which
is the problematical case.  finish_compound_literal just returns the
compound literal without checking narrowing if processing_template_decl.
But finish_decltype_type sees an expr that is not instantiation-dependent,
so it just takes its unlowered_expr_type and returns it, without calling
check_narrowing.

2) decltype is in a template and has dependent expressions
This works, because while finish_compound_literal just returns the compound
literal, finish_decltype_type sees an expr that is instantiation-dependent,
so it creates a DECLTYPE_TYPE.
tsubst_copy_and_build then calls
  RETURN (finish_compound_literal (type, r, complain, cl));
while substituting the CONSTRUCTOR.  Now processing_template_decl is 0, so
finish_compound_literal calls check_narrowing, so we detect it.

3) decltype is not in a template
finish_compound_literal calls check_narrowing and the narrowing is detected.


I think it's better not to blithely instantiate such decltypes just for the
benefit of check_narrowing, but we can still try to check narrowing even in a
template.  check_narrowing calls instantiation_dependent_expression_p, so it
does nothing when there are dependent expressions.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-01-17  Marek Polacek  <polacek@redhat.com>

	PR c++/78244 - narrowing conversion in template not detected.
	* semantics.c (finish_compound_literal): When processing a template,
	try to check narrowing.

	* g++.dg/cpp0x/Wnarrowing15.C: New test.
	* g++.dg/cpp1y/Wnarrowing1.C: New test.

On 1/17/19 2:09 PM, Marek Polacek wrote:
> This patch ought to fix the rest of 78244, a missing narrowing warning in
> decltype.
> 
> As I explained in Bugzilla, there can be three scenarios:
> 
> 1) decltype is in a template and it has no dependent expressions, which
> is the problematical case.  finish_compound_literal just returns the
> compound literal without checking narrowing if processing_template_decl.

This is the sort of thing that we've been gradually fixing: if the 
compound literal isn't dependent at all, we want to do the normal 
processing.  And then usually return a result based on the original 
trees rather than the result of processing.  For instance, 
finish_call_expr.  Something like that ought to work here, too, and be 
more generally applicable; this shouldn't be limited to casting to a 
scalar type, casting to a known class type can also involve narrowing.

The check in the other patch that changes instantiation_dependent_r 
should be more similar to the one in finish_compound_literal.  Or 
perhaps you could set a flag here in finish_compound_literal to indicate 
that it's instantiation-dependent, and just check that in 
instantiation_dependent_r.

Jason

On Thu, Jan 17, 2019 at 04:17:29PM -0500, Jason Merrill wrote:
> On 1/17/19 2:09 PM, Marek Polacek wrote:
> > This patch ought to fix the rest of 78244, a missing narrowing warning in
> > decltype.
> > 
> > As I explained in Bugzilla, there can be three scenarios:
> > 
> > 1) decltype is in a template and it has no dependent expressions, which
> > is the problematical case.  finish_compound_literal just returns the
> > compound literal without checking narrowing if processing_template_decl.
> 
> This is the sort of thing that we've been gradually fixing: if the compound
> literal isn't dependent at all, we want to do the normal processing.  And
> then usually return a result based on the original trees rather than the
> result of processing.  For instance, finish_call_expr.  Something like that
> ought to work here, too, and be more generally applicable; this shouldn't be
> limited to casting to a scalar type, casting to a known class type can also
> involve narrowing.

Great, that works just fine.  I also had to check if the type is
type-dependent, otherwise complete_type could fail.

> The check in the other patch that changes instantiation_dependent_r should
> be more similar to the one in finish_compound_literal.  Or perhaps you could
> set a flag here in finish_compound_literal to indicate that it's
> instantiation-dependent, and just check that in instantiation_dependent_r.

Done, but I feel bad about adding another flag.  But I guess it's cheaper
this way.  Thanks!

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-01-18  Marek Polacek  <polacek@redhat.com>

	PR c++/88815 - narrowing conversion lost in decltype.
	PR c++/78244 - narrowing conversion in template not detected.
	* cp-tree.h (CONSTRUCTOR_IS_DEPENDENT): New.
	* pt.c (instantiation_dependent_r): Consider a CONSTRUCTOR with
	CONSTRUCTOR_IS_DEPENDENT instantiation-dependent.
	* semantics.c (finish_compound_literal): When the compound literal
	isn't instantiation-dependent and the type isn't type-dependent,
	fall back to the normal processing.  Don't only call check_narrowing
	for scalar types.  Set CONSTRUCTOR_IS_DEPENDENT.

	* g++.dg/cpp0x/Wnarrowing15.C: New test.
	* g++.dg/cpp0x/constexpr-decltype3.C: New test.
	* g++.dg/cpp1y/Wnarrowing1.C: New test.

diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
index 5cc8f88d522..778874cccd6 100644
--- gcc/cp/cp-tree.h
+++ gcc/cp/cp-tree.h
@@ -424,6 +424,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
       DECL_FINAL_P (in FUNCTION_DECL)
       QUALIFIED_NAME_IS_TEMPLATE (in SCOPE_REF)
       DECLTYPE_FOR_INIT_CAPTURE (in DECLTYPE_TYPE)
+      CONSTRUCTOR_IS_DEPENDENT (in CONSTRUCTOR)
       TINFO_USED_TEMPLATE_ID (in TEMPLATE_INFO)
       PACK_EXPANSION_SIZEOF_P (in *_PACK_EXPANSION)
       OVL_USING_P (in OVERLOAD)
@@ -4202,6 +4203,11 @@ more_aggr_init_expr_args_p (const aggr_init_expr_arg_iterator *iter)
    B b{1,2}, not B b({1,2}) or B b = {1,2}.  */
 #define CONSTRUCTOR_IS_DIRECT_INIT(NODE) (TREE_LANG_FLAG_0 (CONSTRUCTOR_CHECK (NODE)))
 
+/* True if this CONSTRUCTOR is instantiation-dependent and needs to be
+   substituted.  */
+#define CONSTRUCTOR_IS_DEPENDENT(NODE) \
+  (TREE_LANG_FLAG_1 (CONSTRUCTOR_CHECK (NODE)))
+
 /* True if this CONSTRUCTOR should not be used as a variable initializer
    because it was loaded from a constexpr variable with mutable fields.  */
 #define CONSTRUCTOR_MUTABLE_POISON(NODE) \
diff --git gcc/cp/pt.c gcc/cp/pt.c
index e4f76478f54..ae77bae6b29 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -25800,6 +25800,11 @@ instantiation_dependent_r (tree *tp, int *walk_subtrees,
 	return *tp;
       break;
 
+    case CONSTRUCTOR:
+      if (CONSTRUCTOR_IS_DEPENDENT (*tp))
+	return *tp;
+      break;
+
     default:
       break;
     }
diff --git gcc/cp/semantics.c gcc/cp/semantics.c
index e654750d249..4ff09ad3fb7 100644
--- gcc/cp/semantics.c
+++ gcc/cp/semantics.c
@@ -2795,11 +2795,14 @@ finish_compound_literal (tree type, tree compound_literal,
 	  return error_mark_node;
       }
 
-  if (processing_template_decl)
+  if (instantiation_dependent_expression_p (compound_literal)
+      || dependent_type_p (type))
     {
       TREE_TYPE (compound_literal) = type;
       /* Mark the expression as a compound literal.  */
       TREE_HAS_CONSTRUCTOR (compound_literal) = 1;
+      /* And as instantiation-dependent.  */
+      CONSTRUCTOR_IS_DEPENDENT (compound_literal) = true;
       if (fcl_context == fcl_c99)
 	CONSTRUCTOR_C99_COMPOUND_LITERAL (compound_literal) = 1;
       return compound_literal;
@@ -2822,8 +2825,7 @@ finish_compound_literal (tree type, tree compound_literal,
       && check_array_initializer (NULL_TREE, type, compound_literal))
     return error_mark_node;
   compound_literal = reshape_init (type, compound_literal, complain);
-  if (SCALAR_TYPE_P (type)
-      && !BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
+  if (!BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
       && !check_narrowing (type, compound_literal, complain))
     return error_mark_node;
   if (TREE_CODE (type) == ARRAY_TYPE
diff --git gcc/testsuite/g++.dg/cpp0x/Wnarrowing15.C gcc/testsuite/g++.dg/cpp0x/Wnarrowing15.C
new file mode 100644
index 00000000000..4e7c17dcfca
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/Wnarrowing15.C
@@ -0,0 +1,14 @@
+// PR c++/78244
+// { dg-do compile { target c++11 } }
+
+template <typename T>
+auto f1() -> decltype(int{2.0}, void()) { } // { dg-error "narrowing conversion" }
+
+template <typename T>
+auto f2() -> decltype(int{2.0}) { return 1; } // { dg-error "narrowing conversion" }
+
+template <typename T>
+auto f3() -> decltype(void(), int{2.0}) { return 1; } // { dg-error "narrowing conversion" }
+
+template <typename T>
+auto f4() -> decltype((int{2.0})) { return 1; } // { dg-error "narrowing conversion" }
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-decltype3.C gcc/testsuite/g++.dg/cpp0x/constexpr-decltype3.C
new file mode 100644
index 00000000000..fd05366de50
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-decltype3.C
@@ -0,0 +1,25 @@
+// PR c++/88815
+// { dg-do compile { target c++11 } }
+
+struct true_type {
+    constexpr operator bool() const { return true; }
+};
+
+struct false_type {
+    constexpr operator bool() const { return false; }
+};
+
+template<int (*p)()>
+true_type is_constexpr_impl(decltype(int{(p(), 0U)}));
+
+template<int (*p)()>
+false_type is_constexpr_impl(...);
+
+template<int (*p)()>
+using is_constexpr = decltype(is_constexpr_impl<p>(0));
+
+constexpr int f() { return 0; }
+int g() { return 0; }
+
+static_assert(is_constexpr<f>(), "");
+static_assert(!is_constexpr<g>(), "");
diff --git gcc/testsuite/g++.dg/cpp1y/Wnarrowing1.C gcc/testsuite/g++.dg/cpp1y/Wnarrowing1.C
new file mode 100644
index 00000000000..e1e499542f0
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1y/Wnarrowing1.C
@@ -0,0 +1,5 @@
+// PR c++/78244
+// { dg-do compile { target c++14 } }
+
+template<typename>
+decltype(int{1.1}) v; // { dg-error "narrowing conversion" }

On 1/18/19 9:12 AM, Marek Polacek wrote:
> On Thu, Jan 17, 2019 at 04:17:29PM -0500, Jason Merrill wrote:
>> On 1/17/19 2:09 PM, Marek Polacek wrote:
>>> This patch ought to fix the rest of 78244, a missing narrowing warning in
>>> decltype.
>>>
>>> As I explained in Bugzilla, there can be three scenarios:
>>>
>>> 1) decltype is in a template and it has no dependent expressions, which
>>> is the problematical case.  finish_compound_literal just returns the
>>> compound literal without checking narrowing if processing_template_decl.
>>
>> This is the sort of thing that we've been gradually fixing: if the compound
>> literal isn't dependent at all, we want to do the normal processing.  And
>> then usually return a result based on the original trees rather than the
>> result of processing.  For instance, finish_call_expr.  Something like that
>> ought to work here, too, and be more generally applicable; this shouldn't be
>> limited to casting to a scalar type, casting to a known class type can also
>> involve narrowing.
> 
> Great, that works just fine.  I also had to check if the type is
> type-dependent, otherwise complete_type could fail.
> 
>> The check in the other patch that changes instantiation_dependent_r should
>> be more similar to the one in finish_compound_literal.  Or perhaps you could
>> set a flag here in finish_compound_literal to indicate that it's
>> instantiation-dependent, and just check that in instantiation_dependent_r.
> 
> Done, but I feel bad about adding another flag.  But I guess it's cheaper
> this way.  Thanks!
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
> 
> 2019-01-18  Marek Polacek  <polacek@redhat.com>
> 
> 	PR c++/88815 - narrowing conversion lost in decltype.
> 	PR c++/78244 - narrowing conversion in template not detected.
> 	* cp-tree.h (CONSTRUCTOR_IS_DEPENDENT): New.
> 	* pt.c (instantiation_dependent_r): Consider a CONSTRUCTOR with
> 	CONSTRUCTOR_IS_DEPENDENT instantiation-dependent.
> 	* semantics.c (finish_compound_literal): When the compound literal
> 	isn't instantiation-dependent and the type isn't type-dependent,
> 	fall back to the normal processing.  Don't only call check_narrowing
> 	for scalar types.  Set CONSTRUCTOR_IS_DEPENDENT.
> 
> 	* g++.dg/cpp0x/Wnarrowing15.C: New test.
> 	* g++.dg/cpp0x/constexpr-decltype3.C: New test.
> 	* g++.dg/cpp1y/Wnarrowing1.C: New test.
> 
> diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
> index 5cc8f88d522..778874cccd6 100644
> --- gcc/cp/cp-tree.h
> +++ gcc/cp/cp-tree.h
> @@ -424,6 +424,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
>         DECL_FINAL_P (in FUNCTION_DECL)
>         QUALIFIED_NAME_IS_TEMPLATE (in SCOPE_REF)
>         DECLTYPE_FOR_INIT_CAPTURE (in DECLTYPE_TYPE)
> +      CONSTRUCTOR_IS_DEPENDENT (in CONSTRUCTOR)
>         TINFO_USED_TEMPLATE_ID (in TEMPLATE_INFO)
>         PACK_EXPANSION_SIZEOF_P (in *_PACK_EXPANSION)
>         OVL_USING_P (in OVERLOAD)
> @@ -4202,6 +4203,11 @@ more_aggr_init_expr_args_p (const aggr_init_expr_arg_iterator *iter)
>      B b{1,2}, not B b({1,2}) or B b = {1,2}.  */
>   #define CONSTRUCTOR_IS_DIRECT_INIT(NODE) (TREE_LANG_FLAG_0 (CONSTRUCTOR_CHECK (NODE)))
>   
> +/* True if this CONSTRUCTOR is instantiation-dependent and needs to be
> +   substituted.  */
> +#define CONSTRUCTOR_IS_DEPENDENT(NODE) \
> +  (TREE_LANG_FLAG_1 (CONSTRUCTOR_CHECK (NODE)))
> +
>   /* True if this CONSTRUCTOR should not be used as a variable initializer
>      because it was loaded from a constexpr variable with mutable fields.  */
>   #define CONSTRUCTOR_MUTABLE_POISON(NODE) \
> diff --git gcc/cp/pt.c gcc/cp/pt.c
> index e4f76478f54..ae77bae6b29 100644
> --- gcc/cp/pt.c
> +++ gcc/cp/pt.c
> @@ -25800,6 +25800,11 @@ instantiation_dependent_r (tree *tp, int *walk_subtrees,
>   	return *tp;
>         break;
>   
> +    case CONSTRUCTOR:
> +      if (CONSTRUCTOR_IS_DEPENDENT (*tp))
> +	return *tp;
> +      break;
> +
>       default:
>         break;
>       }
> diff --git gcc/cp/semantics.c gcc/cp/semantics.c
> index e654750d249..4ff09ad3fb7 100644
> --- gcc/cp/semantics.c
> +++ gcc/cp/semantics.c
> @@ -2795,11 +2795,14 @@ finish_compound_literal (tree type, tree compound_literal,
>   	  return error_mark_node;
>         }
>   
> -  if (processing_template_decl)
> +  if (instantiation_dependent_expression_p (compound_literal)
> +      || dependent_type_p (type))
>       {
>         TREE_TYPE (compound_literal) = type;
>         /* Mark the expression as a compound literal.  */
>         TREE_HAS_CONSTRUCTOR (compound_literal) = 1;
> +      /* And as instantiation-dependent.  */
> +      CONSTRUCTOR_IS_DEPENDENT (compound_literal) = true;
>         if (fcl_context == fcl_c99)
>   	CONSTRUCTOR_C99_COMPOUND_LITERAL (compound_literal) = 1;
>         return compound_literal;
> @@ -2822,8 +2825,7 @@ finish_compound_literal (tree type, tree compound_literal,
>         && check_array_initializer (NULL_TREE, type, compound_literal))
>       return error_mark_node;
>     compound_literal = reshape_init (type, compound_literal, complain);
> -  if (SCALAR_TYPE_P (type)
> -      && !BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
> +  if (!BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
>         && !check_narrowing (type, compound_literal, complain))
>       return error_mark_node;

Hmm, I wonder why we would want to call check_narrowing here at all; we 
ought to get the narrowing warning from the normal conversion process. 
Perhaps the issue is that we need to pass LOOKUP_NO_NARROWING to 
digest_init.

Jason

On Mon, Jan 21, 2019 at 03:14:53PM -0500, Jason Merrill wrote:
> On 1/18/19 9:12 AM, Marek Polacek wrote:
> > On Thu, Jan 17, 2019 at 04:17:29PM -0500, Jason Merrill wrote:
> > > On 1/17/19 2:09 PM, Marek Polacek wrote:
> > > > This patch ought to fix the rest of 78244, a missing narrowing warning in
> > > > decltype.
> > > > 
> > > > As I explained in Bugzilla, there can be three scenarios:
> > > > 
> > > > 1) decltype is in a template and it has no dependent expressions, which
> > > > is the problematical case.  finish_compound_literal just returns the
> > > > compound literal without checking narrowing if processing_template_decl.
> > > 
> > > This is the sort of thing that we've been gradually fixing: if the compound
> > > literal isn't dependent at all, we want to do the normal processing.  And
> > > then usually return a result based on the original trees rather than the
> > > result of processing.  For instance, finish_call_expr.  Something like that
> > > ought to work here, too, and be more generally applicable; this shouldn't be
> > > limited to casting to a scalar type, casting to a known class type can also
> > > involve narrowing.
> > 
> > Great, that works just fine.  I also had to check if the type is
> > type-dependent, otherwise complete_type could fail.
> > 
> > > The check in the other patch that changes instantiation_dependent_r should
> > > be more similar to the one in finish_compound_literal.  Or perhaps you could
> > > set a flag here in finish_compound_literal to indicate that it's
> > > instantiation-dependent, and just check that in instantiation_dependent_r.
> > 
> > Done, but I feel bad about adding another flag.  But I guess it's cheaper
> > this way.  Thanks!
> > 
> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
> > 
> > 2019-01-18  Marek Polacek  <polacek@redhat.com>
> > 
> > 	PR c++/88815 - narrowing conversion lost in decltype.
> > 	PR c++/78244 - narrowing conversion in template not detected.
> > 	* cp-tree.h (CONSTRUCTOR_IS_DEPENDENT): New.
> > 	* pt.c (instantiation_dependent_r): Consider a CONSTRUCTOR with
> > 	CONSTRUCTOR_IS_DEPENDENT instantiation-dependent.
> > 	* semantics.c (finish_compound_literal): When the compound literal
> > 	isn't instantiation-dependent and the type isn't type-dependent,
> > 	fall back to the normal processing.  Don't only call check_narrowing
> > 	for scalar types.  Set CONSTRUCTOR_IS_DEPENDENT.
> > 
> > 	* g++.dg/cpp0x/Wnarrowing15.C: New test.
> > 	* g++.dg/cpp0x/constexpr-decltype3.C: New test.
> > 	* g++.dg/cpp1y/Wnarrowing1.C: New test.
> > 
> > diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
> > index 5cc8f88d522..778874cccd6 100644
> > --- gcc/cp/cp-tree.h
> > +++ gcc/cp/cp-tree.h
> > @@ -424,6 +424,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
> >         DECL_FINAL_P (in FUNCTION_DECL)
> >         QUALIFIED_NAME_IS_TEMPLATE (in SCOPE_REF)
> >         DECLTYPE_FOR_INIT_CAPTURE (in DECLTYPE_TYPE)
> > +      CONSTRUCTOR_IS_DEPENDENT (in CONSTRUCTOR)
> >         TINFO_USED_TEMPLATE_ID (in TEMPLATE_INFO)
> >         PACK_EXPANSION_SIZEOF_P (in *_PACK_EXPANSION)
> >         OVL_USING_P (in OVERLOAD)
> > @@ -4202,6 +4203,11 @@ more_aggr_init_expr_args_p (const aggr_init_expr_arg_iterator *iter)
> >      B b{1,2}, not B b({1,2}) or B b = {1,2}.  */
> >   #define CONSTRUCTOR_IS_DIRECT_INIT(NODE) (TREE_LANG_FLAG_0 (CONSTRUCTOR_CHECK (NODE)))
> > +/* True if this CONSTRUCTOR is instantiation-dependent and needs to be
> > +   substituted.  */
> > +#define CONSTRUCTOR_IS_DEPENDENT(NODE) \
> > +  (TREE_LANG_FLAG_1 (CONSTRUCTOR_CHECK (NODE)))
> > +
> >   /* True if this CONSTRUCTOR should not be used as a variable initializer
> >      because it was loaded from a constexpr variable with mutable fields.  */
> >   #define CONSTRUCTOR_MUTABLE_POISON(NODE) \
> > diff --git gcc/cp/pt.c gcc/cp/pt.c
> > index e4f76478f54..ae77bae6b29 100644
> > --- gcc/cp/pt.c
> > +++ gcc/cp/pt.c
> > @@ -25800,6 +25800,11 @@ instantiation_dependent_r (tree *tp, int *walk_subtrees,
> >   	return *tp;
> >         break;
> > +    case CONSTRUCTOR:
> > +      if (CONSTRUCTOR_IS_DEPENDENT (*tp))
> > +	return *tp;
> > +      break;
> > +
> >       default:
> >         break;
> >       }
> > diff --git gcc/cp/semantics.c gcc/cp/semantics.c
> > index e654750d249..4ff09ad3fb7 100644
> > --- gcc/cp/semantics.c
> > +++ gcc/cp/semantics.c
> > @@ -2795,11 +2795,14 @@ finish_compound_literal (tree type, tree compound_literal,
> >   	  return error_mark_node;
> >         }
> > -  if (processing_template_decl)
> > +  if (instantiation_dependent_expression_p (compound_literal)
> > +      || dependent_type_p (type))
> >       {
> >         TREE_TYPE (compound_literal) = type;
> >         /* Mark the expression as a compound literal.  */
> >         TREE_HAS_CONSTRUCTOR (compound_literal) = 1;
> > +      /* And as instantiation-dependent.  */
> > +      CONSTRUCTOR_IS_DEPENDENT (compound_literal) = true;
> >         if (fcl_context == fcl_c99)
> >   	CONSTRUCTOR_C99_COMPOUND_LITERAL (compound_literal) = 1;
> >         return compound_literal;
> > @@ -2822,8 +2825,7 @@ finish_compound_literal (tree type, tree compound_literal,
> >         && check_array_initializer (NULL_TREE, type, compound_literal))
> >       return error_mark_node;
> >     compound_literal = reshape_init (type, compound_literal, complain);
> > -  if (SCALAR_TYPE_P (type)
> > -      && !BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
> > +  if (!BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
> >         && !check_narrowing (type, compound_literal, complain))
> >       return error_mark_node;
> 
> Hmm, I wonder why we would want to call check_narrowing here at all; we
> ought to get the narrowing warning from the normal conversion process.
> Perhaps the issue is that we need to pass LOOKUP_NO_NARROWING to
> digest_init.

reshape_init never ends up calling convert_for_assignment or convert_like_real,
so even if we signalled to check narrowing to it, it wouldn't help.
reshape_init calls check_narrowing just for enum initialization.

Marek

On 1/22/19 4:10 PM, Marek Polacek wrote:
> On Mon, Jan 21, 2019 at 03:14:53PM -0500, Jason Merrill wrote:
>> On 1/18/19 9:12 AM, Marek Polacek wrote:
>>> On Thu, Jan 17, 2019 at 04:17:29PM -0500, Jason Merrill wrote:
>>>> On 1/17/19 2:09 PM, Marek Polacek wrote:
>>>>> This patch ought to fix the rest of 78244, a missing narrowing warning in
>>>>> decltype.
>>>>>
>>>>> As I explained in Bugzilla, there can be three scenarios:
>>>>>
>>>>> 1) decltype is in a template and it has no dependent expressions, which
>>>>> is the problematical case.  finish_compound_literal just returns the
>>>>> compound literal without checking narrowing if processing_template_decl.
>>>>
>>>> This is the sort of thing that we've been gradually fixing: if the compound
>>>> literal isn't dependent at all, we want to do the normal processing.  And
>>>> then usually return a result based on the original trees rather than the
>>>> result of processing.  For instance, finish_call_expr.  Something like that
>>>> ought to work here, too, and be more generally applicable; this shouldn't be
>>>> limited to casting to a scalar type, casting to a known class type can also
>>>> involve narrowing.
>>>
>>> Great, that works just fine.  I also had to check if the type is
>>> type-dependent, otherwise complete_type could fail.
>>>
>>>> The check in the other patch that changes instantiation_dependent_r should
>>>> be more similar to the one in finish_compound_literal.  Or perhaps you could
>>>> set a flag here in finish_compound_literal to indicate that it's
>>>> instantiation-dependent, and just check that in instantiation_dependent_r.
>>>
>>> Done, but I feel bad about adding another flag.  But I guess it's cheaper
>>> this way.  Thanks!
>>>
>>> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>>>
>>> 2019-01-18  Marek Polacek  <polacek@redhat.com>
>>>
>>> 	PR c++/88815 - narrowing conversion lost in decltype.
>>> 	PR c++/78244 - narrowing conversion in template not detected.
>>> 	* cp-tree.h (CONSTRUCTOR_IS_DEPENDENT): New.
>>> 	* pt.c (instantiation_dependent_r): Consider a CONSTRUCTOR with
>>> 	CONSTRUCTOR_IS_DEPENDENT instantiation-dependent.
>>> 	* semantics.c (finish_compound_literal): When the compound literal
>>> 	isn't instantiation-dependent and the type isn't type-dependent,
>>> 	fall back to the normal processing.  Don't only call check_narrowing
>>> 	for scalar types.  Set CONSTRUCTOR_IS_DEPENDENT.
>>>
>>> 	* g++.dg/cpp0x/Wnarrowing15.C: New test.
>>> 	* g++.dg/cpp0x/constexpr-decltype3.C: New test.
>>> 	* g++.dg/cpp1y/Wnarrowing1.C: New test.
>>>
>>> diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
>>> index 5cc8f88d522..778874cccd6 100644
>>> --- gcc/cp/cp-tree.h
>>> +++ gcc/cp/cp-tree.h
>>> @@ -424,6 +424,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
>>>          DECL_FINAL_P (in FUNCTION_DECL)
>>>          QUALIFIED_NAME_IS_TEMPLATE (in SCOPE_REF)
>>>          DECLTYPE_FOR_INIT_CAPTURE (in DECLTYPE_TYPE)
>>> +      CONSTRUCTOR_IS_DEPENDENT (in CONSTRUCTOR)
>>>          TINFO_USED_TEMPLATE_ID (in TEMPLATE_INFO)
>>>          PACK_EXPANSION_SIZEOF_P (in *_PACK_EXPANSION)
>>>          OVL_USING_P (in OVERLOAD)
>>> @@ -4202,6 +4203,11 @@ more_aggr_init_expr_args_p (const aggr_init_expr_arg_iterator *iter)
>>>       B b{1,2}, not B b({1,2}) or B b = {1,2}.  */
>>>    #define CONSTRUCTOR_IS_DIRECT_INIT(NODE) (TREE_LANG_FLAG_0 (CONSTRUCTOR_CHECK (NODE)))
>>> +/* True if this CONSTRUCTOR is instantiation-dependent and needs to be
>>> +   substituted.  */
>>> +#define CONSTRUCTOR_IS_DEPENDENT(NODE) \
>>> +  (TREE_LANG_FLAG_1 (CONSTRUCTOR_CHECK (NODE)))
>>> +
>>>    /* True if this CONSTRUCTOR should not be used as a variable initializer
>>>       because it was loaded from a constexpr variable with mutable fields.  */
>>>    #define CONSTRUCTOR_MUTABLE_POISON(NODE) \
>>> diff --git gcc/cp/pt.c gcc/cp/pt.c
>>> index e4f76478f54..ae77bae6b29 100644
>>> --- gcc/cp/pt.c
>>> +++ gcc/cp/pt.c
>>> @@ -25800,6 +25800,11 @@ instantiation_dependent_r (tree *tp, int *walk_subtrees,
>>>    	return *tp;
>>>          break;
>>> +    case CONSTRUCTOR:
>>> +      if (CONSTRUCTOR_IS_DEPENDENT (*tp))
>>> +	return *tp;
>>> +      break;
>>> +
>>>        default:
>>>          break;
>>>        }
>>> diff --git gcc/cp/semantics.c gcc/cp/semantics.c
>>> index e654750d249..4ff09ad3fb7 100644
>>> --- gcc/cp/semantics.c
>>> +++ gcc/cp/semantics.c
>>> @@ -2795,11 +2795,14 @@ finish_compound_literal (tree type, tree compound_literal,
>>>    	  return error_mark_node;
>>>          }
>>> -  if (processing_template_decl)
>>> +  if (instantiation_dependent_expression_p (compound_literal)
>>> +      || dependent_type_p (type))
>>>        {
>>>          TREE_TYPE (compound_literal) = type;
>>>          /* Mark the expression as a compound literal.  */
>>>          TREE_HAS_CONSTRUCTOR (compound_literal) = 1;
>>> +      /* And as instantiation-dependent.  */
>>> +      CONSTRUCTOR_IS_DEPENDENT (compound_literal) = true;
>>>          if (fcl_context == fcl_c99)
>>>    	CONSTRUCTOR_C99_COMPOUND_LITERAL (compound_literal) = 1;
>>>          return compound_literal;
>>> @@ -2822,8 +2825,7 @@ finish_compound_literal (tree type, tree compound_literal,
>>>          && check_array_initializer (NULL_TREE, type, compound_literal))
>>>        return error_mark_node;
>>>      compound_literal = reshape_init (type, compound_literal, complain);
>>> -  if (SCALAR_TYPE_P (type)
>>> -      && !BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
>>> +  if (!BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
>>>          && !check_narrowing (type, compound_literal, complain))
>>>        return error_mark_node;
>>
>> Hmm, I wonder why we would want to call check_narrowing here at all; we
>> ought to get the narrowing warning from the normal conversion process.
>> Perhaps the issue is that we need to pass LOOKUP_NO_NARROWING to
>> digest_init.
> 
> reshape_init never ends up calling convert_for_assignment or convert_like_real,
> so even if we signalled to check narrowing to it, it wouldn't help.
> reshape_init calls check_narrowing just for enum initialization.

I was talking about digest_init, not reshape_init.  digest_init calls 
convert_for_initialization.

Jason

On Wed, Jan 23, 2019 at 09:00:36AM -0500, Jason Merrill wrote:
> I was talking about digest_init, not reshape_init.  digest_init calls
> convert_for_initialization.

/facepalm

So yes, digest_init calls convert_for_initialization which will end up
calling perform_implicit_conversion_flags which could call convert_like_real
where the narrowing warnings are given, but it doesn't, we go to this case:

  else if (processing_template_decl && conv->kind != ck_identity)
    {
      /* In a template, we are only concerned about determining the
         type of non-dependent expressions, so we do not have to
         perform the actual conversion.  But for initializers, we
         need to be able to perform it at instantiation
         (or instantiate_non_dependent_expr) time.  */
      expr = build1 (IMPLICIT_CONV_EXPR, type, expr);

finish_decltype_type throws away the expression because it's not dependent, and
only uses its type.  So narrowing remains undetected.  Not sure if I should mess
with perform_implicit_conversion_flags.

On Wed, Jan 23, 2019 at 12:57 PM Marek Polacek <polacek@redhat.com> wrote:
>
> On Wed, Jan 23, 2019 at 09:00:36AM -0500, Jason Merrill wrote:
> > I was talking about digest_init, not reshape_init.  digest_init calls
> > convert_for_initialization.
>
> /facepalm
>
> So yes, digest_init calls convert_for_initialization which will end up
> calling perform_implicit_conversion_flags which could call convert_like_real
> where the narrowing warnings are given, but it doesn't, we go to this case:
>
>   else if (processing_template_decl && conv->kind != ck_identity)
>     {
>       /* In a template, we are only concerned about determining the
>          type of non-dependent expressions, so we do not have to
>          perform the actual conversion.  But for initializers, we
>          need to be able to perform it at instantiation
>          (or instantiate_non_dependent_expr) time.  */
>       expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
>
> finish_decltype_type throws away the expression because it's not dependent, and
> only uses its type.  So narrowing remains undetected.  Not sure if I should mess
> with perform_implicit_conversion_flags.

Let's try that; this is a situation where the comment is incorrect.
Perhaps just call check_narrowing here if appropriate, rather than go
through the whole conversion machinery.

Jason

On Wed, Jan 23, 2019 at 03:34:04PM -0500, Jason Merrill wrote:
> On Wed, Jan 23, 2019 at 12:57 PM Marek Polacek <polacek@redhat.com> wrote:
> >
> > On Wed, Jan 23, 2019 at 09:00:36AM -0500, Jason Merrill wrote:
> > > I was talking about digest_init, not reshape_init.  digest_init calls
> > > convert_for_initialization.
> >
> > /facepalm
> >
> > So yes, digest_init calls convert_for_initialization which will end up
> > calling perform_implicit_conversion_flags which could call convert_like_real
> > where the narrowing warnings are given, but it doesn't, we go to this case:
> >
> >   else if (processing_template_decl && conv->kind != ck_identity)
> >     {
> >       /* In a template, we are only concerned about determining the
> >          type of non-dependent expressions, so we do not have to
> >          perform the actual conversion.  But for initializers, we
> >          need to be able to perform it at instantiation
> >          (or instantiate_non_dependent_expr) time.  */
> >       expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
> >
> > finish_decltype_type throws away the expression because it's not dependent, and
> > only uses its type.  So narrowing remains undetected.  Not sure if I should mess
> > with perform_implicit_conversion_flags.
> 
> Let's try that; this is a situation where the comment is incorrect.
> Perhaps just call check_narrowing here if appropriate, rather than go
> through the whole conversion machinery.

I have not been successful.

First, I modified perform_implicit_conversion_flags to go the convert_like
route when dealing with something non-dependent.  That breaks e.g. in
build_value_init:
 346   /* The AGGR_INIT_EXPR tweaking below breaks in templates.  */
 347   gcc_assert (!processing_template_decl
 348               || (SCALAR_TYPE_P (type) || TREE_CODE (type) == ARRAY_TYPE));
Even if I restrict the convert_like way for non-dependent exprs in a template
for scalars, it still breaks elsewhere, e.g. constexpr-template3.C where it
complains about taking the address of an rvalue.

Second, I added check_narrowing to the processing_template_decl case in
perform_implicit_conversion_flags.  That works except it breaks
constexpr-inst1.C -- we no longer get the error.  That's because currently
check_narrowing in finish_compound_literal calls maybe_constant_init, which
calls instantiate_constexpr_fns and we get the desired diagnostic.  But if
I move check_narrowing to perform_implicit_conversion_flags, we no longer
call it in this case -- processing_template_decl is 0 so we call convert_like
but that doesn't do the trick.

So, back to the patch that leaves check_narrowing in finish_compound_literal?

Marek

On 1/24/19 7:17 PM, Marek Polacek wrote:
> On Wed, Jan 23, 2019 at 03:34:04PM -0500, Jason Merrill wrote:
>> On Wed, Jan 23, 2019 at 12:57 PM Marek Polacek <polacek@redhat.com> wrote:
>>>
>>> On Wed, Jan 23, 2019 at 09:00:36AM -0500, Jason Merrill wrote:
>>>> I was talking about digest_init, not reshape_init.  digest_init calls
>>>> convert_for_initialization.
>>>
>>> /facepalm
>>>
>>> So yes, digest_init calls convert_for_initialization which will end up
>>> calling perform_implicit_conversion_flags which could call convert_like_real
>>> where the narrowing warnings are given, but it doesn't, we go to this case:
>>>
>>>    else if (processing_template_decl && conv->kind != ck_identity)
>>>      {
>>>        /* In a template, we are only concerned about determining the
>>>           type of non-dependent expressions, so we do not have to
>>>           perform the actual conversion.  But for initializers, we
>>>           need to be able to perform it at instantiation
>>>           (or instantiate_non_dependent_expr) time.  */
>>>        expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
>>>
>>> finish_decltype_type throws away the expression because it's not dependent, and
>>> only uses its type.  So narrowing remains undetected.  Not sure if I should mess
>>> with perform_implicit_conversion_flags.
>>
>> Let's try that; this is a situation where the comment is incorrect.
>> Perhaps just call check_narrowing here if appropriate, rather than go
>> through the whole conversion machinery.
> 
> I have not been successful.
> 
> First, I modified perform_implicit_conversion_flags to go the convert_like
> route when dealing with something non-dependent.  That breaks e.g. in
> build_value_init:
>   346   /* The AGGR_INIT_EXPR tweaking below breaks in templates.  */
>   347   gcc_assert (!processing_template_decl
>   348               || (SCALAR_TYPE_P (type) || TREE_CODE (type) == ARRAY_TYPE));
> Even if I restrict the convert_like way for non-dependent exprs in a template
> for scalars, it still breaks elsewhere, e.g. constexpr-template3.C where it
> complains about taking the address of an rvalue.
> 
> Second, I added check_narrowing to the processing_template_decl case in
> perform_implicit_conversion_flags.  That works except it breaks
> constexpr-inst1.C -- we no longer get the error.  That's because currently
> check_narrowing in finish_compound_literal calls maybe_constant_init, which
> calls instantiate_constexpr_fns and we get the desired diagnostic.  But if
> I move check_narrowing to perform_implicit_conversion_flags, we no longer
> call it in this case -- processing_template_decl is 0 so we call convert_like
> but that doesn't do the trick.
> 
> So, back to the patch that leaves check_narrowing in finish_compound_literal?

That patch still needs a test for the aggregate case.

Jason

On Fri, Jan 25, 2019 at 10:05:00AM -0500, Jason Merrill wrote:
> On 1/24/19 7:17 PM, Marek Polacek wrote:
> > On Wed, Jan 23, 2019 at 03:34:04PM -0500, Jason Merrill wrote:
> > > On Wed, Jan 23, 2019 at 12:57 PM Marek Polacek <polacek@redhat.com> wrote:
> > > > 
> > > > On Wed, Jan 23, 2019 at 09:00:36AM -0500, Jason Merrill wrote:
> > > > > I was talking about digest_init, not reshape_init.  digest_init calls
> > > > > convert_for_initialization.
> > > > 
> > > > /facepalm
> > > > 
> > > > So yes, digest_init calls convert_for_initialization which will end up
> > > > calling perform_implicit_conversion_flags which could call convert_like_real
> > > > where the narrowing warnings are given, but it doesn't, we go to this case:
> > > > 
> > > >    else if (processing_template_decl && conv->kind != ck_identity)
> > > >      {
> > > >        /* In a template, we are only concerned about determining the
> > > >           type of non-dependent expressions, so we do not have to
> > > >           perform the actual conversion.  But for initializers, we
> > > >           need to be able to perform it at instantiation
> > > >           (or instantiate_non_dependent_expr) time.  */
> > > >        expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
> > > > 
> > > > finish_decltype_type throws away the expression because it's not dependent, and
> > > > only uses its type.  So narrowing remains undetected.  Not sure if I should mess
> > > > with perform_implicit_conversion_flags.
> > > 
> > > Let's try that; this is a situation where the comment is incorrect.
> > > Perhaps just call check_narrowing here if appropriate, rather than go
> > > through the whole conversion machinery.
> > 
> > I have not been successful.
> > 
> > First, I modified perform_implicit_conversion_flags to go the convert_like
> > route when dealing with something non-dependent.  That breaks e.g. in
> > build_value_init:
> >   346   /* The AGGR_INIT_EXPR tweaking below breaks in templates.  */
> >   347   gcc_assert (!processing_template_decl
> >   348               || (SCALAR_TYPE_P (type) || TREE_CODE (type) == ARRAY_TYPE));
> > Even if I restrict the convert_like way for non-dependent exprs in a template
> > for scalars, it still breaks elsewhere, e.g. constexpr-template3.C where it
> > complains about taking the address of an rvalue.
> > 
> > Second, I added check_narrowing to the processing_template_decl case in
> > perform_implicit_conversion_flags.  That works except it breaks
> > constexpr-inst1.C -- we no longer get the error.  That's because currently
> > check_narrowing in finish_compound_literal calls maybe_constant_init, which
> > calls instantiate_constexpr_fns and we get the desired diagnostic.  But if
> > I move check_narrowing to perform_implicit_conversion_flags, we no longer
> > call it in this case -- processing_template_decl is 0 so we call convert_like
> > but that doesn't do the trick.
> > 
> > So, back to the patch that leaves check_narrowing in finish_compound_literal?
> 
> That patch still needs a test for the aggregate case.

Ok, this is a version with Wnarrowing16.C added.

...but we still don't warn for the TYPE_NON_AGGREGATE_CLASS case in
finish_compound_literal, so the nightmare continues.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-01-25  Marek Polacek  <polacek@redhat.com>

	PR c++/88815 - narrowing conversion lost in decltype.
	PR c++/78244 - narrowing conversion in template not detected.
	* cp-tree.h (CONSTRUCTOR_IS_DEPENDENT): New.
	* pt.c (instantiation_dependent_r): Consider a CONSTRUCTOR with
	CONSTRUCTOR_IS_DEPENDENT instantiation-dependent.
	* semantics.c (finish_compound_literal): When the compound literal
	isn't instantiation-dependent and the type isn't type-dependent,
	fall back to the normal processing.  Don't only call check_narrowing
	for scalar types.  Set CONSTRUCTOR_IS_DEPENDENT.

	* g++.dg/cpp0x/Wnarrowing15.C: New test.
	* g++.dg/cpp0x/Wnarrowing16.C: New test.
	* g++.dg/cpp0x/constexpr-decltype3.C: New test.
	* g++.dg/cpp1y/Wnarrowing1.C: New test.

diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
index cd902ce1cf6..77e1425b435 100644
--- gcc/cp/cp-tree.h
+++ gcc/cp/cp-tree.h
@@ -424,6 +424,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
       DECL_FINAL_P (in FUNCTION_DECL)
       QUALIFIED_NAME_IS_TEMPLATE (in SCOPE_REF)
       DECLTYPE_FOR_INIT_CAPTURE (in DECLTYPE_TYPE)
+      CONSTRUCTOR_IS_DEPENDENT (in CONSTRUCTOR)
       TINFO_USED_TEMPLATE_ID (in TEMPLATE_INFO)
       PACK_EXPANSION_SIZEOF_P (in *_PACK_EXPANSION)
       OVL_USING_P (in OVERLOAD)
@@ -4205,6 +4206,11 @@ more_aggr_init_expr_args_p (const aggr_init_expr_arg_iterator *iter)
    B b{1,2}, not B b({1,2}) or B b = {1,2}.  */
 #define CONSTRUCTOR_IS_DIRECT_INIT(NODE) (TREE_LANG_FLAG_0 (CONSTRUCTOR_CHECK (NODE)))
 
+/* True if this CONSTRUCTOR is instantiation-dependent and needs to be
+   substituted.  */
+#define CONSTRUCTOR_IS_DEPENDENT(NODE) \
+  (TREE_LANG_FLAG_1 (CONSTRUCTOR_CHECK (NODE)))
+
 /* True if this CONSTRUCTOR should not be used as a variable initializer
    because it was loaded from a constexpr variable with mutable fields.  */
 #define CONSTRUCTOR_MUTABLE_POISON(NODE) \
diff --git gcc/cp/pt.c gcc/cp/pt.c
index 48c180cc13b..f8b3054533e 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -25815,6 +25815,11 @@ instantiation_dependent_r (tree *tp, int *walk_subtrees,
 	return *tp;
       break;
 
+    case CONSTRUCTOR:
+      if (CONSTRUCTOR_IS_DEPENDENT (*tp))
+	return *tp;
+      break;
+
     default:
       break;
     }
diff --git gcc/cp/semantics.c gcc/cp/semantics.c
index 72191395b47..f1df2561258 100644
--- gcc/cp/semantics.c
+++ gcc/cp/semantics.c
@@ -2795,11 +2795,14 @@ finish_compound_literal (tree type, tree compound_literal,
 	  return error_mark_node;
       }
 
-  if (processing_template_decl)
+  if (instantiation_dependent_expression_p (compound_literal)
+      || dependent_type_p (type))
     {
       TREE_TYPE (compound_literal) = type;
       /* Mark the expression as a compound literal.  */
       TREE_HAS_CONSTRUCTOR (compound_literal) = 1;
+      /* And as instantiation-dependent.  */
+      CONSTRUCTOR_IS_DEPENDENT (compound_literal) = true;
       if (fcl_context == fcl_c99)
 	CONSTRUCTOR_C99_COMPOUND_LITERAL (compound_literal) = 1;
       return compound_literal;
@@ -2822,8 +2825,7 @@ finish_compound_literal (tree type, tree compound_literal,
       && check_array_initializer (NULL_TREE, type, compound_literal))
     return error_mark_node;
   compound_literal = reshape_init (type, compound_literal, complain);
-  if (SCALAR_TYPE_P (type)
-      && !BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
+  if (!BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
       && !check_narrowing (type, compound_literal, complain))
     return error_mark_node;
   if (TREE_CODE (type) == ARRAY_TYPE
diff --git gcc/testsuite/g++.dg/cpp0x/Wnarrowing15.C gcc/testsuite/g++.dg/cpp0x/Wnarrowing15.C
new file mode 100644
index 00000000000..4e7c17dcfca
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/Wnarrowing15.C
@@ -0,0 +1,14 @@
+// PR c++/78244
+// { dg-do compile { target c++11 } }
+
+template <typename T>
+auto f1() -> decltype(int{2.0}, void()) { } // { dg-error "narrowing conversion" }
+
+template <typename T>
+auto f2() -> decltype(int{2.0}) { return 1; } // { dg-error "narrowing conversion" }
+
+template <typename T>
+auto f3() -> decltype(void(), int{2.0}) { return 1; } // { dg-error "narrowing conversion" }
+
+template <typename T>
+auto f4() -> decltype((int{2.0})) { return 1; } // { dg-error "narrowing conversion" }
diff --git gcc/testsuite/g++.dg/cpp0x/Wnarrowing16.C gcc/testsuite/g++.dg/cpp0x/Wnarrowing16.C
new file mode 100644
index 00000000000..21394be1b97
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/Wnarrowing16.C
@@ -0,0 +1,16 @@
+// PR c++/78244
+// { dg-do compile { target c++11 } }
+
+struct S { int d; };
+
+template <typename T>
+auto f1() -> decltype(S{2.0}, void()) { } // { dg-error "narrowing conversion" }
+
+template <typename T>
+auto f2() -> decltype(S{2.0}, 1) { return 1; } // { dg-error "narrowing conversion" }
+
+template <typename T>
+auto f3() -> decltype(void(), S{2.0}, 1) { return 1; } // { dg-error "narrowing conversion" }
+
+template <typename T>
+auto f4() -> decltype((S{2.0}, 1)) { return 1; } // { dg-error "narrowing conversion" }
diff --git gcc/testsuite/g++.dg/cpp0x/constexpr-decltype3.C gcc/testsuite/g++.dg/cpp0x/constexpr-decltype3.C
new file mode 100644
index 00000000000..fd05366de50
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/constexpr-decltype3.C
@@ -0,0 +1,25 @@
+// PR c++/88815
+// { dg-do compile { target c++11 } }
+
+struct true_type {
+    constexpr operator bool() const { return true; }
+};
+
+struct false_type {
+    constexpr operator bool() const { return false; }
+};
+
+template<int (*p)()>
+true_type is_constexpr_impl(decltype(int{(p(), 0U)}));
+
+template<int (*p)()>
+false_type is_constexpr_impl(...);
+
+template<int (*p)()>
+using is_constexpr = decltype(is_constexpr_impl<p>(0));
+
+constexpr int f() { return 0; }
+int g() { return 0; }
+
+static_assert(is_constexpr<f>(), "");
+static_assert(!is_constexpr<g>(), "");
diff --git gcc/testsuite/g++.dg/cpp1y/Wnarrowing1.C gcc/testsuite/g++.dg/cpp1y/Wnarrowing1.C
new file mode 100644
index 00000000000..e1e499542f0
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1y/Wnarrowing1.C
@@ -0,0 +1,5 @@
+// PR c++/78244
+// { dg-do compile { target c++14 } }
+
+template<typename>
+decltype(int{1.1}) v; // { dg-error "narrowing conversion" }

On Fri, Jan 25, 2019 at 4:20 PM Marek Polacek <polacek@redhat.com> wrote:
> On Fri, Jan 25, 2019 at 10:05:00AM -0500, Jason Merrill wrote:
> > On 1/24/19 7:17 PM, Marek Polacek wrote:
> > > On Wed, Jan 23, 2019 at 03:34:04PM -0500, Jason Merrill wrote:
> > > > On Wed, Jan 23, 2019 at 12:57 PM Marek Polacek <polacek@redhat.com> wrote:
> > > > >
> > > > > On Wed, Jan 23, 2019 at 09:00:36AM -0500, Jason Merrill wrote:
> > > > > > I was talking about digest_init, not reshape_init.  digest_init calls
> > > > > > convert_for_initialization.
> > > > >
> > > > > /facepalm
> > > > >
> > > > > So yes, digest_init calls convert_for_initialization which will end up
> > > > > calling perform_implicit_conversion_flags which could call convert_like_real
> > > > > where the narrowing warnings are given, but it doesn't, we go to this case:
> > > > >
> > > > >    else if (processing_template_decl && conv->kind != ck_identity)
> > > > >      {
> > > > >        /* In a template, we are only concerned about determining the
> > > > >           type of non-dependent expressions, so we do not have to
> > > > >           perform the actual conversion.  But for initializers, we
> > > > >           need to be able to perform it at instantiation
> > > > >           (or instantiate_non_dependent_expr) time.  */
> > > > >        expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
> > > > >
> > > > > finish_decltype_type throws away the expression because it's not dependent, and
> > > > > only uses its type.  So narrowing remains undetected.  Not sure if I should mess
> > > > > with perform_implicit_conversion_flags.
> > > >
> > > > Let's try that; this is a situation where the comment is incorrect.
> > > > Perhaps just call check_narrowing here if appropriate, rather than go
> > > > through the whole conversion machinery.
> > >
> > > I have not been successful.
> > >
> > > First, I modified perform_implicit_conversion_flags to go the convert_like
> > > route when dealing with something non-dependent.  That breaks e.g. in
> > > build_value_init:
> > >   346   /* The AGGR_INIT_EXPR tweaking below breaks in templates.  */
> > >   347   gcc_assert (!processing_template_decl
> > >   348               || (SCALAR_TYPE_P (type) || TREE_CODE (type) == ARRAY_TYPE));
> > > Even if I restrict the convert_like way for non-dependent exprs in a template
> > > for scalars, it still breaks elsewhere, e.g. constexpr-template3.C where it
> > > complains about taking the address of an rvalue.
> > >
> > > Second, I added check_narrowing to the processing_template_decl case in
> > > perform_implicit_conversion_flags.  That works except it breaks
> > > constexpr-inst1.C -- we no longer get the error.  That's because currently
> > > check_narrowing in finish_compound_literal calls maybe_constant_init, which
> > > calls instantiate_constexpr_fns and we get the desired diagnostic.  But if
> > > I move check_narrowing to perform_implicit_conversion_flags, we no longer
> > > call it in this case -- processing_template_decl is 0 so we call convert_like
> > > but that doesn't do the trick.
> > >
> > > So, back to the patch that leaves check_narrowing in finish_compound_literal?
> >
> > That patch still needs a test for the aggregate case.
>
> Ok, this is a version with Wnarrowing16.C added.
>
> ...but we still don't warn for the TYPE_NON_AGGREGATE_CLASS case in
> finish_compound_literal, so the nightmare continues.

Alas.  Are you going to keep looking at that, or would you like me to take over?

> -  if (SCALAR_TYPE_P (type)
> -      && !BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
> +  if (!BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
>        && !check_narrowing (type, compound_literal, complain))
>      return error_mark_node;

Does this hunk actually make a difference?  It looks like
check_narrowing only does anything for arithmetic types.

OK with or without this hunk.

Jason

On Sat, Jan 26, 2019 at 07:22:17PM -0500, Jason Merrill wrote:
> On Fri, Jan 25, 2019 at 4:20 PM Marek Polacek <polacek@redhat.com> wrote:
> > On Fri, Jan 25, 2019 at 10:05:00AM -0500, Jason Merrill wrote:
> > > On 1/24/19 7:17 PM, Marek Polacek wrote:
> > > > On Wed, Jan 23, 2019 at 03:34:04PM -0500, Jason Merrill wrote:
> > > > > On Wed, Jan 23, 2019 at 12:57 PM Marek Polacek <polacek@redhat.com> wrote:
> > > > > >
> > > > > > On Wed, Jan 23, 2019 at 09:00:36AM -0500, Jason Merrill wrote:
> > > > > > > I was talking about digest_init, not reshape_init.  digest_init calls
> > > > > > > convert_for_initialization.
> > > > > >
> > > > > > /facepalm
> > > > > >
> > > > > > So yes, digest_init calls convert_for_initialization which will end up
> > > > > > calling perform_implicit_conversion_flags which could call convert_like_real
> > > > > > where the narrowing warnings are given, but it doesn't, we go to this case:
> > > > > >
> > > > > >    else if (processing_template_decl && conv->kind != ck_identity)
> > > > > >      {
> > > > > >        /* In a template, we are only concerned about determining the
> > > > > >           type of non-dependent expressions, so we do not have to
> > > > > >           perform the actual conversion.  But for initializers, we
> > > > > >           need to be able to perform it at instantiation
> > > > > >           (or instantiate_non_dependent_expr) time.  */
> > > > > >        expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
> > > > > >
> > > > > > finish_decltype_type throws away the expression because it's not dependent, and
> > > > > > only uses its type.  So narrowing remains undetected.  Not sure if I should mess
> > > > > > with perform_implicit_conversion_flags.
> > > > >
> > > > > Let's try that; this is a situation where the comment is incorrect.
> > > > > Perhaps just call check_narrowing here if appropriate, rather than go
> > > > > through the whole conversion machinery.
> > > >
> > > > I have not been successful.
> > > >
> > > > First, I modified perform_implicit_conversion_flags to go the convert_like
> > > > route when dealing with something non-dependent.  That breaks e.g. in
> > > > build_value_init:
> > > >   346   /* The AGGR_INIT_EXPR tweaking below breaks in templates.  */
> > > >   347   gcc_assert (!processing_template_decl
> > > >   348               || (SCALAR_TYPE_P (type) || TREE_CODE (type) == ARRAY_TYPE));
> > > > Even if I restrict the convert_like way for non-dependent exprs in a template
> > > > for scalars, it still breaks elsewhere, e.g. constexpr-template3.C where it
> > > > complains about taking the address of an rvalue.
> > > >
> > > > Second, I added check_narrowing to the processing_template_decl case in
> > > > perform_implicit_conversion_flags.  That works except it breaks
> > > > constexpr-inst1.C -- we no longer get the error.  That's because currently
> > > > check_narrowing in finish_compound_literal calls maybe_constant_init, which
> > > > calls instantiate_constexpr_fns and we get the desired diagnostic.  But if
> > > > I move check_narrowing to perform_implicit_conversion_flags, we no longer
> > > > call it in this case -- processing_template_decl is 0 so we call convert_like
> > > > but that doesn't do the trick.
> > > >
> > > > So, back to the patch that leaves check_narrowing in finish_compound_literal?
> > >
> > > That patch still needs a test for the aggregate case.
> >
> > Ok, this is a version with Wnarrowing16.C added.
> >
> > ...but we still don't warn for the TYPE_NON_AGGREGATE_CLASS case in
> > finish_compound_literal, so the nightmare continues.
> 
> Alas.  Are you going to keep looking at that, or would you like me to take over?

I think I should keep poking at it, but I welcome any advice.  But perhaps we
should leave this for GCC 10?

I'll add the new test to the PR.

> > -  if (SCALAR_TYPE_P (type)
> > -      && !BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
> > +  if (!BRACE_ENCLOSED_INITIALIZER_P (compound_literal)
> >        && !check_narrowing (type, compound_literal, complain))
> >      return error_mark_node;
> 
> Does this hunk actually make a difference?  It looks like
> check_narrowing only does anything for arithmetic types.

No difference (I ran the testsuite with some additional checking).

> OK with or without this hunk.

Thanks for the reviews.

Marek