c++: Fix class NTTP with template arguments [PR92948]

This PR points out an ICE with an alias template and class NTTP, but I
found that there are more issues.  Trouble arise when we use a
(non-type) template parameter as an argument to the template arg list of
a template that accepts a class NTTP and a conversion to a class is
involved, e.g.

  struct A { A(int) { } };
  template<A a> struct B { };

  template<int X> void fn () {
      B<X> b;
  }

Normally for such a conversion we create a TARGET_EXPR with an
AGGR_INIT_EXPR that calls a __ct_comp with some arguments, but not in
this case: when converting X to type A in convert_nontype_argument we
are in a template and the template parameter 'X' is value-dependent, and
AGGR_INIT_EXPR don't work in templates.  So it just creates a TARGET_EXPR
that contains "A::A(*this, X)", but with that overload resolution fails:
  error: no matching function for call to 'A::A(A*, int)'
That happens because finish_call_expr for a BASELINK creates a dummy
object, so we have 'this' twice.  I thought for the value-dependent case
we could use IMPLICIT_CONV_EXPR, as in the patch below.  Note that I
only do this when we convert to a different type than the type of the
expr.  The point is to avoid the call to a converting ctor.

The second issue was an ICE in tsubst_copy: when there's a conversion
like the above involved then
 /* Wrapper to make a C++20 template parameter object const.  */
  op = tsubst_copy (op, args, complain, in_decl);
might not produce a _ZT... VAR_DECL with const type, so the assert
  gcc_assert (CP_TYPE_CONST_P (TREE_TYPE (op)));
fails.  Allowing IMPLICIT_CONV_EXPR there probably makes sense.

And since convert_nontype_argument uses value_dependent_expression_p
a lot, I used a dedicated bool to speed things up.

Bootstrapped/regtested on x86_64-linux, ok for trunk and maybe 9?

2020-01-28  Marek Polacek  <polacek@redhat.com>

	PR c++/92948 - Fix class NTTP with template arguments.
	* pt.c (convert_nontype_argument): Use IMPLICIT_CONV_EXPR when
	converting a value-dependent expression to a class type.
	(tsubst_copy) <case VIEW_CONVERT_EXPR>: Allow IMPLICIT_CONV_EXPR
	as the result of the tsubst_copy call.

	* g++.dg/cpp2a/nontype-class28.C: New test.
	* g++.dg/cpp2a/nontype-class29.C: New test.
---
 gcc/cp/pt.c                                  | 36 +++++++++++--------
 gcc/testsuite/g++.dg/cpp2a/nontype-class28.C | 37 ++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/nontype-class29.C | 26 ++++++++++++++
 3 files changed, 85 insertions(+), 14 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class28.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class29.C


base-commit: ad690d79cfbb905c5546c9333c5fd089d906505b

On 1/28/20 6:34 PM, Marek Polacek wrote:
> This PR points out an ICE with an alias template and class NTTP, but I
> found that there are more issues.  Trouble arise when we use a
> (non-type) template parameter as an argument to the template arg list of
> a template that accepts a class NTTP and a conversion to a class is
> involved, e.g.
> 
>    struct A { A(int) { } };
>    template<A a> struct B { };
> 
>    template<int X> void fn () {
>        B<X> b;
>    }
> 
> Normally for such a conversion we create a TARGET_EXPR with an
> AGGR_INIT_EXPR that calls a __ct_comp with some arguments, but not in
> this case: when converting X to type A in convert_nontype_argument we
> are in a template and the template parameter 'X' is value-dependent, and
> AGGR_INIT_EXPR don't work in templates.  So it just creates a TARGET_EXPR
> that contains "A::A(*this, X)", but with that overload resolution fails:
>    error: no matching function for call to 'A::A(A*, int)'
> That happens because finish_call_expr for a BASELINK creates a dummy
> object, so we have 'this' twice.  I thought for the value-dependent case
> we could use IMPLICIT_CONV_EXPR, as in the patch below.  Note that I
> only do this when we convert to a different type than the type of the
> expr.  The point is to avoid the call to a converting ctor.
> 
> The second issue was an ICE in tsubst_copy: when there's a conversion
> like the above involved then
>   /* Wrapper to make a C++20 template parameter object const.  */
>    op = tsubst_copy (op, args, complain, in_decl);
> might not produce a _ZT... VAR_DECL with const type, so the assert
>    gcc_assert (CP_TYPE_CONST_P (TREE_TYPE (op)));
> fails.  Allowing IMPLICIT_CONV_EXPR there probably makes sense.
> 
> And since convert_nontype_argument uses value_dependent_expression_p
> a lot, I used a dedicated bool to speed things up.
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk and maybe 9?

OK for both.

The const/non-const issue with class NTTP is a pain, I'm wondering if it 
would have been better to make the template parameter const and deal 
with hiding the const where needed, or if that would be a similar pain 
in a different set of places.

> 2020-01-28  Marek Polacek  <polacek@redhat.com>
> 
> 	PR c++/92948 - Fix class NTTP with template arguments.
> 	* pt.c (convert_nontype_argument): Use IMPLICIT_CONV_EXPR when
> 	converting a value-dependent expression to a class type.
> 	(tsubst_copy) <case VIEW_CONVERT_EXPR>: Allow IMPLICIT_CONV_EXPR
> 	as the result of the tsubst_copy call.
> 
> 	* g++.dg/cpp2a/nontype-class28.C: New test.
> 	* g++.dg/cpp2a/nontype-class29.C: New test.
> ---
>   gcc/cp/pt.c                                  | 36 +++++++++++--------
>   gcc/testsuite/g++.dg/cpp2a/nontype-class28.C | 37 ++++++++++++++++++++
>   gcc/testsuite/g++.dg/cpp2a/nontype-class29.C | 26 ++++++++++++++
>   3 files changed, 85 insertions(+), 14 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class28.C
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class29.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index e889c800aca..fccf4e95453 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -7074,7 +7074,8 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>     if (non_dep)
>       expr = instantiate_non_dependent_expr_internal (expr, complain);
>   
> -  if (value_dependent_expression_p (expr))
> +  const bool val_dep_p = value_dependent_expression_p (expr);
> +  if (val_dep_p)
>       expr = canonicalize_expr_argument (expr, complain);
>   
>     /* 14.3.2/5: The null pointer{,-to-member} conversion is applied
> @@ -7100,10 +7101,17 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>   	     a conversion function with a value-dependent argument, which
>   	     could invoke taking the address of a temporary representing
>   	     the result of the conversion.  */
> -	  if (COMPOUND_LITERAL_P (expr)
> -	      && CONSTRUCTOR_IS_DEPENDENT (expr)
> -	      && MAYBE_CLASS_TYPE_P (expr_type)
> -	      && TYPE_HAS_CONVERSION (expr_type))
> +	  if (!same_type_ignoring_top_level_qualifiers_p (type, expr_type)
> +	      && ((COMPOUND_LITERAL_P (expr)
> +		   && CONSTRUCTOR_IS_DEPENDENT (expr)
> +		   && MAYBE_CLASS_TYPE_P (expr_type)
> +		   && TYPE_HAS_CONVERSION (expr_type))
> +		  /* Similarly, converting e.g. an integer to a class
> +		     involves a constructor call.  convert_like would
> +		     create a TARGET_EXPR, but in a template we can't
> +		     use AGGR_INIT_EXPR, and the TARGET_EXPR would lead
> +		     to a bogus error.  */
> +		  || (val_dep_p && MAYBE_CLASS_TYPE_P (type))))
>   	    {
>   	      expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
>   	      IMPLICIT_CONV_EXPR_NONTYPE_ARG (expr) = true;
> @@ -7189,8 +7197,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>   
>         /* Notice that there are constant expressions like '4 % 0' which
>   	 do not fold into integer constants.  */
> -      if (TREE_CODE (expr) != INTEGER_CST
> -	  && !value_dependent_expression_p (expr))
> +      if (TREE_CODE (expr) != INTEGER_CST && !val_dep_p)
>   	{
>   	  if (complain & tf_error)
>   	    {
> @@ -7265,7 +7272,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>   	Here, we do not care about functions, as they are invalid anyway
>   	for a parameter of type pointer-to-object.  */
>   
> -      if (value_dependent_expression_p (expr))
> +      if (val_dep_p)
>   	/* Non-type template parameters are OK.  */
>   	;
>         else if (cxx_dialect >= cxx11 && integer_zerop (expr))
> @@ -7333,8 +7340,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>   	    }
>   	}
>   
> -      if (TYPE_REF_OBJ_P (TREE_TYPE (expr))
> -	  && value_dependent_expression_p (expr))
> +      if (TYPE_REF_OBJ_P (TREE_TYPE (expr)) && val_dep_p)
>   	/* OK, dependent reference.  We don't want to ask whether a DECL is
>   	   itself value-dependent, since what we want here is its address.  */;
>         else
> @@ -7412,7 +7418,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>   
>         /* [temp.arg.nontype] bullet 1 says the pointer to member
>            expression must be a pointer-to-member constant.  */
> -      if (!value_dependent_expression_p (expr)
> +      if (!val_dep_p
>   	  && !check_valid_ptrmem_cst_expr (type, expr, complain))
>   	return NULL_TREE;
>   
> @@ -7429,7 +7435,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>       {
>         /* [temp.arg.nontype] bullet 1 says the pointer to member
>            expression must be a pointer-to-member constant.  */
> -      if (!value_dependent_expression_p (expr)
> +      if (!val_dep_p
>   	  && !check_valid_ptrmem_cst_expr (type, expr, complain))
>   	return NULL_TREE;
>   
> @@ -7452,7 +7458,7 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>       {
>         /* Replace the argument with a reference to the corresponding template
>   	 parameter object.  */
> -      if (!value_dependent_expression_p (expr))
> +      if (!val_dep_p)
>   	expr = get_template_parm_object (expr, complain);
>         if (expr == error_mark_node)
>   	return NULL_TREE;
> @@ -16406,7 +16412,9 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>   		}
>   	      else
>   		{
> -		  gcc_assert (CP_TYPE_CONST_P (TREE_TYPE (op)));
> +		  gcc_assert (CP_TYPE_CONST_P (TREE_TYPE (op))
> +			      || (TREE_CODE (op) == IMPLICIT_CONV_EXPR
> +				  && IMPLICIT_CONV_EXPR_NONTYPE_ARG (op)));
>   		  return op;
>   		}
>   	    }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class28.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class28.C
> new file mode 100644
> index 00000000000..80b9b499355
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class28.C
> @@ -0,0 +1,37 @@
> +// PR c++/92948 - Fix class NTTP with template arguments.
> +// { dg-do compile { target c++2a } }
> +
> +struct A {
> +  constexpr A(int) { }
> +};
> +
> +template<A>
> +struct B {
> +  using U = unsigned;
> +};
> +
> +template<A a>
> +using U = B<a>;
> +
> +template<int X, typename Y = typename B<X>::U>
> +void foo()
> +{
> +}
> +
> +template<int X, typename Y = typename U<X>::U>
> +void foo2()
> +{
> +}
> +
> +template<typename Y = typename B<1>::U>
> +void foo3()
> +{
> +}
> +
> +void
> +g ()
> +{
> +  foo<1>();
> +  foo2<1>();
> +  foo3<>();
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class29.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class29.C
> new file mode 100644
> index 00000000000..2d2e9690a06
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class29.C
> @@ -0,0 +1,26 @@
> +// PR c++/92948 - Fix class NTTP with template arguments.
> +// { dg-do run { target c++2a } }
> +
> +struct A {
> +  int i;
> +  constexpr A(int n) : i(n) { }
> +};
> +
> +template<A a>
> +struct B {
> +  static constexpr int i = a.i;
> +};
> +
> +template<int X>
> +void foo()
> +{
> +  B<X> b;
> +  if (b.i != 42)
> +    __builtin_abort ();
> +}
> +
> +int
> +main ()
> +{
> +  foo<42>();
> +}
> 
> base-commit: ad690d79cfbb905c5546c9333c5fd089d906505b
>