c++: Avoid a suspicious -Wnoexcept warning [PR93805]

In this PR we're emitting -Wnoexcept warnings about potentially-throwing NSDMIs
when computing the noexcept specification of a class's defaulted default
constructor.  Alhough these warnings are in some sense valid, this patch takes
the route of suppressing them, because:

  1. the warning message is confusing in its current form;
  2. warning for 'struct C { B b = B(); };' but not for 'struct C { B b; };'
     is inconsistent; and
  3. emitting a warning here arguably doesn't fall under the umbrella of
     -Wnoexcept, whose documentation says it warns only when a
     noexcept-expression evaluates to false, but there are noexcept-expressions
     here.

Tested on x86_64-pc-linux-gnu, does this look OK to commit?

gcc/cp/ChangeLog:

	PR c++/93805
	* method.c (walk_field_subobs): Pass tf_none as the complain argument to
	expr_noexcept_p.

gcc/testsuite/ChangeLog:

	PR c++/93805
	* g++.dg/warn/Wnoexcept2.C: New test.
---
 gcc/cp/method.c                        |  2 +-
 gcc/testsuite/g++.dg/warn/Wnoexcept2.C | 15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wnoexcept2.C

On 3/22/20 5:14 PM, Patrick Palka wrote:
> In this PR we're emitting -Wnoexcept warnings about potentially-throwing NSDMIs
> when computing the noexcept specification of a class's defaulted default
> constructor.  Alhough these warnings are in some sense valid, this patch takes
> the route of suppressing them, because:
> 
>    1. the warning message is confusing in its current form;
>    2. warning for 'struct C { B b = B(); };' but not for 'struct C { B b; };'
>       is inconsistent; and
>    3. emitting a warning here arguably doesn't fall under the umbrella of
>       -Wnoexcept, whose documentation says it warns only when a
>       noexcept-expression evaluates to false, but there are noexcept-expressions
>       here.
> 
> Tested on x86_64-pc-linux-gnu, does this look OK to commit?

Let's add a comment to maybe_noexcept_warning suggesting that we might 
in future want to do something like walk_subtrees in the case of a 
defaulted function.  OK with that change.

Jason

> gcc/cp/ChangeLog:
> 
> 	PR c++/93805
> 	* method.c (walk_field_subobs): Pass tf_none as the complain argument to
> 	expr_noexcept_p.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/93805
> 	* g++.dg/warn/Wnoexcept2.C: New test.
> ---
>   gcc/cp/method.c                        |  2 +-
>   gcc/testsuite/g++.dg/warn/Wnoexcept2.C | 15 +++++++++++++++
>   2 files changed, 16 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/warn/Wnoexcept2.C
> 
> diff --git a/gcc/cp/method.c b/gcc/cp/method.c
> index c131fd41536..41b9ff86bdd 100644
> --- a/gcc/cp/method.c
> +++ b/gcc/cp/method.c
> @@ -1988,7 +1988,7 @@ walk_field_subobs (tree fields, special_function_kind sfk, tree fnname,
>   		  if (nsdmi == error_mark_node)
>   		    *spec_p = error_mark_node;
>   		  else if (*spec_p != error_mark_node
> -			   && !expr_noexcept_p (nsdmi, complain))
> +			   && !expr_noexcept_p (nsdmi, tf_none))
>   		    *spec_p = noexcept_false_spec;
>   		}
>   	      /* Don't do the normal processing.  */
> diff --git a/gcc/testsuite/g++.dg/warn/Wnoexcept2.C b/gcc/testsuite/g++.dg/warn/Wnoexcept2.C
> new file mode 100644
> index 00000000000..60541be3575
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wnoexcept2.C
> @@ -0,0 +1,15 @@
> +// PR c++/93805
> +// { dg-do compile { target c++11 } }
> +// { dg-additional-options "-Wnoexcept" }
> +
> +struct B
> +{
> +  B() {}
> +};
> +
> +struct C
> +{
> +  B b = B();
> +};
> +
> +C c; // { dg-bogus "noexcept-expression" }
>

On Mon, 23 Mar 2020, Jason Merrill wrote:

> On 3/22/20 5:14 PM, Patrick Palka wrote:
> > In this PR we're emitting -Wnoexcept warnings about potentially-throwing
> > NSDMIs
> > when computing the noexcept specification of a class's defaulted default
> > constructor.  Alhough these warnings are in some sense valid, this patch
> > takes
> > the route of suppressing them, because:
> > 
> >    1. the warning message is confusing in its current form;
> >    2. warning for 'struct C { B b = B(); };' but not for 'struct C { B b;
> > };'
> >       is inconsistent; and
> >    3. emitting a warning here arguably doesn't fall under the umbrella of
> >       -Wnoexcept, whose documentation says it warns only when a
> >       noexcept-expression evaluates to false, but there are
> > noexcept-expressions
> >       here.
> > 
> > Tested on x86_64-pc-linux-gnu, does this look OK to commit?
> 
> Let's add a comment to maybe_noexcept_warning suggesting that we might in
> future want to do something like walk_subtrees in the case of a defaulted
> function.  OK with that change.
> 
> Jason

Thanks, patch committed with a comment to that effect added to
maybe_noexcept_warning.  Here is the final patch:

-- >8 --

gcc/cp/ChangeLog:

	PR c++/93805
	* except.c (maybe_noexcept_warning): Add TODO comment.
	* method.c (walk_field_subobs): Pass tf_none to expr_noexcept_p.

gcc/testsuite/ChangeLog:

	PR c++/93805
	* g++.dg/warn/Wnoexcept2.C: New test.
---
 gcc/cp/ChangeLog                       |  6 ++++++
 gcc/cp/except.c                        |  5 ++++-
 gcc/cp/method.c                        |  2 +-
 gcc/testsuite/ChangeLog                |  5 +++++
 gcc/testsuite/g++.dg/warn/Wnoexcept2.C | 15 +++++++++++++++
 5 files changed, 31 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wnoexcept2.C

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 3340f47d506..59db03c0b07 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2020-03-23  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/93805
+	* except.c (maybe_noexcept_warning): Add TODO.
+	* method.c (walk_field_subobs): Pass tf_none to expr_noexcept_p.
+
 2020-03-23  nathans  <nathan@acm.org>
 
 	PR c++/94044
diff --git a/gcc/cp/except.c b/gcc/cp/except.c
index 262ba5d309c..7e93c51f9ea 100644
--- a/gcc/cp/except.c
+++ b/gcc/cp/except.c
@@ -1160,7 +1160,10 @@ struct GTY(()) pending_noexcept {
 static GTY(()) vec<pending_noexcept, va_gc> *pending_noexcept_checks;
 
 /* FN is a FUNCTION_DECL that caused a noexcept-expr to be false.  Warn if
-   it can't throw.  */
+   it can't throw.
+
+   TODO: Consider extending -Wnoexcept to do something like walk_subtrees in the
+   case of a defaulted function that obtained a noexcept(false) spec.  */
 
 static void
 maybe_noexcept_warning (tree fn)
diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index c131fd41536..41b9ff86bdd 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -1988,7 +1988,7 @@ walk_field_subobs (tree fields, special_function_kind sfk, tree fnname,
 		  if (nsdmi == error_mark_node)
 		    *spec_p = error_mark_node;
 		  else if (*spec_p != error_mark_node
-			   && !expr_noexcept_p (nsdmi, complain))
+			   && !expr_noexcept_p (nsdmi, tf_none))
 		    *spec_p = noexcept_false_spec;
 		}
 	      /* Don't do the normal processing.  */
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 1d053e07721..5f079f1fca9 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-03-23  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/93805
+	* g++.dg/warn/Wnoexcept2.C: New test.
+
 2020-03-23  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/91993
diff --git a/gcc/testsuite/g++.dg/warn/Wnoexcept2.C b/gcc/testsuite/g++.dg/warn/Wnoexcept2.C
new file mode 100644
index 00000000000..60541be3575
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wnoexcept2.C
@@ -0,0 +1,15 @@
+// PR c++/93805
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-Wnoexcept" }
+
+struct B
+{
+  B() {}
+};
+
+struct C
+{
+  B b = B();
+};
+
+C c; // { dg-bogus "noexcept-expression" }