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.  Although 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 no
     noexcept-expressions here.

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

@@ -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

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)

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.  */

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

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" }