c++: Fix -Wnoexcept handling of system headers (PR90992).

The immediate issue here was that the second warning didn't depend on the
first one, so if the first location was in a system header, we'd
mysteriously give the second by itself.

It's also the case that the thing we care about being in a system header is
the function that we want to suggest adding 'noexcept' to, not the
noexcept-expression; it's useful to suggest adding noexcept to a user
function to satisfy a noexcept-expression in a system header.

	PR c++/90992
	* except.c (maybe_noexcept_warning): Check DECL_IN_SYSTEM_HEADER and
	temporarily enable -Wsystem-headers.  Change second warning to
	conditional inform.

1 files changed, 10 insertions, 6 deletions

diff --git a/gcc/cp/except.c b/gcc/cp/except.c
index 0b40234..788b96d 100644
--- a/gcc/cp/except.c
+++ b/gcc/cp/except.c
@@ -1165,13 +1165,17 @@ static GTY(()) vec<pending_noexcept, va_gc> *pending_noexcept_checks;
 static void
 maybe_noexcept_warning (tree fn)
 {
-  if (TREE_NOTHROW (fn))
+  if (TREE_NOTHROW (fn)
+      && (!DECL_IN_SYSTEM_HEADER (fn)
+	  || global_dc->dc_warn_system_headers))
     {
-      warning (OPT_Wnoexcept, "noexcept-expression evaluates to %<false%> "
-	       "because of a call to %qD", fn);
-      warning_at (DECL_SOURCE_LOCATION (fn), OPT_Wnoexcept,
-		  "but %qD does not throw; perhaps "
-		  "it should be declared %<noexcept%>", fn);
+      temp_override<bool> s (global_dc->dc_warn_system_headers, true);
+      auto_diagnostic_group d;
+      if (warning (OPT_Wnoexcept, "noexcept-expression evaluates to %<false%> "
+		   "because of a call to %qD", fn))
+	inform (DECL_SOURCE_LOCATION (fn),
+		"but %qD does not throw; perhaps "
+		"it should be declared %<noexcept%>", fn);
     }
 }