c++: Avoid calls in non-evaluated contexts affect whether function can or can't throw [PR94326]

The following testcase FAILs -fcompare-debug, because if we emit a
-Wreturn-local-addr warning, we tsubst decltype in order to print the
warning and as that function could throw, set_flags_from_callee during that
sets cp_function_chain->can_throw and later on we don't set TREE_NOTHROW
on foo.  While with -w or -Wno-return-local-addr, tsubst isn't called during
the warning_at, cp_function_chain->can_throw is kept clear and TREE_NOTHROW
is set on foo.
It isn't just a matter of the warning though, in
int foo ();
int bar () { return sizeof (foo ()); }
int baz () { return sizeof (int); }
I don't really see why we should mark only baz as TREE_NOTHROW and not bar
too, when neither can really throw.

2020-03-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94326
	* call.c (set_flags_from_callee): Don't update
	cp_function_chain->can_throw or current_function_returns_abnormally
	if cp_unevaluated_operand.

	* g++.dg/other/pr94326.C: New test.

1 files changed, 19 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/other/pr94326.C b/gcc/testsuite/g++.dg/other/pr94326.C
new file mode 100644
index 0000000..4069c03
--- /dev/null
+++ b/gcc/testsuite/g++.dg/other/pr94326.C
@@ -0,0 +1,19 @@
+// PR c++/94326
+// { dg-do compile { target c++11 } }
+// { dg-options "-fcompare-debug" }
+
+template <typename = int> struct A {
+  const int &foo() { return 0; }	// { dg-warning "returning reference to temporary" }
+  template <typename _Kt> void bar(_Kt) { foo(); }
+};
+struct B {
+  A<> b;
+  template <typename _Kt> auto baz(_Kt p1) -> decltype(b.bar(p1)) {
+    b.bar(p1);
+  }
+};
+struct C {};
+void operator<(C, int) {
+  B a;
+  a.baz(C{});
+}