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

[Jakub Jelinek via Gcc-patches]

Hi!

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.

Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux, ok for
trunk?

2020-03-26  Jakub Jelinek  <ja...@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.

--- gcc/cp/call.c.jj    2020-03-25 08:05:07.153731580 +0100
+++ gcc/cp/call.c       2020-03-26 15:03:42.432909693 +0100
@@ -333,11 +333,14 @@ set_flags_from_callee (tree call)
           && internal_fn_flags (CALL_EXPR_IFN (call)) & ECF_NOTHROW)
     nothrow = true;
 
-  if (!nothrow && at_function_scope_p () && cfun && cp_function_chain)
-    cp_function_chain->can_throw = 1;
+  if (cfun && cp_function_chain && !cp_unevaluated_operand)
+    {
+      if (!nothrow && at_function_scope_p ())
+       cp_function_chain->can_throw = 1;
 
-  if (decl && TREE_THIS_VOLATILE (decl) && cfun && cp_function_chain)
-    current_function_returns_abnormally = 1;
+      if (decl && TREE_THIS_VOLATILE (decl))
+       current_function_returns_abnormally = 1;
+    }
 
   TREE_NOTHROW (call) = nothrow;
 }
--- gcc/testsuite/g++.dg/other/pr94326.C.jj     2020-03-26 15:14:23.609400216 
+0100
+++ gcc/testsuite/g++.dg/other/pr94326.C        2020-03-26 15:14:54.162947065 
+0100
@@ -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{});
+}

        Jakub