[PATCH] c++: Fix up mangling ICE with void{} [PR106863]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

We ICE on the following testcase during mangling, finish_compound_literal
returns for void{} void_node and the mangler doesn't handle it.
Handling void_node in the mangler seems problematic to me, because
we don't know for which case it has been created.
The following patch arranges to mangle just void{} the same as void()
if that is what we want to use, by doing what we do for void() when
processing void{}.
The code does that only if processing_template_decl, because otherwise
build_functional_cast will return void_node, so calling it looks like
wasted effort to me.  But if you want to call it unconditionally,
I can certainly do that too.

Or do you want to mangle it differently?  How?

clang++ doesn't support DR2351, so I can't check what they are doing.

Bootstrapped/regtested on x86_64-linux and i686-linux.

2022-10-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106863
	* semantics.cc (finish_compound_literal): For void{}, if
	processing_template_decl return build_functional_cast of NULL_TREE
	to VOID_TYPE rather than void_node.

	* g++.dg/cpp0x/dr2351-2.C: New test.

--- gcc/cp/semantics.cc.jj	2022-10-10 09:31:57.410985121 +0200
+++ gcc/cp/semantics.cc	2022-10-18 15:24:08.726026118 +0200
@@ -3164,7 +3164,12 @@ finish_compound_literal (tree type, tree
     {
       /* DR2351 */
       if (VOID_TYPE_P (type) && CONSTRUCTOR_NELTS (compound_literal) == 0)
-	return void_node;
+	{
+	  if (!processing_template_decl)
+	    return void_node;
+	  location_t loc = cp_expr_loc_or_input_loc (compound_literal);
+	  return build_functional_cast (loc, type, NULL_TREE, complain);
+	}
       else if (VOID_TYPE_P (type)
 	       && processing_template_decl
 	       && maybe_zero_constructor_nelts (compound_literal))
--- gcc/testsuite/g++.dg/cpp0x/dr2351-2.C.jj	2022-10-18 15:27:01.146690132 +0200
+++ gcc/testsuite/g++.dg/cpp0x/dr2351-2.C	2022-10-18 15:27:39.909164970 +0200
@@ -0,0 +1,16 @@
+// DR2351
+// { dg-do compile { target c++11 } }
+
+void bar (int);
+
+template <typename T>
+auto foo (T t) -> decltype (bar (t), void{})
+{
+  return bar (t);
+}
+
+int
+main ()
+{
+  foo (0);
+}

	Jakub