C++ PATCH for c++/64547 (constexpr fn returning void)

[Jason Merrill]

In C++14 a constexpr function doesn't need to return a value.

Tested x86_64-pc-linux-gnu, applying to trunk.

commit 9675a7bde41b5430197854d8c1822c8f4d95b95e
Author: Jason Merrill <ja...@redhat.com>
Date:   Fri Jan 9 01:46:16 2015 -0500

    	PR c++/64547
    	* constexpr.c (cxx_eval_call_expression): A call to a void
    	function doesn't need to return a value.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 9a0d518..650250b 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1386,6 +1386,8 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 		   value by evaluating *this, but we don't bother; there's
 		   no need to put such a call in the hash table.  */
 		result = lval ? ctx->object : ctx->ctor;
+	      else if (VOID_TYPE_P (TREE_TYPE (res)))
+		result = void_node;
 	      else
 		{
 		  result = *ctx->values->get (slot ? slot : res);
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-void2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-void2.C
new file mode 100644
index 0000000..321a35e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-void2.C
@@ -0,0 +1,21 @@
+// PR c++/64547
+// { dg-do compile { target c++14 } }
+
+struct X
+{
+    int x;
+    constexpr int get() const {return x;}
+    constexpr void set(int foo) {x = foo;}
+};
+
+constexpr int bar()
+{
+    X x{42};
+    x.set(666);
+    return x.get();
+}
+
+int main()
+{
+    constexpr int foo = bar();
+}