re PR c++/64547 (A non-const constexpr function is rejected incorrectly)

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

From-SVN: r219466

gcc/cp/ChangeLog

+2015-01-12  Jason Merrill  <jason@redhat.com>
+
+	PR c++/64547
+	* constexpr.c (cxx_eval_call_expression): A call to a void
+	function doesn't need to return a value.
+
 2015-01-09  Michael Collison  <michael.collison@linaro.org>
 
 	* call.c: Include hash-set.h, machmode.h, vec.h, double-int.h,

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);

gcc/testsuite/g++.dg/cpp1y/constexpr-void2.C

+// 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();
+}