variant (visit): Make visit constexpr.

	* include/std/variant (visit): Make visit constexpr. Also cleanup
	__get_alternative and __storage, since we don't support reference/void
	alternatives any more.
	* testsuite/20_util/variant/compile.cc: Add tests.

From-SVN: r243295

libstdc++-v3/ChangeLog

+2016-11-27  Tim Shen  <timshen@google.com>
+
+	* include/std/variant (visit): Make visit constexpr. Also cleanup
+	__get_alternative and __storage, since we don't support reference/void
+	alternatives any more.
+	* testsuite/20_util/variant/compile.cc: Add tests.
+
 2016-12-07  Tim Shen  <timshen@google.com>
 
 	* include/bits/enable_special_members.h: Make

libstdc++-v3/testsuite/20_util/variant/compile.cc

@@ -275,6 +275,22 @@ void test_visit()
     };
     visit(Visitor(), variant<int, char>(), variant<float, double>());
   }
+  {
+    struct Visitor
+    {
+      constexpr bool operator()(const int&) { return true; }
+      constexpr bool operator()(const nonliteral&) { return false; }
+    };
+    static_assert(visit(Visitor(), variant<int, nonliteral>(0)), "");
+  }
+  {
+    struct Visitor
+    {
+      constexpr bool operator()(const int&) { return true; }
+      constexpr bool operator()(const nonliteral&) { return false; }
+    };
+    static_assert(visit(Visitor(), variant<int, nonliteral>(0)), "");
+  }
 }
 
 void test_constexpr()