Implement LWG 2946, 3075 and 3076. Reviewed as https://reviews.llvm.org/D48616

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@336132 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.ops/string_rfind/string_size.pass.cpp

@@ -14,6 +14,7 @@
 #include <string>
 #include <cassert>
 
+#include "test_macros.h"
 #include "min_allocator.h"
 
 template <class S>
@@ -154,4 +155,11 @@ int main()
     test1<S>();
     }
 #endif
+
+#if TEST_STD_VER > 3
+    {   // LWG 2946
+    std::string s = " !";
+    assert(s.rfind({"abc", 1}) == std::string::npos);
+    }
+#endif
 }