[libc++] Prepare string.modifiers tests for constexpr

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D119329

libcxx/test/std/strings/basic.string/string.modifiers/string_append/string.pass.cpp

@@ -18,7 +18,7 @@
 #include "min_allocator.h"
 
 template <class S>
-void
+TEST_CONSTEXPR_CXX20 void
 test(S s, S str, S expected)
 {
     s.append(str);
@@ -26,9 +26,8 @@ test(S s, S str, S expected)
     assert(s == expected);
 }
 
-int main(int, char**)
-{
-    {
+bool test() {
+  {
     typedef std::string S;
     test(S(), S(), S());
     test(S(), S("12345"), S("12345"));
@@ -50,9 +49,9 @@ int main(int, char**)
     test(S("12345678901234567890"), S("1234567890"), S("123456789012345678901234567890"));
     test(S("12345678901234567890"), S("12345678901234567890"),
          S("1234567890123456789012345678901234567890"));
-    }
+  }
 #if TEST_STD_VER >= 11
-    {
+  {
     typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
     test(S(), S(), S());
     test(S(), S("12345"), S("12345"));
@@ -74,16 +73,26 @@ int main(int, char**)
     test(S("12345678901234567890"), S("1234567890"), S("123456789012345678901234567890"));
     test(S("12345678901234567890"), S("12345678901234567890"),
          S("1234567890123456789012345678901234567890"));
-    }
+  }
 #endif
 
 #if TEST_STD_VER > 3
-    {   // LWG 2946
+  {   // LWG 2946
     std::string s;
     s.append({"abc", 1});
     assert(s.size() == 1);
     assert(s == "a");
-    }
+  }
+#endif
+
+  return true;
+}
+
+int main(int, char**)
+{
+  test();
+#if TEST_STD_VER > 17
+  // static_assert(test());
 #endif
 
   return 0;