[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_assign/pointer.pass.cpp

@@ -18,7 +18,7 @@
 #include "min_allocator.h"
 
 template <class S>
-void
+TEST_CONSTEXPR_CXX20 void
 test(S s, const typename S::value_type* str, S expected)
 {
     s.assign(str);
@@ -26,9 +26,8 @@ test(S s, const typename S::value_type* str, S expected)
     assert(s == expected);
 }
 
-int main(int, char**)
-{
-    {
+bool test() {
+  {
     typedef std::string S;
     test(S(), "", S());
     test(S(), "12345", S("12345"));
@@ -42,9 +41,9 @@ int main(int, char**)
     test(S("12345678901234567890"), "12345", S("12345"));
     test(S("12345678901234567890"), "12345678901234567890",
          S("12345678901234567890"));
-    }
+  }
 #if TEST_STD_VER >= 11
-    {
+  {
     typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
     test(S(), "", S());
     test(S(), "12345", S("12345"));
@@ -58,10 +57,10 @@ int main(int, char**)
     test(S("12345678901234567890"), "12345", S("12345"));
     test(S("12345678901234567890"), "12345678901234567890",
          S("12345678901234567890"));
-    }
+  }
 #endif
 
-    { // test assignment to self
+  { // test assignment to self
     typedef std::string S;
     S s_short = "123/";
     S s_long  = "Lorem ipsum dolor sit amet, consectetur/";
@@ -73,7 +72,17 @@ int main(int, char**)
 
     s_long.assign(s_long.c_str() + 30);
     assert(s_long == "nsectetur/");
-    }
+  }
+
+  return true;
+}
+
+int main(int, char**)
+{
+  test();
+#if TEST_STD_VER > 17
+  // static_assert(test());
+#endif
 
   return 0;
 }