[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/string.pass.cpp

@@ -19,7 +19,7 @@
 #include "test_allocator.h"
 
 template <class S>
-void
+TEST_CONSTEXPR_CXX20 void
 test(S s, S str, S expected)
 {
     s.assign(str);
@@ -28,7 +28,7 @@ test(S s, S str, S expected)
 }
 
 template <class S>
-void
+TEST_CONSTEXPR_CXX20 void
 testAlloc(S s, S str, const typename S::allocator_type& a)
 {
     s.assign(str);
@@ -37,9 +37,8 @@ testAlloc(S s, S str, const typename S::allocator_type& a)
     assert(s.get_allocator() == a);
 }
 
-int main(int, char**)
-{
-    {
+bool test() {
+  {
     typedef std::string S;
     test(S(), S(), S());
     test(S(), S("12345"), S("12345"));
@@ -66,19 +65,19 @@ int main(int, char**)
     testAlloc(S(), S("12345"), std::allocator<char>());
     testAlloc(S(), S("1234567890"), std::allocator<char>());
     testAlloc(S(), S("12345678901234567890"), std::allocator<char>());
-    }
+  }
 
-    { //  LWG#5579 make sure assign takes the allocators where appropriate
+  { //  LWG#5579 make sure assign takes the allocators where appropriate
     typedef other_allocator<char> A;  // has POCCA --> true
     typedef std::basic_string<char, std::char_traits<char>, A> S;
     testAlloc(S(A(5)), S(A(3)), A(3));
     testAlloc(S(A(5)), S("1"), A());
     testAlloc(S(A(5)), S("1", A(7)), A(7));
     testAlloc(S(A(5)), S("1234567890123456789012345678901234567890123456789012345678901234567890", A(7)), A(7));
-    }
+  }
 
 #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"));
@@ -105,13 +104,23 @@ int main(int, char**)
     testAlloc(S(), S("12345"), min_allocator<char>());
     testAlloc(S(), S("1234567890"), min_allocator<char>());
     testAlloc(S(), S("12345678901234567890"), min_allocator<char>());
-    }
+  }
 #endif
 #if TEST_STD_VER > 14
-    {
+  {
     typedef std::string S;
     static_assert(noexcept(S().assign(S())), "");  // LWG#2063
-    }
+  }
+#endif
+
+  return true;
+}
+
+int main(int, char**)
+{
+  test();
+#if TEST_STD_VER > 17
+  // static_assert(test());
 #endif
 
   return 0;