[JSC] String#toWellFormed should return stringified value

https://bugs.webkit.org/show_bug.cgi?id=251757
rdar://problem/105104668

Reviewed by Justin Michaud and Mark Lam.

We should return stringified value instead of original thisValue for the fast path.

* JSTests/stress/string-to-well-formed-number.js: Added.
(shouldBe):
* Source/JavaScriptCore/runtime/StringPrototype.cpp:
(JSC::JSC_DEFINE_HOST_FUNCTION):

Canonical link: https://commits.webkit.org/260043@main

JSTests/stress/string-to-well-formed-number.js

@@ -0,0 +1,7 @@
+function shouldBe(actual, expected) {
+    if (actual !== expected)
+        throw new Error('bad value: ' + actual);
+}
+
+shouldBe(String.prototype.toWellFormed.call(1), '1');
+shouldBe(String.prototype.toWellFormed.call(42.5), '42.5');

Source/JavaScriptCore/runtime/StringPrototype.cpp

@@ -1825,17 +1825,20 @@ JSC_DEFINE_HOST_FUNCTION(stringProtoFuncToWellFormed, (JSGlobalObject* globalObj
     if (thisValue.isString() && asString(thisValue)->is8Bit())
         return JSValue::encode(thisValue);
 
-    String string = thisValue.toWTFString(globalObject);
+    JSString* stringValue = thisValue.toString(globalObject);
     RETURN_IF_EXCEPTION(scope, { });
 
-    if (string.is8Bit())
-        return JSValue::encode(thisValue);
+    if (stringValue->is8Bit())
+        return JSValue::encode(stringValue);
+
+    String string = stringValue->value(globalObject);
+    RETURN_IF_EXCEPTION(scope, { });
 
     const UChar* characters = string.characters16();
     unsigned length = string.length();
     auto firstIllFormedIndex = illFormedIndex(characters, length);
     if (!firstIllFormedIndex)
-        return JSValue::encode(thisValue);
+        return JSValue::encode(stringValue);
 
     Vector<UChar> buffer;
     buffer.reserveInitialCapacity(length);