Improve is/toWellFormedString coverage.

test/built-ins/String/prototype/isWellFormed/prop-desc.js

@@ -11,7 +11,6 @@ info: |
 includes: [propertyHelper.js]
 features: [String.prototype.isWellFormed]
 ---*/
-assert.sameValue(typeof String.prototype.isWellFormed, 'function');
 
 assert.sameValue(
   typeof String.prototype.isWellFormed,

test/built-ins/String/prototype/isWellFormed/to-string-primitive.js

@@ -0,0 +1,32 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-string.prototype.iswellformed
+description: >
+  The method should coerce the receiver to a string.
+info: |
+  String.prototype.isWellFormed ( )
+
+  2. Let S be ? ToString(O).
+  …
+
+features: [String.prototype.isWellFormed]
+---*/
+
+const tests = [
+  [true, Boolean.prototype],
+  [1, Number.prototype],
+  [1n, BigInt.prototype],
+];
+
+for (const [v, proto] of tests) {
+  proto.toString = function() {
+    throw new Test262Error(`should not call toString on the prototype for ${typeof v}`);
+  }
+  let result = String.prototype.isWellFormed.call(v);
+  delete proto.toString;
+  assert.sameValue(result, true, `isWellFormed for ${typeof v}`);
+}
+
+Symbol.prototype.toString = function() { throw new TypeError("should not call toString on the prototype for Symbol"); }
+assert.throws(TypeError, () => String.prototype.isWellFormed.call(Symbol()), `Built-in result for Symbol`);

test/built-ins/String/prototype/toWellFormed/prop-desc.js

@@ -11,10 +11,9 @@ info: |
 includes: [propertyHelper.js]
 features: [String.prototype.toWellFormed]
 ---*/
-assert.sameValue(typeof String.prototype.toWellFormed, 'function');
 
 assert.sameValue(
-  typeof String.prototype.isWellFormed,
+  typeof String.prototype.toWellFormed,
   'function',
   'The value of `typeof String.prototype.toWellFormed` is "function"'
 );

test/built-ins/String/prototype/toWellFormed/to-string-primitive.js

@@ -0,0 +1,32 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-string.prototype.towellformed
+description: >
+  The method should coerce the receiver to a string.
+info: |
+  String.prototype.toWellFormed ( )
+
+  2. Let S be ? ToString(O).
+  …
+
+features: [String.prototype.toWellFormed]
+---*/
+
+const tests = [
+  [true, "true", Boolean.prototype],
+  [1, "1", Number.prototype],
+  [1n, "1", BigInt.prototype],
+];
+
+for (const [v, expected, proto] of tests) {
+  proto.toString = function() {
+    throw new Test262Error(`should not call toString on the prototype for ${typeof v}`);
+  }
+  let result = String.prototype.toWellFormed.call(v);
+  delete proto.toString;
+  assert.sameValue(result, expected, `toWellFormed for ${typeof v}`);
+}
+
+Symbol.prototype.toString = function() { throw new TypeError("should not call toString on the prototype for Symbol"); }
+assert.throws(TypeError, () => String.prototype.toWellFormed.call(Symbol()), `Built-in result for Symbol`);