test for String.prototype.indexOf first parameter type coercion

harness/typeCoercion.js

@@ -129,9 +129,15 @@ function testCoercibleToPrimitiveWithMethod(hint, method, test) {
     [methodNames[0]]: method,
     [methodNames[1]]: function() { throw new Test262Error(); },
   });
-  test({
+  if (hint === "number") {
-    [methodNames[1]]: method,
+    // The default valueOf returns an object, which is unsuitable.
-  });
+    // The default toString returns a String, which is suitable.
+    // Therefore this test only works for valueOf falling back to toString.
+    test({
+      // this is toString:
+      [methodNames[1]]: method,
+    });
+  }
 
   // GetMethod: if func is undefined or null, return undefined.
   test({
@@ -228,3 +234,52 @@ function testNotCoercibleToPrimitive(hint, test) {
   testUnsuitableMethod(function() { return Object(1); });
   testUnsuitableMethod(function() { return {}; });
 }
+
+function testCoercibleToString(test) {
+  function testPrimitiveValue(value, expectedString) {
+    test(value, expectedString);
+    // ToPrimitive
+    testPrimitiveWrappers(value, "string", function(value) {
+      test(value, expectedString);
+    });
+  }
+
+  testPrimitiveValue(undefined, "undefined");
+  testPrimitiveValue(null, "null");
+  testPrimitiveValue(true, "true");
+  testPrimitiveValue(false, "false");
+  testPrimitiveValue(0, "0");
+  testPrimitiveValue(-0, "0");
+  testPrimitiveValue(Infinity, "Infinity");
+  testPrimitiveValue(-Infinity, "-Infinity");
+  testPrimitiveValue(123.456, "123.456");
+  testPrimitiveValue(-123.456, "-123.456");
+  testPrimitiveValue("", "");
+  testPrimitiveValue("foo", "foo");
+
+  if (typeof BigInt !== "undefined") {
+    // BigInt -> TypeError
+    testPrimitiveValue(BigInt(0), "0");
+  }
+
+  // toString of a few objects
+  test([], "");
+  test(["foo", "bar"], "foo,bar");
+  test({}, "[object Object]");
+}
+
+function testNotCoercibleToString(test) {
+  function testPrimitiveValue(value) {
+    test(TypeError, value);
+    // ToPrimitive
+    testPrimitiveWrappers(value, "string", function(value) {
+      test(TypeError, value);
+    });
+  }
+
+  // Symbol -> TypeError
+  testPrimitiveValue(Symbol("1"));
+
+  // ToPrimitive
+  testNotCoercibleToPrimitive("string", test);
+}

test/built-ins/String/prototype/indexOf/searchstring-tostring.js

@@ -0,0 +1,25 @@
+// Copyright (C) 2017 Josh Wolfe. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-string.prototype.indexof
+description: String.prototype.indexOf type coercion for searchString parameter
+info: >
+  String.prototype.indexOf ( searchString [ , position ] )
+
+  3. Let searchStr be ? ToString(searchString).
+
+includes: [typeCoercion.js]
+---*/
+
+testCoercibleToString(function(value, expectedString) {
+  if (expectedString.length === 0) {
+    assert.sameValue(("x_x_x").indexOf(value), 0);
+  } else {
+    assert.sameValue(expectedString.indexOf("\x00"), -1, "sanity check");
+    assert.sameValue(("\x00\x00" + expectedString + "\x00\x00").indexOf(value), 2);
+  }
+});
+
+testNotCoercibleToString(function(error, value) {
+  assert.throws(error, function() { "".indexOf(value); });
+});