[es7] implement exponentiation operator proposal

Implements Stage 4 proposal from http://rwaldron.github.io/exponentiation-operator/,
without adding any knowledge of the feature to compiler backends.

BUG=v8:3915
LOG=Y
R=adamk@chromium.org, rossberg@chromium.org, littledan@chromium.org

Review URL: https://codereview.chromium.org/1678303002

Cr-Commit-Position: refs/heads/master@{#34890}

src/bootstrapper.cc

@@ -2276,6 +2276,7 @@ EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(promise_extra)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_tailcalls)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_instanceof)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_restrictive_declarations)
+EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_exponentiation_operator)
 
 void InstallPublicSymbol(Factory* factory, Handle<Context> native_context,
                          const char* name, Handle<Symbol> value) {
@@ -2978,6 +2979,7 @@ bool Genesis::InstallExperimentalNatives() {
   static const char* harmony_object_own_property_descriptors_natives[] = {
       nullptr};
   static const char* harmony_array_prototype_values_natives[] = {nullptr};
+  static const char* harmony_exponentiation_operator_natives[] = {nullptr};
 
   for (int i = ExperimentalNatives::GetDebuggerCount();
        i < ExperimentalNatives::GetBuiltinsCount(); i++) {

src/contexts.h

@@ -116,6 +116,7 @@ enum BindingFlags {
   V(MAP_GET_METHOD_INDEX, JSFunction, map_get)                                \
   V(MAP_HAS_METHOD_INDEX, JSFunction, map_has)                                \
   V(MAP_SET_METHOD_INDEX, JSFunction, map_set)                                \
+  V(MATH_POW_METHOD_INDEX, JSFunction, math_pow)                              \
   V(MESSAGE_GET_COLUMN_NUMBER_INDEX, JSFunction, message_get_column_number)   \
   V(MESSAGE_GET_LINE_NUMBER_INDEX, JSFunction, message_get_line_number)       \
   V(MESSAGE_GET_SOURCE_LINE_INDEX, JSFunction, message_get_source_line)       \

src/flag-definitions.h

@@ -205,7 +205,8 @@ DEFINE_IMPLICATION(es_staging, harmony_tailcalls)
   V(harmony_simd, "harmony simd")                                       \
   V(harmony_do_expressions, "harmony do-expressions")                   \
   V(harmony_tailcalls, "harmony tail calls")                            \
-  V(harmony_regexp_property, "harmony unicode regexp property classes")
+  V(harmony_regexp_property, "harmony unicode regexp property classes") \
+  V(harmony_exponentiation_operator, "harmony exponentiation operator `**`")
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED(V)                                                    \

src/js/math.js

@@ -202,6 +202,10 @@ function CubeRoot(x) {
 
 // -------------------------------------------------------------------
 
+%InstallToContext([
+  "math_pow", MathPowJS,
+]);
+
 %AddNamedProperty(GlobalMath, toStringTagSymbol, "Math", READ_ONLY | DONT_ENUM);
 
 // Set up math constants.

src/parsing/parser-base.h

@@ -121,6 +121,12 @@ class ParserBase : public Traits {
   bool allow_##name() const { return allow_##name##_; } \
   void set_allow_##name(bool allow) { allow_##name##_ = allow; }
 
+#define SCANNER_ACCESSORS(name)                                  \
+  bool allow_##name() const { return scanner_->allow_##name(); } \
+  void set_allow_##name(bool allow) {                            \
+    return scanner_->set_allow_##name(allow);                    \
+  }
+
   ALLOW_ACCESSORS(lazy);
   ALLOW_ACCESSORS(natives);
   ALLOW_ACCESSORS(harmony_sloppy);
@@ -131,6 +137,7 @@ class ParserBase : public Traits {
   ALLOW_ACCESSORS(harmony_do_expressions);
   ALLOW_ACCESSORS(harmony_function_name);
   ALLOW_ACCESSORS(harmony_function_sent);
+  SCANNER_ACCESSORS(harmony_exponentiation_operator);
 #undef ALLOW_ACCESSORS
 
   uintptr_t stack_limit() const { return stack_limit_; }
@@ -2009,6 +2016,11 @@ ParserBase<Traits>::ParseAssignmentExpression(bool accept_IN,
     Traits::SetFunctionNameFromIdentifierRef(right, expression);
   }
 
+  if (op == Token::ASSIGN_EXP) {
+    DCHECK(!is_destructuring_assignment);
+    return Traits::RewriteAssignExponentiation(expression, right, pos);
+  }
+
   ExpressionT result = factory()->NewAssignment(op, expression, right, pos);
 
   if (is_destructuring_assignment) {
@@ -2118,8 +2130,11 @@ ParserBase<Traits>::ParseBinaryExpression(int prec, bool accept_IN,
       ArrowFormalParametersUnexpectedToken(classifier);
       Token::Value op = Next();
       int pos = position();
+
+      const bool is_right_associative = op == Token::EXP;
+      const int next_prec = is_right_associative ? prec1 : prec1 + 1;
       ExpressionT y =
-          ParseBinaryExpression(prec1 + 1, accept_IN, classifier, CHECK_OK);
+          ParseBinaryExpression(next_prec, accept_IN, classifier, CHECK_OK);
       Traits::RewriteNonPattern(classifier, CHECK_OK);
 
       if (this->ShortcutNumericLiteralBinaryExpression(&x, y, op, pos,
@@ -2147,6 +2162,9 @@ ParserBase<Traits>::ParseBinaryExpression(int prec, bool accept_IN,
             x = factory()->NewUnaryOperation(Token::NOT, x, pos);
           }
         }
+
+      } else if (op == Token::EXP) {
+        x = Traits::RewriteExponentiation(x, y, pos);
       } else {
         // We have a "normal" binary operation.
         x = factory()->NewBinaryOperation(op, x, y, pos);
@@ -2192,6 +2210,12 @@ ParserBase<Traits>::ParseUnaryExpression(ExpressionClassifier* classifier,
       }
     }
 
+    if (peek() == Token::EXP) {
+      ReportUnexpectedToken(Next());
+      *ok = false;
+      return this->EmptyExpression();
+    }
+
     // Allow Traits do rewrite the expression.
     return this->BuildUnaryExpression(expression, op, pos, factory());
   } else if (Token::IsCountOp(op)) {

src/parsing/parser.cc

@@ -439,6 +439,14 @@ bool ParserTraits::ShortcutNumericLiteralBinaryExpression(
         *x = factory->NewNumberLiteral(value, pos, has_dot);
         return true;
       }
+      case Token::EXP: {
+        double value = std::pow(x_val, y_val);
+        int int_value = static_cast<int>(value);
+        *x = factory->NewNumberLiteral(
+            int_value == value && value != -0.0 ? int_value : value, pos,
+            has_dot);
+        return true;
+      }
       default:
         break;
     }
@@ -785,6 +793,8 @@ Parser::Parser(ParseInfo* info)
   set_allow_harmony_function_sent(FLAG_harmony_function_sent);
   set_allow_harmony_restrictive_declarations(
       FLAG_harmony_restrictive_declarations);
+  set_allow_harmony_exponentiation_operator(
+      FLAG_harmony_exponentiation_operator);
   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
     use_counts_[feature] = 0;
@@ -4681,6 +4691,7 @@ PreParser::PreParseResult Parser::ParseLazyFunctionBodyWithPreParser(
     SET_ALLOW(harmony_do_expressions);
     SET_ALLOW(harmony_function_name);
     SET_ALLOW(harmony_function_sent);
+    SET_ALLOW(harmony_exponentiation_operator);
 #undef SET_ALLOW
   }
   PreParser::PreParseResult result = reusable_preparser_->PreParseLazyFunction(
@@ -5371,6 +5382,16 @@ void ParserTraits::RewriteDestructuringAssignments() {
   parser_->RewriteDestructuringAssignments();
 }
 
+Expression* ParserTraits::RewriteExponentiation(Expression* left,
+                                                Expression* right, int pos) {
+  return parser_->RewriteExponentiation(left, right, pos);
+}
+
+Expression* ParserTraits::RewriteAssignExponentiation(Expression* left,
+                                                      Expression* right,
+                                                      int pos) {
+  return parser_->RewriteAssignExponentiation(left, right, pos);
+}
 
 void ParserTraits::RewriteNonPattern(Type::ExpressionClassifier* classifier,
                                      bool* ok) {
@@ -5460,6 +5481,60 @@ void Parser::RewriteDestructuringAssignments() {
   }
 }
 
+Expression* Parser::RewriteExponentiation(Expression* left, Expression* right,
+                                          int pos) {
+  ZoneList<Expression*>* args = new (zone()) ZoneList<Expression*>(2, zone());
+  args->Add(left, zone());
+  args->Add(right, zone());
+  return factory()->NewCallRuntime(Context::MATH_POW_METHOD_INDEX, args, pos);
+}
+
+Expression* Parser::RewriteAssignExponentiation(Expression* left,
+                                                Expression* right, int pos) {
+  ZoneList<Expression*>* args = new (zone()) ZoneList<Expression*>(2, zone());
+  if (left->IsVariableProxy()) {
+    VariableProxy* lhs = left->AsVariableProxy();
+
+    Expression* result;
+    DCHECK_NOT_NULL(lhs->raw_name());
+    result =
+        this->ExpressionFromIdentifier(lhs->raw_name(), lhs->position(),
+                                       lhs->end_position(), scope_, factory());
+    args->Add(left, zone());
+    args->Add(right, zone());
+    Expression* call =
+        factory()->NewCallRuntime(Context::MATH_POW_METHOD_INDEX, args, pos);
+    return factory()->NewAssignment(Token::ASSIGN, result, call, pos);
+  } else if (left->IsProperty()) {
+    Property* prop = left->AsProperty();
+    auto temp_obj = scope_->NewTemporary(ast_value_factory()->empty_string());
+    auto temp_key = scope_->NewTemporary(ast_value_factory()->empty_string());
+    Expression* assign_obj = factory()->NewAssignment(
+        Token::ASSIGN, factory()->NewVariableProxy(temp_obj), prop->obj(),
+        RelocInfo::kNoPosition);
+    Expression* assign_key = factory()->NewAssignment(
+        Token::ASSIGN, factory()->NewVariableProxy(temp_key), prop->key(),
+        RelocInfo::kNoPosition);
+    args->Add(factory()->NewProperty(factory()->NewVariableProxy(temp_obj),
+                                     factory()->NewVariableProxy(temp_key),
+                                     left->position()),
+              zone());
+    args->Add(right, zone());
+    Expression* call =
+        factory()->NewCallRuntime(Context::MATH_POW_METHOD_INDEX, args, pos);
+    Expression* target = factory()->NewProperty(
+        factory()->NewVariableProxy(temp_obj),
+        factory()->NewVariableProxy(temp_key), RelocInfo::kNoPosition);
+    Expression* assign =
+        factory()->NewAssignment(Token::ASSIGN, target, call, pos);
+    return factory()->NewBinaryOperation(
+        Token::COMMA, assign_obj,
+        factory()->NewBinaryOperation(Token::COMMA, assign_key, assign, pos),
+        pos);
+  }
+  UNREACHABLE();
+  return nullptr;
+}
 
 Expression* Parser::RewriteSpreads(ArrayLiteral* lit) {
   // Array literals containing spreads are rewritten using do expressions, e.g.

src/parsing/parser.h

@@ -636,6 +636,11 @@ class ParserTraits {
   // Rewrite all DestructuringAssignments in the current FunctionState.
   V8_INLINE void RewriteDestructuringAssignments();
 
+  V8_INLINE Expression* RewriteExponentiation(Expression* left,
+                                              Expression* right, int pos);
+  V8_INLINE Expression* RewriteAssignExponentiation(Expression* left,
+                                                    Expression* right, int pos);
+
   V8_INLINE void QueueDestructuringAssignmentForRewriting(
       Expression* assignment);
   V8_INLINE void QueueNonPatternForRewriting(Expression* expr);
@@ -1043,6 +1048,11 @@ class Parser : public ParserBase<ParserTraits> {
 
   V8_INLINE void RewriteDestructuringAssignments();
 
+  V8_INLINE Expression* RewriteExponentiation(Expression* left,
+                                              Expression* right, int pos);
+  V8_INLINE Expression* RewriteAssignExponentiation(Expression* left,
+                                                    Expression* right, int pos);
+
   friend class NonPatternRewriter;
   V8_INLINE Expression* RewriteSpreads(ArrayLiteral* lit);
 

src/parsing/preparser.h

@@ -902,6 +902,16 @@ class PreParserTraits {
 
   inline void RewriteDestructuringAssignments() {}
 
+  inline PreParserExpression RewriteExponentiation(PreParserExpression left,
+                                                   PreParserExpression right,
+                                                   int pos) {
+    return left;
+  }
+  inline PreParserExpression RewriteAssignExponentiation(
+      PreParserExpression left, PreParserExpression right, int pos) {
+    return left;
+  }
+
   inline void QueueDestructuringAssignmentForRewriting(PreParserExpression) {}
   inline void QueueNonPatternForRewriting(PreParserExpression) {}
 

src/parsing/scanner.cc

@@ -40,7 +40,8 @@ Scanner::Scanner(UnicodeCache* unicode_cache)
     : unicode_cache_(unicode_cache),
       bookmark_c0_(kNoBookmark),
       octal_pos_(Location::invalid()),
-      found_html_comment_(false) {
+      found_html_comment_(false),
+      allow_harmony_exponentiation_operator_(false) {
   bookmark_current_.literal_chars = &bookmark_current_literal_;
   bookmark_current_.raw_literal_chars = &bookmark_current_raw_literal_;
   bookmark_next_.literal_chars = &bookmark_next_literal_;
@@ -565,7 +566,14 @@ void Scanner::Scan() {
 
       case '*':
         // * *=
-        token = Select('=', Token::ASSIGN_MUL, Token::MUL);
+        Advance();
+        if (c0_ == '*' && allow_harmony_exponentiation_operator()) {
+          token = Select('=', Token::ASSIGN_EXP, Token::EXP);
+        } else if (c0_ == '=') {
+          token = Select(Token::ASSIGN_MUL);
+        } else {
+          token = Token::MUL;
+        }
         break;
 
       case '%':

src/parsing/scanner.h

@@ -450,6 +450,12 @@ class Scanner {
 
   bool FoundHtmlComment() const { return found_html_comment_; }
 
+#define DECLARE_ACCESSORS(name)                                \
+  inline bool allow_##name() const { return allow_##name##_; } \
+  inline void set_allow_##name(bool allow) { allow_##name##_ = allow; }
+  DECLARE_ACCESSORS(harmony_exponentiation_operator)
+#undef ACCESSOR
+
  private:
   // The current and look-ahead token.
   struct TokenDesc {
@@ -758,6 +764,8 @@ class Scanner {
 
   // Whether this scanner encountered an HTML comment.
   bool found_html_comment_;
+
+  bool allow_harmony_exponentiation_operator_;
 };
 
 }  // namespace internal

src/parsing/token.h

@@ -63,6 +63,7 @@ namespace internal {
   T(ASSIGN_MUL, "*=", 2)                                             \
   T(ASSIGN_DIV, "/=", 2)                                             \
   T(ASSIGN_MOD, "%=", 2)                                             \
+  T(ASSIGN_EXP, "**=", 2)                                            \
                                                                      \
   /* Binary operators sorted by precedence. */                       \
   /* IsBinaryOp() relies on this block of enum values */             \
@@ -82,6 +83,7 @@ namespace internal {
   T(MUL, "*", 13)                                                    \
   T(DIV, "/", 13)                                                    \
   T(MOD, "%", 13)                                                    \
+  T(EXP, "**", 14)                                                   \
                                                                      \
   /* Compare operators sorted by precedence. */                      \
   /* IsCompareOp() relies on this block of enum values */            \
@@ -214,12 +216,10 @@ class Token {
   }
 
   static bool IsAssignmentOp(Value tok) {
-    return INIT <= tok && tok <= ASSIGN_MOD;
+    return INIT <= tok && tok <= ASSIGN_EXP;
   }
 
-  static bool IsBinaryOp(Value op) {
-    return COMMA <= op && op <= MOD;
-  }
+  static bool IsBinaryOp(Value op) { return COMMA <= op && op <= EXP; }
 
   static bool IsTruncatingBinaryOp(Value op) {
     return BIT_OR <= op && op <= ROR;

test/cctest/test-parsing.cc

@@ -1509,6 +1509,7 @@ enum ParserFlag {
   kNoLegacyConst,
   kAllowHarmonyFunctionSent,
   kAllowHarmonyRestrictiveDeclarations,
+  kAllowHarmonyExponentiationOperator
 };
 
 enum ParserSyncTestResult {
@@ -1529,6 +1530,8 @@ void SetParserFlags(i::ParserBase<Traits>* parser,
       flags.Contains(kAllowHarmonyFunctionSent));
   parser->set_allow_harmony_restrictive_declarations(
       flags.Contains(kAllowHarmonyRestrictiveDeclarations));
+  parser->set_allow_harmony_exponentiation_operator(
+      flags.Contains(kAllowHarmonyExponentiationOperator));
 }
 
 
@@ -7322,3 +7325,95 @@ TEST(FunctionDeclarationError) {
   RunParserSyncTest(sloppy_context, sloppy_data, kSuccess, restrictive_flags,
                     arraysize(restrictive_flags));
 }
+
+TEST(ExponentiationOperator) {
+  // clang-format off
+  const char* context_data[][2] = {
+    { "var O = { p: 1 }, x = 10; ; if (", ") { foo(); }" },
+    { "var O = { p: 1 }, x = 10; ; (", ")" },
+    { "var O = { p: 1 }, x = 10; foo(", ")" },
+    { NULL, NULL }
+  };
+  const char* data[] = {
+    "(delete O.p) ** 10",
+    "(delete x) ** 10",
+    "(~O.p) ** 10",
+    "(~x) ** 10",
+    "(!O.p) ** 10",
+    "(!x) ** 10",
+    "(+O.p) ** 10",
+    "(+x) ** 10",
+    "(-O.p) ** 10",
+    "(-x) ** 10",
+    "(typeof O.p) ** 10",
+    "(typeof x) ** 10",
+    "(void 0) ** 10",
+    "(void O.p) ** 10",
+    "(void x) ** 10",
+    "++O.p ** 10",
+    "++x ** 10",
+    "--O.p ** 10",
+    "--x ** 10",
+    "O.p++ ** 10",
+    "x++ ** 10",
+    "O.p-- ** 10",
+    "x-- ** 10",
+    NULL
+  };
+  // clang-format on
+
+  static const ParserFlag always_flags[] = {
+      kAllowHarmonyExponentiationOperator};
+  RunParserSyncTest(context_data, data, kSuccess, NULL, 0, always_flags,
+                    arraysize(always_flags));
+}
+
+TEST(ExponentiationOperatorErrors) {
+  // clang-format off
+  const char* context_data[][2] = {
+    { "var O = { p: 1 }, x = 10; ; if (", ") { foo(); }" },
+    { "var O = { p: 1 }, x = 10; ; (", ")" },
+    { "var O = { p: 1 }, x = 10; foo(", ")" },
+    { NULL, NULL }
+  };
+  const char* error_data[] = {
+    "delete O.p ** 10",
+    "delete x ** 10",
+    "~O.p ** 10",
+    "~x ** 10",
+    "!O.p ** 10",
+    "!x ** 10",
+    "+O.p ** 10",
+    "+x ** 10",
+    "-O.p ** 10",
+    "-x ** 10",
+    "typeof O.p ** 10",
+    "typeof x ** 10",
+    "void ** 10",
+    "void O.p ** 10",
+    "void x ** 10",
+    "++delete O.p ** 10",
+    "--delete O.p ** 10",
+    "++~O.p ** 10",
+    "++~x ** 10",
+    "--!O.p ** 10",
+    "--!x ** 10",
+    "++-O.p ** 10",
+    "++-x ** 10",
+    "--+O.p ** 10",
+    "--+x ** 10",
+    "[ x ] **= [ 2 ]",
+    "[ x **= 2 ] = [ 2 ]",
+    "{ x } **= { x: 2 }",
+    "{ x: x **= 2 ] = { x: 2 }",
+    // TODO(caitp): a Call expression as LHS should be an early ReferenceError!
+    // "Array() **= 10",
+    NULL
+  };
+  // clang-format on
+
+  static const ParserFlag always_flags[] = {
+      kAllowHarmonyExponentiationOperator};
+  RunParserSyncTest(context_data, error_data, kError, NULL, 0, always_flags,
+                    arraysize(always_flags));
+}

test/mjsunit/harmony/exponentiation-operator.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony-exponentiation-operator --harmony-proxies
+
+function TestBasic() {
+  assertEquals(-(8 ** 2), -64);
+  assertEquals(+(8 ** 2), 64);
+  assertEquals(~(8 ** 2), -65);
+  assertEquals(!(8 ** 2), false);
+
+  assertEquals(2 ** -2, 0.25);
+
+  var o = { p: 1 };
+  assertEquals(2 ** delete o.p, 2);
+
+  assertEquals(2 ** void 1, NaN);
+
+  assertEquals(2 ** typeof 1, NaN);
+
+  var s = "2";
+  var n = 2;
+
+  assertEquals(2 ** "2", 4);
+  assertEquals(2 ** +"2", 4);
+  assertEquals(2 ** +s, 4);
+  assertEquals(2 ** s, 4);
+  assertEquals(2 ** 2, 4);
+  assertEquals(2 ** +2, 4);
+  assertEquals(2 ** +n, 4);
+  assertEquals(2 ** n, 4);
+
+  assertEquals(2 ** -"2", 0.25);
+  assertEquals(2 ** -s, 0.25);
+  assertEquals(2 ** -2, 0.25);
+  assertEquals(2 ** -n, 0.25);
+
+  assertEquals(2 ** ~"2", 0.125);
+  assertEquals(2 ** ~s, 0.125);
+  assertEquals(2 ** ~2, 0.125);
+  assertEquals(2 ** ~n, 0.125);
+
+  assertEquals(2 ** !"2", 1);
+  assertEquals(2 ** !s, 1);
+  assertEquals(2 ** !2, 1);
+  assertEquals(2 ** !n, 1);
+
+ var exponent = 2;
+ assertEquals(2 ** 3, 8);
+ assertEquals(3 * 2 ** 3, 24);
+ assertEquals(2 ** ++exponent, 8);
+ assertEquals(2 ** -1 * 2, 1);
+ assertEquals(2 ** 2 * 4, 16);
+ assertEquals(2 ** 2 / 2, 2);
+ assertEquals(2 ** (3 ** 2), 512);
+ assertEquals(2 ** 3 ** 2, 512);
+ assertEquals(2 * 3 ** 2, 18);
+ assertEquals(16 / 2 ** 2, 4);
+}
+TestBasic();
+
+
+function TestAssignment() {
+  var base = -5;
+  assertEquals(base **= 3, -125);
+  assertEquals(base, -125);
+}
+TestAssignment();
+
+
+function TestPrecedence() {
+  var base = 4;
+  assertEquals(--base ** 2, 9);  // 3 ** 2
+  assertEquals(++base ** 2, 16); // 4 ** 2
+  assertEquals(base++ ** 2, 16); // 4 ** 2
+  assertEquals(base-- ** 2, 25); // 5 ** 2
+
+  assertEquals(4, base);
+  assertEquals(--base ** --base ** 2,
+               Math.pow(3, Math.pow(2, 2)));
+
+  assertEquals(2, base);
+  assertEquals(++base ** ++base ** 2,
+               Math.pow(3, Math.pow(4, 2)));
+
+  base = 4;
+  assertEquals(base-- ** base-- ** 2,
+               Math.pow(4, Math.pow(3, 2)));
+
+  assertEquals(2, base);
+  assertEquals(base++ ** base++ ** 2,
+               Math.pow(2, Math.pow(3, 2)));
+}
+TestPrecedence();
+
+
+function TestInvariants() {
+  assertEquals(NaN, 2 ** NaN);
+  assertEquals(NaN, (+0) ** NaN);
+  assertEquals(NaN, (-0) ** NaN);
+  assertEquals(NaN, Infinity ** NaN);
+  assertEquals(NaN, (-Infinity) ** NaN);
+
+  assertEquals(1, NaN ** +0);
+  assertEquals(1, NaN ** -0);
+
+  assertEquals(NaN, NaN ** NaN);
+  assertEquals(NaN, NaN ** 2.2);
+  assertEquals(NaN, NaN ** 1);
+  assertEquals(NaN, NaN ** -1);
+  assertEquals(NaN, NaN ** -2.2);
+  assertEquals(NaN, NaN ** Infinity);
+  assertEquals(NaN, NaN ** -Infinity);
+
+  assertEquals(Infinity, 1.1 ** Infinity);
+  assertEquals(Infinity, (-1.1) ** Infinity);
+  assertEquals(Infinity, 2 ** Infinity);
+  assertEquals(Infinity, (-2) ** Infinity);
+
+  // Because +0 == -0, we need to compare 1/{+,-}0 to {+,-}Infinity
+  assertEquals(+Infinity, 1/1.1 ** -Infinity);
+  assertEquals(+Infinity, 1/(-1.1) ** -Infinity);
+  assertEquals(+Infinity, 1/2 ** -Infinity);
+  assertEquals(+Infinity, 1/(-2) ** -Infinity);
+
+  assertEquals(NaN, 1 ** Infinity);
+  assertEquals(NaN, 1 ** -Infinity);
+  assertEquals(NaN, (-1) ** Infinity);
+  assertEquals(NaN, (-1) ** -Infinity);
+
+  assertEquals(+0, 0.1 ** Infinity);
+  assertEquals(+0, (-0.1) ** Infinity);
+  assertEquals(+0, 0.999 ** Infinity);
+  assertEquals(+0, (-0.999) ** Infinity);
+
+  assertEquals(Infinity, 0.1 ** -Infinity);
+  assertEquals(Infinity, (-0.1) ** -Infinity);
+  assertEquals(Infinity, 0.999 ** -Infinity);
+  assertEquals(Infinity, (-0.999) ** -Infinity);
+
+  assertEquals(Infinity, Infinity ** 0.1);
+  assertEquals(Infinity, Infinity ** 2);
+
+  assertEquals(+Infinity, 1/Infinity ** -0.1);
+  assertEquals(+Infinity, 1/Infinity ** -2);
+
+  assertEquals(-Infinity, (-Infinity) ** 3);
+  assertEquals(-Infinity, (-Infinity) ** 13);
+
+  assertEquals(Infinity, (-Infinity) ** 3.1);
+  assertEquals(Infinity, (-Infinity) ** 2);
+
+  assertEquals(-Infinity, 1/(-Infinity) ** -3);
+  assertEquals(-Infinity, 1/(-Infinity) ** -13);
+
+  assertEquals(+Infinity, 1/(-Infinity) ** -3.1);
+  assertEquals(+Infinity, 1/(-Infinity) ** -2);
+
+  assertEquals(+Infinity, 1/(+0) ** 1.1);
+  assertEquals(+Infinity, 1/(+0) ** 2);
+
+  assertEquals(Infinity, (+0) ** -1.1);
+  assertEquals(Infinity, (+0) ** -2);
+
+  assertEquals(-Infinity, 1/(-0) ** 3);
+  assertEquals(-Infinity, 1/(-0) ** 13);
+
+  assertEquals(+Infinity, 1/(-0) ** 3.1);
+  assertEquals(+Infinity, 1/(-0) ** 2);
+
+  assertEquals(-Infinity, (-0) ** -3);
+  assertEquals(-Infinity, (-0) ** -13);
+
+  assertEquals(Infinity, (-0) ** -3.1);
+  assertEquals(Infinity, (-0) ** -2);
+
+  assertEquals(NaN, (-0.00001) ** 1.1);
+  assertEquals(NaN, (-0.00001) ** -1.1);
+  assertEquals(NaN, (-1.1) ** 1.1);
+  assertEquals(NaN, (-1.1) ** -1.1);
+  assertEquals(NaN, (-2) ** 1.1);
+  assertEquals(NaN, (-2) ** -1.1);
+  assertEquals(NaN, (-1000) ** 1.1);
+  assertEquals(NaN, (-1000) ** -1.1);
+
+  assertEquals(+Infinity, 1/(-0) ** 0.5);
+  assertEquals(+Infinity, 1/(-0) ** 0.6);
+  assertEquals(-Infinity, 1/(-0) ** 1);
+  assertEquals(-Infinity, 1/(-0) ** 10000000001);
+
+  assertEquals(+Infinity, (-0) ** -0.5);
+  assertEquals(+Infinity, (-0) ** -0.6);
+  assertEquals(-Infinity, (-0) ** -1);
+  assertEquals(-Infinity, (-0) ** -10000000001);
+
+  assertEquals(4, 16 ** 0.5);
+  assertEquals(NaN, (-16) ** 0.5);
+  assertEquals(0.25, 16 ** -0.5);
+  assertEquals(NaN, (-16) ** -0.5);
+}
+TestInvariants();
+
+
+function TestOperationOrder() {
+  var log = [];
+  var handler = {
+    get(t, n) {
+      var result = Reflect.get(t, n);
+      var str = typeof result === "object" ? "[object Object]" : String(result);
+      log.push(`[[Get]](${String(n)}) -> ${str}`);
+      return result;
+    },
+    set(t, n, v) {
+      var result = Reflect.set(t, n, v);
+      log.push(`[[Set]](${String(n)}, ${String(v)}) -> ${String(result)}`);
+      return result;
+    },
+    has() { assertUnreachable("trap 'has' invoked"); },
+    deleteProperty() { assertUnreachable("trap 'deleteProperty' invoked"); },
+    ownKeys() { assertUnreachable("trap 'ownKeys' invoked"); },
+    apply() { assertUnreachable("trap 'apply' invoked"); },
+    construct() { assertUnreachable("trap 'construct' invoked"); },
+    getPrototypeOf() { assertUnreachable("trap 'getPrototypeOf' invoked"); },
+    setPrototypeOf() { assertUnreachable("trap 'setPrototypeOf' invoked"); },
+    isExtensible() { assertUnreachable("trap 'isExtensible' invoked"); },
+    preventExtensions() {
+      assertUnreachable("trap 'preventExtensions' invoked"); },
+    getOwnPropertyDescriptor() {
+      assertUnreachable("trap 'getOwnPropertyDescriptor' invoked"); },
+    defineProperty() { assertUnreachable("trap 'defineProperty' invoked"); },
+  };
+  var P = new Proxy({ x: 2 }, handler);
+
+  assertEquals(256, P.x **= "8");
+  assertEquals([
+    "[[Get]](x) -> 2",
+    "[[Set]](x, 256) -> true"
+  ], log);
+
+  log = [];
+  var O = new Proxy({ p: P }, handler);
+  assertEquals(65536, O.p.x **= 2 );
+  assertEquals([
+    "[[Get]](p) -> [object Object]",
+    "[[Get]](x) -> 256",
+    "[[Set]](x, 65536) -> true"
+  ], log);
+}
+TestOperationOrder();
+
+
+function TestOverrideMathPow() {
+  var MathPow = MathPow;
+  Math.pow = function(a, b) {
+    assertUnreachable(`Math.pow(${String(a)}, ${String(b)}) invoked`);
+  }
+
+  TestBasic();
+  TestAssignment();
+  TestInvariants();
+  TestOperationOrder();
+
+  Math.pow = MathPow;
+}
+TestOverrideMathPow();
+
+function TestBadAssignmentLHS() {
+  assertThrows("if (false) { 17 **= 10; }", ReferenceError);
+  assertThrows("if (false) { '17' **= 10; }", ReferenceError);
+  assertThrows("if (false) { /17/ **= 10; }", ReferenceError);
+  assertThrows("if (false) { ({ valueOf() { return 17; } } **= 10); }",
+               ReferenceError);
+  // TODO(caitp): a Call expression as LHS should be an early ReferenceError!
+  // assertThrows("if (false) { Array() **= 10; }", ReferenceError);
+  assertThrows(() => Array() **= 10, ReferenceError);
+}
+TestBadAssignmentLHS();