Normative: add #x in obj syntax (#2290)

See tc39/proposal-private-fields-in-in#7

spec.html

@@ -7818,6 +7818,7 @@ <h1>Static Semantics: IsFunctionDefinition</h1>
           RelationalExpression `&gt;=` ShiftExpression
           RelationalExpression `instanceof` ShiftExpression
           RelationalExpression `in` ShiftExpression
+          PrivateIdentifier `in` ShiftExpression
 
         EqualityExpression :
           EqualityExpression `==` RelationalExpression
@@ -8562,6 +8563,7 @@ <h1>Static Semantics: AssignmentTargetType</h1>
           RelationalExpression `&gt;=` ShiftExpression
           RelationalExpression `instanceof` ShiftExpression
           RelationalExpression `in` ShiftExpression
+          PrivateIdentifier `in` ShiftExpression
 
         EqualityExpression :
           EqualityExpression `==` RelationalExpression
@@ -17188,6 +17190,7 @@ <h2>Syntax</h2>
         RelationalExpression[?In, ?Yield, ?Await] `&gt;=` ShiftExpression[?Yield, ?Await]
         RelationalExpression[?In, ?Yield, ?Await] `instanceof` ShiftExpression[?Yield, ?Await]
         [+In] RelationalExpression[+In, ?Yield, ?Await] `in` ShiftExpression[?Yield, ?Await]
+        [+In] PrivateIdentifier `in` ShiftExpression[?Yield, ?Await]
     </emu-grammar>
     <emu-note>
       <p>The <sub>[In]</sub> grammar parameter is needed to avoid confusing the `in` operator in a relational expression with the `in` operator in a `for` statement.</p>
@@ -17252,6 +17255,23 @@ <h1>Runtime Semantics: Evaluation</h1>
         1. If Type(_rval_) is not Object, throw a *TypeError* exception.
         1. Return ? HasProperty(_rval_, ? ToPropertyKey(_lval_)).
       </emu-alg>
+      <emu-grammar>RelationalExpression : PrivateIdentifier `in` ShiftExpression</emu-grammar>
+      <emu-alg>
+        1. Let _privateIdentifier_ be the StringValue of |PrivateIdentifier|.
+        1. Let _rref_ be the result of evaluating |ShiftExpression|.
+        1. Let _rval_ be ? GetValue(_rref_).
+        1. If Type(_rval_) is not Object, throw a *TypeError* exception.
+        1. Let _env_ be the running execution context's PrivateEnvironment.
+        1. Let _privateNameBinding_ be ? ResolveBinding(_privateIdentifier_, _env_).
+        1. Let _privateName_ be ? GetValue(_privateNameBinding_).
+        1. Assert: _privateName_ is a Private Name.
+        1. If _privateName_.[[Kind]] is *"field"*, then
+          1. If ! PrivateFieldFind(_privateName_, _rval_) is not ~empty~, return *true*.
+        1. Else,
+          1. Assert: _privateName_.[[Kind]] is *"method"* or *"accessor"*.
+          1. If PrivateBrandCheck(_rval_, _privateName_) is not an abrupt completion, return *true*.
+        1. Return *false*.
+      </emu-alg>
     </emu-clause>
 
     <emu-clause id="sec-instanceofoperator" aoid="InstanceofOperator">
@@ -21307,6 +21327,14 @@ <h1>Static Semantics: AllPrivateIdentifiersValid</h1>
         1. Append to _newNames_ the elements of PrivateBoundIdentifiers of |ClassBody|.
         1. Return AllPrivateIdentifiersValid of |ClassElementList| with argument _newNames_.
       </emu-alg>
+
+      <emu-grammar>RelationalExpression : PrivateIdentifier `in` ShiftExpression</emu-grammar>
+      <emu-alg>
+        1. If StringValue of |PrivateIdentifier| is in _names_, return *true*.
+        1. Return *false*.
+      </emu-alg>
+
+      <p>For all other grammatical productions, recurse on subexpressions/substatements, passing in the _names_ of the caller. If all pieces return *true*, then return *true*. If any returns *false*, return *false*.</p>
     </emu-clause>
 
     <emu-clause id="sec-class-definitions-static-semantics-privateboundidentifiers" type="sdo" aoid="PrivateBoundIdentifiers">
@@ -22158,6 +22186,7 @@ <h1>Expression Rules</h1>
             RelationalExpression `&gt;=` ShiftExpression
             RelationalExpression `instanceof` ShiftExpression
             RelationalExpression `in` ShiftExpression
+            PrivateIdentifier `in` ShiftExpression
 
           ShiftExpression :
             ShiftExpression `&lt;&lt;` AdditiveExpression