Normative: Return false in [[PreventExtensions]] for variable length TypedArrays (#3453)

Fixes #3385

Author: syg

spec.html

@@ -14708,7 +14708,20 @@ <h1>
       <h1>TypedArray Exotic Objects</h1>
       <p>A TypedArray is an exotic object that performs special handling of integer index property keys.</p>
       <p>TypedArrays have the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], [[ContentType]], and [[TypedArrayName]] internal slots.</p>
-      <p>An object is a <dfn id="typedarray" oldids="integer-indexed-exotic-object" variants="TypedArrays">TypedArray</dfn> if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in <emu-xref href="#sec-ordinary-object-internal-methods-and-internal-slots"></emu-xref>. These methods are installed by TypedArrayCreate.</p>
+      <p>An object is a <dfn id="typedarray" oldids="integer-indexed-exotic-object" variants="TypedArrays">TypedArray</dfn> if its [[PreventExtensions]], [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]], internal methods use the definitions in this section, and its other essential internal methods use the definitions found in <emu-xref href="#sec-ordinary-object-internal-methods-and-internal-slots"></emu-xref>. These methods are installed by TypedArrayCreate.</p>
+
+      <emu-clause id="sec-typedarray-preventextensions" type="internal method">
+        <h1>[[PreventExtensions]] ( ): a normal completion containing a Boolean</h1>
+        <dl class="header">
+          <dt>for</dt>
+          <dd>a TypedArray _O_</dd>
+        </dl>
+        <emu-alg>
+          1. NOTE: The extensibility-related invariants specified in <emu-xref href="#sec-invariants-of-the-essential-internal-methods"></emu-xref> do not allow this method to return *true* when _O_ can gain (or lose and then regain) properties, which might occur for properties with integer index names when its underlying buffer is resized.
+          1. If IsTypedArrayFixedLength(_O_) is *false*, return *false*.
+          1. Return OrdinaryPreventExtensions(_O_).
+        </emu-alg>
+      </emu-clause>
 
       <emu-clause id="sec-typedarray-getownproperty" oldids="sec-integer-indexed-exotic-objects-getownproperty-p" type="internal method">
         <h1>
@@ -14936,6 +14949,7 @@ <h1>
         <emu-alg>
           1. Let _internalSlotsList_ be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
           1. Let _A_ be MakeBasicObject(_internalSlotsList_).
+          1. Set _A_.[[PreventExtensions]] as specified in <emu-xref href="#sec-typedarray-preventextensions"></emu-xref>.
           1. Set _A_.[[GetOwnProperty]] as specified in <emu-xref href="#sec-typedarray-getownproperty"></emu-xref>.
           1. Set _A_.[[HasProperty]] as specified in <emu-xref href="#sec-typedarray-hasproperty"></emu-xref>.
           1. Set _A_.[[DefineOwnProperty]] as specified in <emu-xref href="#sec-typedarray-defineownproperty"></emu-xref>.
@@ -15015,6 +15029,22 @@ <h1>
         </emu-alg>
       </emu-clause>
 
+      <emu-clause id="sec-istypedarrayfixedlength" type="abstract operation">
+        <h1>
+          IsTypedArrayFixedLength (
+            _O_: a TypedArray,
+          ): a Boolean
+        </h1>
+        <dl class="header">
+        </dl>
+        <emu-alg>
+          1. If _O_.[[ArrayLength]] is ~auto~, return *false*.
+          1. Let _buffer_ be _O_.[[ViewedArrayBuffer]].
+          1. If IsFixedLengthArrayBuffer(_buffer_) is *false* and IsSharedArrayBuffer(_buffer_) is *false*, return *false*.
+          1. Return *true*.
+        </emu-alg>
+      </emu-clause>
+
       <emu-clause id="sec-isvalidintegerindex" type="abstract operation">
         <h1>
           IsValidIntegerIndex (