diff --git a/JSTests/stress/class-field-initializer-nested-function-parameters.js b/JSTests/stress/class-field-initializer-nested-function-parameters.js
new file mode 100644
index 0000000000000..d1be2644a90cc
--- /dev/null
+++ b/JSTests/stress/class-field-initializer-nested-function-parameters.js
@@ -0,0 +1,303 @@
+// FieldDefinition: it is a Syntax Error if ContainsArguments of Initializer is true, or if Initializer Contains SuperCall.
+// Both rules look into an arrow function and stop at any other function. The parser used to stop only at the body of
+// such a function, so `arguments` and `super()` in its parameters were a SyntaxError.
+
+function shouldBe(actual, expected) {
+    if (actual !== expected)
+        throw new Error(`bad value: ${String(actual)}, expected: ${String(expected)}`);
+}
+
+function shouldThrowSyntaxError(script, message) {
+    let error;
+    try {
+        (0, eval)(script);
+    } catch (e) {
+        error = e;
+    }
+    if (!(error instanceof SyntaxError))
+        throw new Error(`Expected SyntaxError for: ${script}` + (error ? `, but got ${error}` : ", but nothing was thrown"));
+    if (error.message !== message)
+        throw new Error(`Expected "${message}" for: ${script}, but got "${error.message}"`);
+}
+
+function shouldNotThrowSyntaxError(script) {
+    try {
+        (0, eval)(script);
+    } catch (e) {
+        if (e instanceof SyntaxError)
+            throw new Error(`Unexpected SyntaxError for: ${script}: ${e.message}`);
+        throw e;
+    }
+}
+
+// [text before the expression in the field initializer, text after it]
+const fields = [
+    ["(class { f = (", "); })"],
+    ["(class { static f = (", "); })"],
+    ["(class { #f = (", "); })"],
+    ["(class { static #f = (", "); })"],
+    ["(class { ['f'] = (", "); })"],
+    ["(class extends Object { f = (", "); })"],
+    // A class field initializer in another one.
+    ["(class { g = class { f = (", "); }; })"],
+    // The expression is in an arrow function in the initializer.
+    ["(class { f = () => (", "); })"],
+    ["(class { f = (a = (", ")) => a; })"],
+    ["(class { f = async () => { await (", "); }; })"],
+];
+// Only for valid code: `arguments` in an initializer in a class static block gets the error of the static block.
+const fieldInStaticBlock = ["(class { static { class C { f = (", "); } } })"];
+
+// `arguments` in the parameters of a function that is not an arrow function.
+const functionsWithOwnArguments = [
+    "function (p = arguments) { }",
+    "function (p = arguments.length) { }",
+    "function (p = arguments[0]) { }",
+    "function (p = () => arguments) { }",
+    "function (p = () => () => arguments.length) { }",
+    "function ({ p = arguments }) { }",
+    "function ([p = arguments]) { }",
+    "function ({ [arguments.length]: p }) { }",
+    "function (...[p = arguments]) { }",
+    "function (a, b = a, c = arguments.length) { }",
+    "function (p = `${arguments.length}`) { }",
+    "function (p = { [arguments.length]: 1 }) { }",
+    "function (p = typeof arguments) { }",
+    "function named(p = arguments) { }",
+    "function* (p = arguments) { }",
+    "async function (p = arguments) { }",
+    "async function* (p = arguments) { }",
+    "{ m(p = arguments) { } }",
+    "{ *m(p = arguments) { } }",
+    "{ async m(p = arguments) { } }",
+    "{ async *m(p = arguments) { } }",
+    "{ set m(p = arguments) { } }",
+    "{ set m({ p = arguments }) { } }",
+    "{ ['m'](p = arguments) { } }",
+    "class { constructor(p = arguments) { } }",
+    "class extends Object { constructor(p = arguments) { super(); } }",
+    "class { m(p = arguments) { } }",
+    "class { static m(p = arguments) { } }",
+    "class { #m(p = arguments) { } }",
+    "class { static #m(p = arguments) { } }",
+    "class { *m(p = arguments) { } }",
+    "class { async m(p = arguments) { } }",
+    "class { set m(p = arguments) { } }",
+    "class { static set m(p = arguments) { } }",
+    // A function in the parameters of another function.
+    "function (p = function (q = arguments) { }) { }",
+    "function (p = { m(q = arguments) { } }) { }",
+    "function (p = () => function (q = arguments) { }) { }",
+    // The parameters belong to the function, and so does a class heritage or a computed key in them.
+    "function (p = class extends arguments { }) { }",
+    "function (p = class { [arguments]() { } }) { }",
+    "function (p = class { [arguments] = 1; }) { }",
+    "function (p = class { static [arguments.length] = 1; }) { }",
+    // The body was valid before too.
+    "function () { arguments; }",
+    "function (p = arguments) { arguments; () => arguments; }",
+];
+
+for (const [before, after] of [...fields, fieldInStaticBlock]) {
+    for (const expression of functionsWithOwnArguments)
+        shouldNotThrowSyntaxError(before + expression + after);
+}
+
+// `super()` in the parameters of the constructor of a derived class.
+for (const [before, after] of [...fields, fieldInStaticBlock]) {
+    for (const expression of [
+        "class extends Object { constructor(p = super()) { } }",
+        "class extends Object { constructor(p = () => super()) { p(); } }",
+        "class extends Object { constructor(p = super(), q = () => super()) { } }",
+        "class extends Object { constructor({ p = super() }) { } }",
+        "class extends Object { constructor(p = class extends super() { }) { } }",
+        "class extends Object { constructor(p = class { [super()]() { } }) { } }",
+        "class extends Object { constructor(p = { [super()]: 1 }) { } }",
+        "class extends Object { constructor(p = super()) { } static m(q = arguments) { } }",
+    ])
+        shouldNotThrowSyntaxError(before + expression + after);
+}
+
+// Still a SyntaxError: the initializer itself, an arrow function, a computed key and a class heritage are part of the initializer.
+const argumentsMessage = "Unexpected identifier 'arguments'. Cannot reference 'arguments' in class field initializer.";
+const superCallMessage = "Unexpected token '('. super call is not valid in class field initializer context.";
+const superMessage = "super is not valid in this context.";
+
+for (const [before, after] of fields) {
+    for (const expression of [
+        "arguments",
+        "arguments.length",
+        "() => arguments",
+        "(p = arguments) => p",
+        "(p = arguments.length) => p",
+        "([p = arguments]) => p",
+        "(p = () => arguments) => p",
+        "async (p = arguments) => p",
+        "async () => arguments",
+        "{ [arguments]() { } }",
+        "{ [arguments.length]: function (p) { } }",
+        "{ get [arguments]() { return 1; } }",
+        "class { [arguments]() { } }",
+        "class { static [arguments.length](p) { } }",
+        "class extends arguments { }",
+        "class extends (() => arguments) { }",
+        // A class field initializer in the parameters is an initializer again.
+        "function (p = class { g = arguments; }) { }",
+        "function (p = class { g = () => arguments; }) { }",
+        "function (p = class { static g = arguments.length; }) { }",
+        "function (p = class { g = (q = arguments) => q; }) { }",
+        "{ m(p = class { g = arguments; }) { } }",
+        "function () { class C { g = arguments; } }",
+        // After the function, the rest of the initializer is checked as before.
+        "[function (p = arguments) { }, arguments]",
+        "[{ m(p = arguments) { } }, () => arguments]",
+        "function (p = arguments) { }(arguments)",
+        "(a = function (p = arguments) { }, b = arguments) => a",
+        "async (a = function (p = arguments) { }) => arguments",
+        "{ m(p = arguments) { }, [arguments]: 1 }",
+        "class { m(p = arguments) { } [arguments]() { } }",
+        "() => { (function (p = arguments) { }); return arguments; }",
+    ])
+        shouldThrowSyntaxError(before + expression + after, argumentsMessage);
+
+    // And what is valid only in the initializer is still valid after the function: in global code, `new.target` in an arrow function.
+    for (const expression of [
+        "[function (p = arguments) { }, () => new.target]",
+        "[() => 1, function (p = arguments) { }]",
+        "(a = function (p = arguments) { }) => function (q = arguments) { }",
+    ])
+        shouldNotThrowSyntaxError(before + expression + after);
+
+    shouldThrowSyntaxError(before + "function (p = class { static { arguments; } }) { }" + after, "Cannot use 'arguments' as an identifier in static block.");
+    // The parser reads "({ p = arguments })" as an expression first, and that fails before it gets to `arguments`.
+    shouldThrowSyntaxError(before + "({ p = arguments }) => p" + after, "Unexpected token '='. Expected a ':' following the property name 'p'.");
+}
+
+for (const [before, after] of [
+    ["(class extends Object { f = (", "); })"],
+    ["(class extends Object { static f = (", "); })"],
+    ["(class extends Object { constructor() { super(); } f = (", "); })"],
+    ["(class extends Object { constructor(p = class extends Object { f = (", "); }) { super(); } })"],
+]) {
+    for (const expression of [
+        "super()",
+        "() => super()",
+        "(p = super()) => p",
+        "{ [super()]: 1 }",
+        "class extends super() { }",
+        "class { [super()]() { } }",
+        "class extends Object { constructor(p = class { g = super(); }) { } }",
+        "class extends Object { constructor(p = class { g = () => super(); }) { } }",
+        // After the constructor, the rest of the initializer is checked as before.
+        "[class extends Object { constructor(p = super()) { } }, super()]",
+        "[class extends Object { constructor(p = super()) { } }, () => super()]",
+    ])
+        shouldThrowSyntaxError(before + expression + after, superCallMessage);
+
+    // Only the constructor of a derived class can call super().
+    for (const expression of [
+        "function (p = super()) { }",
+        "function (p = () => super()) { }",
+        "{ m(p = super()) { } }",
+        "{ set m(p = super()) { } }",
+        "class { constructor(p = super()) { } }",
+        "class extends Object { m(p = super()) { } }",
+        "class extends Object { static m(p = super()) { } }",
+        "class extends Object { set m(p = super()) { } }",
+        "class extends Object { constructor(p = function (q = super()) { }) { } }",
+        "class extends Object { constructor(p = function () { super(); }) { } }",
+    ])
+        shouldThrowSyntaxError(before + expression + after, superMessage);
+}
+
+// `yield` and `await` in these parameters are what they are without the class field.
+shouldThrowSyntaxError("(function* () { class C { f = function (p = yield) { }; } })", "Unexpected keyword 'yield'. Cannot use yield expression out of generator.");
+shouldThrowSyntaxError("(function* () { class C { f = function* (p = yield) { }; } })", "Unexpected keyword 'yield'. Cannot use yield expression within parameters.");
+shouldThrowSyntaxError("(function* () { class C { f = { *m(p = yield) { } }; } })", "Unexpected keyword 'yield'. Cannot use yield expression within parameters.");
+shouldThrowSyntaxError("(function* () { class C { f = (p = yield) => p; } })", "Unexpected keyword 'yield'. Cannot use yield expression inside class field initializer expression.");
+shouldThrowSyntaxError("(async function () { class C { f = async function (p = await 1) { }; } })", "Cannot use 'await' within a parameter default expression.");
+shouldNotThrowSyntaxError("(async function () { class C { f = function (p = await) { }; } })");
+
+// The values.
+class A {
+    length = function (p = arguments.length) { return p; };
+    object = function (p = arguments) { return p === arguments; };
+    arrow = function (p = () => arguments.length) { return p(); };
+    pattern = function ({ p = arguments.length }, q) { return p; };
+    rest = function (...[p = arguments.length]) { return p; };
+    method = { m(p = arguments.length) { return p; } };
+    setter = { set m(p = arguments.length) { this.value = p; } };
+    static staticField = function (p = arguments[1]) { return p; };
+    #privateField = function (p = arguments.length) { return p; };
+    callPrivateField() { return this.#privateField(undefined, 1, 2, 3); }
+    generator = function* (p = arguments.length) { yield p; };
+    asyncFunction = async function (p = arguments.length) { return p; };
+    inArrow = () => function (p = arguments.length) { return p; };
+    nested = new (class { constructor(p = arguments.length) { this.p = p; } })(undefined, 1);
+    nestedFunction = function (p = function (q = arguments.length) { return q; }) { return p(undefined, 1, 2) + arguments.length; };
+    newTarget = function (p = new.target, q = () => new.target) { return [p, q()]; };
+    evalInParameters = function (p = eval("arguments.length"), q = () => eval("arguments.length")) { return p + q(); };
+    evalInInitializer = eval("(function (p = arguments.length) { return p; })");
+}
+
+let asyncResult;
+for (let i = 0; i < testLoopCount; ++i) {
+    const a = new A();
+    shouldBe(a.length(), 0);
+    shouldBe(a.length(undefined, 2), 2);
+    shouldBe(a.length(7, 2), 7);
+    shouldBe(a.object(), true);
+    shouldBe(a.arrow(undefined, 1, 2), 3);
+    shouldBe(a.pattern({}, 1), 2);
+    shouldBe(a.pattern({ p: 5 }), 5);
+    shouldBe(a.rest(), 0);
+    shouldBe(a.rest(undefined, 1), 2);
+    shouldBe(a.method.m(undefined, 1, 2, 3), 4);
+    a.setter.m = undefined;
+    shouldBe(a.setter.value, 1);
+    shouldBe(A.staticField(undefined, "second"), "second");
+    shouldBe(a.callPrivateField(), 4);
+    shouldBe(a.generator(undefined, 1).next().value, 2);
+    shouldBe(a.inArrow()(undefined, 1, 2), 3);
+    shouldBe(a.nested.p, 2);
+    shouldBe(a.nestedFunction(undefined, 1), 5);
+    shouldBe(a.newTarget()[0], undefined);
+    shouldBe(a.newTarget()[1], undefined);
+    shouldBe(new a.newTarget()[0], a.newTarget);
+    shouldBe(new a.newTarget()[1], a.newTarget);
+    shouldBe(a.evalInParameters(undefined, undefined, 1), 6);
+    shouldBe(a.evalInInitializer(undefined, 1, 2), 3);
+    if (!i)
+        a.asyncFunction(undefined, 1, 2).then(value => { asyncResult = value; });
+}
+drainMicrotasks();
+shouldBe(asyncResult, 3);
+
+class Base {
+    constructor(value) { this.value = value; }
+}
+class B {
+    Derived = class extends Base {
+        constructor(p = super("parameters"), q = this.value) {
+            shouldBe(p, this);
+            this.q = q;
+        }
+    };
+    DerivedWithArrow = class extends Base {
+        constructor(p = () => super("arrow")) {
+            shouldBe(p(), this);
+        }
+    };
+    static Derived = class extends Base {
+        constructor(p = arguments.length, q = super(p)) { }
+    };
+}
+
+for (let i = 0; i < testLoopCount; ++i) {
+    const b = new B();
+    const derived = new b.Derived();
+    shouldBe(derived.value, "parameters");
+    shouldBe(derived.q, "parameters");
+    shouldBe(new b.DerivedWithArrow().value, "arrow");
+    shouldBe(new B.Derived(undefined, undefined, 2).value, 3);
+}
diff --git a/Source/JavaScriptCore/parser/Parser.cpp b/Source/JavaScriptCore/parser/Parser.cpp
index d3f28f7d139f3..4749744293203 100644
--- a/Source/JavaScriptCore/parser/Parser.cpp
+++ b/Source/JavaScriptCore/parser/Parser.cpp
@@ -2364,7 +2364,6 @@ template <class TreeBuilder> TreeFunctionBody Parser<LexerType>::parseFunctionBo
     TreeBuilder& context, SyntaxChecker& syntaxChecker, const JSTokenLocation& startLocation, int startColumn, unsigned functionStart, int functionNameStart, int parametersStart,
     ConstructorKind constructorKind, SuperBinding superBinding, FunctionBodyType bodyType, unsigned parameterCount)
 {
-    SetForScope overrideParsingClassFieldInitializer(m_parserState.isParsingClassFieldInitializer, bodyType != StandardFunctionBodyBlock && m_parserState.isParsingClassFieldInitializer);
     SetForScope maybeUnmaskAsync(m_parserState.classFieldInitMasksAsync, !isAsyncFunctionParseMode(m_parseMode) && m_parserState.classFieldInitMasksAsync);
     bool isArrowFunctionBodyExpression = bodyType == ArrowFunctionBodyExpression;
     if (!isArrowFunctionBodyExpression) {
@@ -2599,6 +2598,9 @@ template <class TreeBuilder> bool Parser<LexerType>::parseFunctionInfo(TreeBuild
     functionScope->setConstructorKind(constructorKind);
 
     SetForScope functionParsePhasePoisoner(m_parserState.functionParsePhase, FunctionParsePhase::Body);
+    // The early errors of a class field initializer (ContainsArguments, Contains SuperCall) look into an arrow function and
+    // stop at any other function: at its parameters as well as its body.
+    SetForScope overrideParsingClassFieldInitializer(m_parserState.isParsingClassFieldInitializer, isArrowFunctionParseMode(mode) && m_parserState.isParsingClassFieldInitializer);
     int functionNameStart = m_token.m_startPosition.offset;
     const Identifier* lastFunctionName = m_parserState.lastFunctionName;
     m_parserState.lastFunctionName = nullptr;