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fix(es/parser): emit TS2371 for object defaults in declare signatures #12095

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c76a9ea
fix(es/parser): emit TS2371 for object defaults in declare signatures
Ayush7614 Jul 31, 2026
a15012a
fix(es/parser): do not emit TS2371 for declare const defaults
Ayush7614 Jul 31, 2026
a767462
fix(es/parser): cover type-signature TS2371 without duplicates
Ayush7614 Jul 31, 2026
732b90e
fix(es/parser): cover type-signature TS2371 without duplicates
Ayush7614 Jul 31, 2026
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6 changes: 6 additions & 0 deletions .changeset/ts2371-declare-object-defaults.md
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---
swc_core: patch
swc_ecma_parser: patch
---

fix(es/parser): emit TS2371 for object-pattern defaults in declare signatures
34 changes: 10 additions & 24 deletions crates/swc_ecma_parser/src/parser/class_and_fn.rs
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Expand Up @@ -355,16 +355,7 @@ impl<I: Tokens> Parser<I> {
if p.syntax().typescript() && body.is_none() {
// Declare functions cannot have assignment pattern in parameters
for param in &params {
// TODO: Search deeply for assignment pattern using a Visitor

let span = match &param.pat {
Pat::Assign(ref p) => Some(p.span()),
_ => None,
};

if let Some(span) = span {
p.emit_err(span, SyntaxError::TS2371)
}
p.emit_ts2371_for_param_initializers(&param.pat);
}
}

Expand Down Expand Up @@ -1249,22 +1240,17 @@ impl<I: Tokens> Parser<I> {
if self.syntax().typescript() && body.is_none() {
// Declare constructors cannot have assignment pattern in parameters
for param in &params {
// TODO: Search deeply for assignment pattern using a Visitor

let span = match *param {
ParamOrTsParamProp::Param(ref param) => match param.pat {
Pat::Assign(ref p) => Some(p.span()),
_ => None,
},
match param {
ParamOrTsParamProp::Param(param) => {
self.emit_ts2371_for_param_initializers(&param.pat);
}
ParamOrTsParamProp::TsParamProp(TsParamProp {
param: TsParamPropParam::Assign(ref p),
param: TsParamPropParam::Assign(p),
..
}) => Some(p.span()),
_ => None,
};

if let Some(span) = span {
self.emit_err(span, SyntaxError::TS2371)
}) => {
self.emit_err(p.span(), SyntaxError::TS2371);
}
_ => {}
}
}
}
Expand Down
9 changes: 8 additions & 1 deletion crates/swc_ecma_parser/src/parser/object.rs
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Expand Up @@ -75,7 +75,14 @@ impl<I: Tokens> Parser<I> {
};

let value = if self.input_mut().eat(Token::Eq) {
self.allow_in_expr(Self::parse_assignment_expr).map(Some)?
let right = self.allow_in_expr(Self::parse_assignment_expr)?;
// Ambient / declare signatures cannot have parameter initializers
// (TS2371). Object shorthand defaults use AssignPatProp and would
// otherwise skip the InDeclare check used for AssignPat.
if self.ctx().contains(Context::InDeclare) {
self.emit_err(self.span(start), SyntaxError::TS2371);
}

@chatgpt-codex-connector chatgpt-codex-connector Bot Jul 31, 2026

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P2 Badge Don't report TS2371 for ambient destructuring bindings

This check runs for every object binding pattern parsed under Context::InDeclare, not just function/constructor parameters. For example, declare const { a = 1 }: { a?: number }; reaches this branch while parsing the variable declarator, and TypeScript accepts that ambient destructuring binding, but SWC will now emit the parameter-only TS2371 diagnostic. Narrow the eager check to parameter/signature parsing (or rely on the post-parse parameter traversal) so declare variable bindings are not rejected.

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Some(right)
} else {
let ctx = self.ctx();
if self.ctx().is_reserved_word(&key.sym) {
Expand Down
43 changes: 43 additions & 0 deletions crates/swc_ecma_parser/src/parser/pat.rs
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Expand Up @@ -77,6 +77,49 @@ impl<I: Tokens> Parser<I> {
}
}

/// Emit TS2371 for any parameter initializer nested in a binding pattern.
///
/// Used for ambient / declare function and constructor signatures where
/// top-level AssignPat checks alone miss object shorthand defaults and
/// nested defaults.
pub(crate) fn emit_ts2371_for_param_initializers(&mut self, pat: &Pat) {
match pat {
Pat::Assign(a) => {
self.emit_err(a.span(), SyntaxError::TS2371);

@chatgpt-codex-connector chatgpt-codex-connector Bot Jul 31, 2026

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P2 Badge Avoid re-emitting TS2371 for parsed declare defaults

For declare function/method signatures parsed under Context::InParameters, nested Pat::Assign defaults have already reported TS2371 from parse_binding_element before this post-parse walk runs. This line reports the same span a second time for cases such as declare function f([x = 1]): void; or declare function f({ a: b = 1 }): void;, so consumers and the newly added stderr fixtures will see duplicate TS2371 diagnostics unless the walker skips already-checked assign patterns in that context or the eager check is centralized here.

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self.emit_ts2371_for_param_initializers(&a.left);
}
Pat::Array(arr) => {
for elem in arr.elems.iter().flatten() {
self.emit_ts2371_for_param_initializers(elem);
}
}
Pat::Object(obj) => {
for prop in &obj.props {
match prop {
ObjectPatProp::KeyValue(KeyValuePatProp { value, .. })
| ObjectPatProp::Rest(RestPat { arg: value, .. }) => {
self.emit_ts2371_for_param_initializers(value);
}
ObjectPatProp::Assign(AssignPatProp {
span,
value: Some(_),
..
}) => {
self.emit_err(*span, SyntaxError::TS2371);
}
ObjectPatProp::Assign(AssignPatProp { value: None, .. }) => {}
#[cfg(swc_ast_unknown)]
_ => {}
}
}
}
Pat::Rest(r) => self.emit_ts2371_for_param_initializers(&r.arg),
Pat::Ident(_) | Pat::Invalid(_) | Pat::Expr(_) => {}
#[cfg(swc_ast_unknown)]
_ => {}
}
}

fn assign_pat_type_ann(&mut self, pat: &mut Pat, span: Span, type_ann: Box<TsType>) {
let type_ann = Some(Box::new(TsTypeAnn { span, type_ann }));

Expand Down
2 changes: 2 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/array-still-errors/input.ts
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declare function bar([x = 1]: number[]): void;
declare function baz([x, y = 2]: number[]): void;
12 changes: 12 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/array-still-errors/input.ts.swc-stderr
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x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/array-still-errors/input.ts:1:1]
1 | declare function bar([x = 1]: number[]): void;
: ^^^^^
2 | declare function baz([x, y = 2]: number[]): void;
`----
x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/array-still-errors/input.ts:2:1]
1 | declare function bar([x = 1]: number[]): void;
2 | declare function baz([x, y = 2]: number[]): void;
: ^^^^^
`----
4 changes: 4 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/declare-method/input.ts
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declare class C {
method({ a = 1 }: { a?: number }): void;
constructor({ a = 1 }: { a?: number });
}
14 changes: 14 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/declare-method/input.ts.swc-stderr
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x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/declare-method/input.ts:2:1]
1 | declare class C {
2 | method({ a = 1 }: { a?: number }): void;
: ^^^^^
3 | constructor({ a = 1 }: { a?: number });
`----
x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/declare-method/input.ts:3:1]
2 | method({ a = 1 }: { a?: number }): void;
3 | constructor({ a = 1 }: { a?: number });
: ^^^^^
4 | }
`----
1 change: 1 addition & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/key-value-default/input.ts
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declare function foo({ a: b = 1 }: { a?: number }): void;
5 changes: 5 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/key-value-default/input.ts.swc-stderr
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x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/key-value-default/input.ts:1:1]
1 | declare function foo({ a: b = 1 }: { a?: number }): void;
: ^^^^^
`----
4 changes: 4 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/mixed/input.ts
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declare function top(a = 1): void;
declare function obj({ a = 1 }): void;
declare function arr([a = 1]): void;
declare function nested({ a: { b = 1 } }): void;
26 changes: 26 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/mixed/input.ts.swc-stderr
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x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/mixed/input.ts:1:1]
1 | declare function top(a = 1): void;
: ^^^^^
2 | declare function obj({ a = 1 }): void;
`----
x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/mixed/input.ts:2:1]
1 | declare function top(a = 1): void;
2 | declare function obj({ a = 1 }): void;
: ^^^^^
3 | declare function arr([a = 1]): void;
`----
x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/mixed/input.ts:3:1]
2 | declare function obj({ a = 1 }): void;
3 | declare function arr([a = 1]): void;
: ^^^^^
4 | declare function nested({ a: { b = 1 } }): void;
`----
x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/mixed/input.ts:4:1]
3 | declare function arr([a = 1]): void;
4 | declare function nested({ a: { b = 1 } }): void;
: ^^^^^
`----
2 changes: 2 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/nested-object/input.ts
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declare function foo({ a: { b = 1 } }: { a: { b?: number } }): void;
declare function bar({ a: [x = 1] }: { a: number[] }): void;
12 changes: 12 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/nested-object/input.ts.swc-stderr
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x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/nested-object/input.ts:1:1]
1 | declare function foo({ a: { b = 1 } }: { a: { b?: number } }): void;
: ^^^^^
2 | declare function bar({ a: [x = 1] }: { a: number[] }): void;
`----
x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/nested-object/input.ts:2:1]
1 | declare function foo({ a: { b = 1 } }: { a: { b?: number } }): void;
2 | declare function bar({ a: [x = 1] }: { a: number[] }): void;
: ^^^^^
`----
2 changes: 2 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/object-shorthand/input.ts
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declare function foo({ a = 1 }: { a?: number }): void;
declare function bar({ a = 1, b = 2 }: { a?: number; b?: number }): void;
18 changes: 18 additions & 0 deletions crates/swc_ecma_parser/tests/typescript-errors/ts2371/object-shorthand/input.ts.swc-stderr
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x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/object-shorthand/input.ts:1:1]
1 | declare function foo({ a = 1 }: { a?: number }): void;
: ^^^^^
2 | declare function bar({ a = 1, b = 2 }: { a?: number; b?: number }): void;
`----
x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/object-shorthand/input.ts:2:1]
1 | declare function foo({ a = 1 }: { a?: number }): void;
2 | declare function bar({ a = 1, b = 2 }: { a?: number; b?: number }): void;
: ^^^^^
`----
x A parameter initializer is only allowed in a function or constructor implementation
,-[$DIR/tests/typescript-errors/ts2371/object-shorthand/input.ts:2:1]
1 | declare function foo({ a = 1 }: { a?: number }): void;
2 | declare function bar({ a = 1, b = 2 }: { a?: number; b?: number }): void;
: ^^^^^
`----
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