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Fix Element.matches strict narrowing #2487

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Fix Element.matches strict narrowing #2487

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62a9488
Fix Element.matches strict narrowing
cjc0013 May 28, 2026
f49b6ab
Refine Element.matches narrowing map
cjc0013 Jun 30, 2026
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11 changes: 8 additions & 3 deletions baselines/dom.generated.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -14051,9 +14051,12 @@ interface Element extends Node, ARIAMixin, Animatable, ChildNode, NonDocumentTyp
*
* [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/matches)
*/
matches<K extends keyof HTMLElementTagNameMap>(selectors: K): this is HTMLElementTagNameMap[K];
matches<K extends keyof SVGElementTagNameMap>(selectors: K): this is SVGElementTagNameMap[K];
matches<K extends keyof MathMLElementTagNameMap>(selectors: K): this is MathMLElementTagNameMap[K];
matches<K extends keyof ElementMatchesMap<HTMLElementTagNameMap, this>>(selectors: K): this is Extract<HTMLElementTagNameMap[K], this>;
matches<K extends keyof ElementMatchesMap<SVGElementTagNameMap, this>>(selectors: K): this is Extract<SVGElementTagNameMap[K], this>;
matches<K extends keyof ElementMatchesMap<MathMLElementTagNameMap, this>>(selectors: K): this is Extract<MathMLElementTagNameMap[K], this>;
matches<K extends keyof HTMLElementTagNameMap>(selectors: K): boolean;
matches<K extends keyof SVGElementTagNameMap>(selectors: K): boolean;
matches<K extends keyof MathMLElementTagNameMap>(selectors: K): boolean;
matches(selectors: string): boolean;
/**
* The **`releasePointerCapture()`** method of the Element interface releases (stops) pointer capture that was previously set for a specific (PointerEvent) pointer.
Expand Down Expand Up @@ -43880,6 +43883,8 @@ interface MathMLElementTagNameMap {
"semantics": MathMLElement;
}

type ElementMatchesMap<T, U> = { [K in keyof T as T[K] extends U ? U extends T[K] ? never : K : never]: T[K] };

/** @deprecated Directly use HTMLElementTagNameMap or SVGElementTagNameMap as appropriate, instead. */
type ElementTagNameMap = HTMLElementTagNameMap & Pick<SVGElementTagNameMap, Exclude<keyof SVGElementTagNameMap, keyof HTMLElementTagNameMap>>;

Expand Down
11 changes: 8 additions & 3 deletions baselines/ts5.5/dom.generated.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -14038,9 +14038,12 @@ interface Element extends Node, ARIAMixin, Animatable, ChildNode, NonDocumentTyp
*
* [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/matches)
*/
matches<K extends keyof HTMLElementTagNameMap>(selectors: K): this is HTMLElementTagNameMap[K];
matches<K extends keyof SVGElementTagNameMap>(selectors: K): this is SVGElementTagNameMap[K];
matches<K extends keyof MathMLElementTagNameMap>(selectors: K): this is MathMLElementTagNameMap[K];
matches<K extends keyof ElementMatchesMap<HTMLElementTagNameMap, this>>(selectors: K): this is Extract<HTMLElementTagNameMap[K], this>;
matches<K extends keyof ElementMatchesMap<SVGElementTagNameMap, this>>(selectors: K): this is Extract<SVGElementTagNameMap[K], this>;
matches<K extends keyof ElementMatchesMap<MathMLElementTagNameMap, this>>(selectors: K): this is Extract<MathMLElementTagNameMap[K], this>;
matches<K extends keyof HTMLElementTagNameMap>(selectors: K): boolean;
matches<K extends keyof SVGElementTagNameMap>(selectors: K): boolean;
matches<K extends keyof MathMLElementTagNameMap>(selectors: K): boolean;
matches(selectors: string): boolean;
/**
* The **`releasePointerCapture()`** method of the Element interface releases (stops) pointer capture that was previously set for a specific (PointerEvent) pointer.
Expand Down Expand Up @@ -43854,6 +43857,8 @@ interface MathMLElementTagNameMap {
"semantics": MathMLElement;
}

type ElementMatchesMap<T, U> = { [K in keyof T as T[K] extends U ? U extends T[K] ? never : K : never]: T[K] };

/** @deprecated Directly use HTMLElementTagNameMap or SVGElementTagNameMap as appropriate, instead. */
type ElementTagNameMap = HTMLElementTagNameMap & Pick<SVGElementTagNameMap, Exclude<keyof SVGElementTagNameMap, keyof HTMLElementTagNameMap>>;

Expand Down
11 changes: 8 additions & 3 deletions baselines/ts5.6/dom.generated.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -14048,9 +14048,12 @@ interface Element extends Node, ARIAMixin, Animatable, ChildNode, NonDocumentTyp
*
* [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/matches)
*/
matches<K extends keyof HTMLElementTagNameMap>(selectors: K): this is HTMLElementTagNameMap[K];
matches<K extends keyof SVGElementTagNameMap>(selectors: K): this is SVGElementTagNameMap[K];
matches<K extends keyof MathMLElementTagNameMap>(selectors: K): this is MathMLElementTagNameMap[K];
matches<K extends keyof ElementMatchesMap<HTMLElementTagNameMap, this>>(selectors: K): this is Extract<HTMLElementTagNameMap[K], this>;
matches<K extends keyof ElementMatchesMap<SVGElementTagNameMap, this>>(selectors: K): this is Extract<SVGElementTagNameMap[K], this>;
matches<K extends keyof ElementMatchesMap<MathMLElementTagNameMap, this>>(selectors: K): this is Extract<MathMLElementTagNameMap[K], this>;
matches<K extends keyof HTMLElementTagNameMap>(selectors: K): boolean;
matches<K extends keyof SVGElementTagNameMap>(selectors: K): boolean;
matches<K extends keyof MathMLElementTagNameMap>(selectors: K): boolean;
matches(selectors: string): boolean;
/**
* The **`releasePointerCapture()`** method of the Element interface releases (stops) pointer capture that was previously set for a specific (PointerEvent) pointer.
Expand Down Expand Up @@ -43877,6 +43880,8 @@ interface MathMLElementTagNameMap {
"semantics": MathMLElement;
}

type ElementMatchesMap<T, U> = { [K in keyof T as T[K] extends U ? U extends T[K] ? never : K : never]: T[K] };

/** @deprecated Directly use HTMLElementTagNameMap or SVGElementTagNameMap as appropriate, instead. */
type ElementTagNameMap = HTMLElementTagNameMap & Pick<SVGElementTagNameMap, Exclude<keyof SVGElementTagNameMap, keyof HTMLElementTagNameMap>>;

Expand Down
11 changes: 8 additions & 3 deletions baselines/ts5.9/dom.generated.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -14048,9 +14048,12 @@ interface Element extends Node, ARIAMixin, Animatable, ChildNode, NonDocumentTyp
*
* [MDN Reference](https://developer.mozilla.org/docs/Web/API/Element/matches)
*/
matches<K extends keyof HTMLElementTagNameMap>(selectors: K): this is HTMLElementTagNameMap[K];
matches<K extends keyof SVGElementTagNameMap>(selectors: K): this is SVGElementTagNameMap[K];
matches<K extends keyof MathMLElementTagNameMap>(selectors: K): this is MathMLElementTagNameMap[K];
matches<K extends keyof ElementMatchesMap<HTMLElementTagNameMap, this>>(selectors: K): this is Extract<HTMLElementTagNameMap[K], this>;
matches<K extends keyof ElementMatchesMap<SVGElementTagNameMap, this>>(selectors: K): this is Extract<SVGElementTagNameMap[K], this>;
matches<K extends keyof ElementMatchesMap<MathMLElementTagNameMap, this>>(selectors: K): this is Extract<MathMLElementTagNameMap[K], this>;
matches<K extends keyof HTMLElementTagNameMap>(selectors: K): boolean;
matches<K extends keyof SVGElementTagNameMap>(selectors: K): boolean;
matches<K extends keyof MathMLElementTagNameMap>(selectors: K): boolean;
matches(selectors: string): boolean;
/**
* The **`releasePointerCapture()`** method of the Element interface releases (stops) pointer capture that was previously set for a specific (PointerEvent) pointer.
Expand Down Expand Up @@ -43877,6 +43880,8 @@ interface MathMLElementTagNameMap {
"semantics": MathMLElement;
}

type ElementMatchesMap<T, U> = { [K in keyof T as T[K] extends U ? U extends T[K] ? never : K : never]: T[K] };

/** @deprecated Directly use HTMLElementTagNameMap or SVGElementTagNameMap as appropriate, instead. */
type ElementTagNameMap = HTMLElementTagNameMap & Pick<SVGElementTagNameMap, Exclude<keyof SVGElementTagNameMap, keyof HTMLElementTagNameMap>>;

Expand Down
15 changes: 14 additions & 1 deletion src/build/emitter.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -645,7 +645,12 @@ export function emitWebIdl(
const paramName = m.signature[0].param![0].name;
for (const mapName of tagNameMapNames) {
printer.printLine(
`matches<K extends keyof ${mapName}>(${paramName}: K): this is ${mapName}[K];`,
`matches<K extends keyof ElementMatchesMap<${mapName}, this>>(${paramName}: K): this is Extract<${mapName}[K], this>;`,
);
}
for (const mapName of tagNameMapNames) {
printer.printLine(
`matches<K extends keyof ${mapName}>(${paramName}: K): boolean;`,
);
}
printer.printLine(`matches(${paramName}: string): boolean;`);
Expand Down Expand Up @@ -715,6 +720,13 @@ export function emitWebIdl(
printer.printLine("");
}

function emitElementMatchesMap() {
printer.printLine(
"type ElementMatchesMap<T, U> = { [K in keyof T as T[K] extends U ? U extends T[K] ? never : K : never]: T[K] };",

@saschanaz saschanaz Jun 28, 2026

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This is hard to parse, can you use something like ElementMap and ElementType than T and U? And also with parentheses.

It also kinda works weird, keyof ElementMatchesMap<HTMLElementTagNameMap, HTMLDivElement> is somehow "object" | "col" | "colgroup" | "embed" | "hr" | "iframe" | "img" | "input" | "legend" | "table" | "tbody" | "td" | "tfoot" | "th" | "thead" | "tr" ? It should be "div", right?

@cjc0013 cjc0013 Jun 30, 2026 •
edited
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Thanks, that makes sense. I updated the helper to make the strict-narrowing intent explicit with clearer names and a parenthesized mapped-type condition.

I also changed the narrowing map so same-type matches and structurally unrelated matches fall back to boolean. That covers the HTMLDivElement case you pointed out, so HTMLDivElement.matches("div") stays boolean and HTMLDivElement.matches("object") no longer gets a bogus type-guard path.

I added fixture coverage for the original false-branch regression, HTMLElement narrowing to HTMLDivElement, HTMLDivElement.matches("div") staying boolean, and HTMLDivElement.matches("object") not narrowing to HTMLObjectElement.

);
printer.printLine("");
}

/// Emit overloads for the createEvent method
function emitCreateEventOverloads(m: Browser.Method) {
if (matchParamMethodSignature(m, "createEvent", "Event", "string")) {
Expand Down Expand Up @@ -1664,6 +1676,7 @@ export function emitWebIdl(
"MathMLElementTagNameMap",
tagNameToEleName.mathMLResult,
);
emitElementMatchesMap();
emitDeprecatedHTMLOrSVGElementTagNameMap();
emitNamedConstructors();
}
Expand Down
19 changes: 19 additions & 0 deletions unittests/files/matches.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
declare const element: Element;
declare const htmlElement: HTMLElement;
declare const htmlTableCellElement: HTMLTableCellElement;

if (element.matches("dt")) {
const narrowed: HTMLElement = element;
}

if (!htmlElement.matches("dt")) {
htmlElement.id;
}

if (htmlElement.matches("td")) {
const narrowed: HTMLTableCellElement = htmlElement;
}

if (!htmlTableCellElement.matches("th")) {
htmlTableCellElement.id;
}
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