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Control flow analysis for destructured rests #62350

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910ec1d
Control flow analysis for destructured rests
Andarist Aug 28, 2025
9d225ba
fmt
Andarist Aug 28, 2025
5035583
fixed the thing
Andarist Aug 28, 2025
ec0b88f
just reuse existing `CheckMode.RestBindingElement`
Andarist Aug 29, 2025
b43faa7
revert default param
Andarist Aug 29, 2025
a7818f8
remove `BindingElementGrandparent`
Andarist Aug 29, 2025
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32 changes: 20 additions & 12 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1339,6 +1339,7 @@ export const enum CheckMode {
// e.g. in `const { a, ...rest } = foo`, when checking the type of `foo` to determine the type of `rest`,
// we need to preserve generic types instead of substituting them for constraints
TypeOnly = 1 << 6, // Called from getTypeOfExpression, diagnostics may be omitted
SkipConstraintsSubstitution = 1 << 7,
}

/** @internal */
Expand Down Expand Up @@ -30365,21 +30366,27 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
return contextualType && !isGenericType(contextualType);
}

function getNarrowableTypeForReference(type: Type, reference: Node, checkMode?: CheckMode) {
if (isNoInferType(type)) {
type = (type as SubstitutionType).baseType;
}
function shouldSubstituteConstraints(type: Type, reference: Node, checkMode?: CheckMode) {
// When the type of a reference is or contains an instantiable type with a union type constraint, and
// when the reference is in a constraint position (where it is known we'll obtain the apparent type) or
// has a contextual type containing no top-level instantiables (meaning constraints will determine
// assignability), we substitute constraints for all instantiables in the type of the reference to give
// control flow analysis an opportunity to narrow it further. For example, for a reference of a type
// parameter type 'T extends string | undefined' with a contextual type 'string', we substitute
// 'string | undefined' to give control flow analysis the opportunity to narrow to type 'string'.
const substituteConstraints = !(checkMode && checkMode & CheckMode.Inferential) &&
return !(checkMode && checkMode & CheckMode.Inferential) &&
someType(type, isGenericTypeWithUnionConstraint) &&
(isConstraintPosition(type, reference) || hasContextualTypeWithNoGenericTypes(reference, checkMode));
return substituteConstraints ? mapType(type, getBaseConstraintOrType) : type;
}

function getNarrowableTypeForReference(type: Type, reference: Node, checkMode = CheckMode.Normal) {
if (isNoInferType(type)) {
type = (type as SubstitutionType).baseType;
}
if (checkMode & CheckMode.SkipConstraintsSubstitution) {
return type;
}
return shouldSubstituteConstraints(type, reference, checkMode) ? mapType(type, getBaseConstraintOrType) : type;
}

function isExportOrExportExpression(location: Node) {
Expand Down Expand Up @@ -30811,7 +30818,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
const type = getTypeOfSymbol(symbol);
const declaration = symbol.valueDeclaration;
if (declaration) {
// If we have a non-rest binding element with no initializer declared as a const variable or a const-like
// If we have a binding element with no initializer declared as a const variable or a const-like
// parameter (a parameter for which there are no assignments in the function body), and if the parent type
// for the destructuring is a union type, one or more of the binding elements may represent discriminant
// properties, and we want the effects of conditional checks on such discriminants to affect the types of
Expand All @@ -30834,19 +30841,20 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
// the binding pattern AST instance for '{ kind, payload }' as a pseudo-reference and narrow this reference
// as if it occurred in the specified location. We then recompute the narrowed binding element type by
// destructuring from the narrowed parent type.
if (isBindingElement(declaration) && !declaration.initializer && !declaration.dotDotDotToken && declaration.parent.elements.length >= 2) {
if (isBindingElement(declaration) && !declaration.initializer && declaration.parent.elements.length >= 2) {
const parent = declaration.parent.parent;
const rootDeclaration = getRootDeclaration(parent);
if (rootDeclaration.kind === SyntaxKind.VariableDeclaration && getCombinedNodeFlagsCached(rootDeclaration) & NodeFlags.Constant || rootDeclaration.kind === SyntaxKind.Parameter) {
const links = getNodeLinks(parent);
if (!(links.flags & NodeCheckFlags.InCheckIdentifier)) {
links.flags |= NodeCheckFlags.InCheckIdentifier;
const parentType = getTypeForBindingElementParent(parent, CheckMode.Normal);
const parentTypeConstraint = parentType && mapType(parentType, getBaseConstraintOrType);
const parentType = getTypeForBindingElementParent(parent, shouldSubstituteConstraints(type, location) ? CheckMode.Normal : CheckMode.SkipConstraintsSubstitution);
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When obtaining the parent type we don't want to get its constraints substituted based on its contextual type or it being in a constraint position or something. This would lead to returning its constraint and then using that for getBindingElementTypeFromParentType even when the current location isn't at the constraint position.

const parentNarrowableType = parentType && getNarrowableTypeForReference(parentType, location);

links.flags &= ~NodeCheckFlags.InCheckIdentifier;
if (parentTypeConstraint && parentTypeConstraint.flags & TypeFlags.Union && !(rootDeclaration.kind === SyntaxKind.Parameter && isSomeSymbolAssigned(rootDeclaration))) {
if (parentNarrowableType && parentNarrowableType.flags & TypeFlags.Union && !(rootDeclaration.kind === SyntaxKind.Parameter && isSomeSymbolAssigned(rootDeclaration))) {
const pattern = declaration.parent;
const narrowedType = getFlowTypeOfReference(pattern, parentTypeConstraint, parentTypeConstraint, /*flowContainer*/ undefined, location.flowNode);
const narrowedType = getFlowTypeOfReference(pattern, parentNarrowableType, parentNarrowableType, /*flowContainer*/ undefined, location.flowNode);
if (narrowedType.flags & TypeFlags.Never) {
return neverType;
}
Expand Down
4 changes: 2 additions & 2 deletions tests/baselines/reference/arrayDestructuringInSwitch1.symbols
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Original file line number Diff line number Diff line change
Expand Up @@ -31,9 +31,9 @@ export function evaluate(expression: Expression): boolean {

case 'and': {
return operands.every((child) => evaluate(child));
>operands.every : Symbol(Array.every, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
>operands.every : Symbol(Array.every, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
>operands : Symbol(operands, Decl(arrayDestructuringInSwitch1.ts, 5, 20))
>every : Symbol(Array.every, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
>every : Symbol(Array.every, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
>child : Symbol(child, Decl(arrayDestructuringInSwitch1.ts, 8, 31))
>evaluate : Symbol(evaluate, Decl(arrayDestructuringInSwitch1.ts, 1, 84))
>child : Symbol(child, Decl(arrayDestructuringInSwitch1.ts, 8, 31))
Expand Down
16 changes: 8 additions & 8 deletions tests/baselines/reference/arrayDestructuringInSwitch1.types
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Expand Up @@ -46,12 +46,12 @@ export function evaluate(expression: Expression): boolean {
return operands.every((child) => evaluate(child));
>operands.every((child) => evaluate(child)) : boolean
> : ^^^^^^^
>operands.every : { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; } | { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; }
> : ^^^ ^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^ ^ ^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^ ^^ ^^^ ^^^ ^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^ ^ ^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^ ^^ ^^^ ^^^ ^^^
>operands : Expression[] | [Expression]
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^
>every : { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; } | { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; }
> : ^^^ ^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^ ^ ^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^ ^^ ^^^ ^^^ ^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^ ^ ^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^ ^^ ^^^ ^^^ ^^^
>operands.every : { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; }
> : ^^^ ^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^ ^ ^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^ ^^ ^^^ ^^^ ^^^
>operands : Expression[]
> : ^^^^^^^^^^^^
>every : { <S extends Expression>(predicate: (value: Expression, index: number, array: Expression[]) => value is S, thisArg?: any): this is S[]; (predicate: (value: Expression, index: number, array: Expression[]) => unknown, thisArg?: any): boolean; }
> : ^^^ ^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^ ^^^ ^ ^^^ ^^^ ^^^^^^^^^^^^^^ ^^ ^^ ^^^^^^^^^^^^^^^^^^^ ^^ ^^^ ^^^ ^^^
>(child) => evaluate(child) : (child: Expression) => boolean
> : ^ ^^^^^^^^^^^^^^^^^^^^^^^^
>child : Expression
Expand All @@ -76,8 +76,8 @@ export function evaluate(expression: Expression): boolean {
> : ^ ^^ ^^^^^
>operands[0] : Expression
> : ^^^^^^^^^^
>operands : Expression[] | [Expression]
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^
>operands : [Expression]
> : ^^^^^^^^^^^^
>0 : 0
> : ^
}
Expand Down
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