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Improve inference by not considering thisless functions to be context-sensitive #62243

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4e92f82
Improve inference by not considering thisless functions to be context…
Andarist Aug 6, 2025
c7f90c9
cleanup `ContainerFlags.IsControlFlowContainer`
Andarist Aug 6, 2025
c29601c
add tests
Andarist Aug 6, 2025
cf4544d
fix propagation condition
Andarist Aug 7, 2025
5fed81c
add test cases
Andarist Aug 9, 2025
c069fbb
fmt
Andarist Aug 9, 2025
07e36ce
Prefer `returnMapper`-based contextual type instantiations when candi…
Andarist Aug 11, 2025
53047af
Don't skip generic functions when arity is not alright
Andarist Aug 12, 2025
ee1c607
Revert "Don't skip generic functions when arity is not alright"
Andarist Aug 12, 2025
f4ced6e
actually, just remove `SkipGenericFunctions` in `checkAndAggregateYie…
Andarist Aug 12, 2025
4543ff1
Merge remote-tracking branch 'origin/main' into no-context-sensitivit…
Andarist Oct 26, 2025
73d04f5
Merge remote-tracking branch 'origin/main' into no-context-sensitivit…
Andarist Dec 5, 2025
e22b095
Merge remote-tracking branch 'origin/main' into no-context-sensitivit…
Andarist Dec 5, 2025
48f2995
`hasContextSensitiveYieldExpression`
Andarist Dec 8, 2025
c0ea121
add comment
Andarist Dec 9, 2025
ec8f425
tweak `instantiateContextualType` further
Andarist Dec 9, 2025
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25 changes: 17 additions & 8 deletions src/compiler/binder.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1014,6 +1014,7 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
const saveExceptionTarget = currentExceptionTarget;
const saveActiveLabelList = activeLabelList;
const saveHasExplicitReturn = hasExplicitReturn;
const saveSeenThisKeyword = seenThisKeyword;
const isImmediatelyInvoked = (
containerFlags & ContainerFlags.IsFunctionExpression &&
!hasSyntacticModifier(node, ModifierFlags.Async) &&
Expand All @@ -1036,19 +1037,22 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
currentContinueTarget = undefined;
activeLabelList = undefined;
hasExplicitReturn = false;
seenThisKeyword = false;
bindChildren(node);
// Reset all reachability check related flags on node (for incremental scenarios)
node.flags &= ~NodeFlags.ReachabilityAndEmitFlags;
// Reset flags (for incremental scenarios)
node.flags &= ~(NodeFlags.ReachabilityAndEmitFlags | NodeFlags.ContainsThis);
if (!(currentFlow.flags & FlowFlags.Unreachable) && containerFlags & ContainerFlags.IsFunctionLike && nodeIsPresent((node as FunctionLikeDeclaration | ClassStaticBlockDeclaration).body)) {
node.flags |= NodeFlags.HasImplicitReturn;
if (hasExplicitReturn) node.flags |= NodeFlags.HasExplicitReturn;
(node as FunctionLikeDeclaration | ClassStaticBlockDeclaration).endFlowNode = currentFlow;
}
if (seenThisKeyword) {
node.flags |= NodeFlags.ContainsThis;
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There's trailing whitespace at the end of this line. Remove the extra spaces.

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node.flags |= NodeFlags.ContainsThis;
node.flags |= NodeFlags.ContainsThis;

Copilot uses AI. Check for mistakes.
}
if (node.kind === SyntaxKind.SourceFile) {
node.flags |= emitFlags;
(node as SourceFile).endFlowNode = currentFlow;
}

if (currentReturnTarget) {
addAntecedent(currentReturnTarget, currentFlow);
currentFlow = finishFlowLabel(currentReturnTarget);
Expand All @@ -1065,12 +1069,15 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
currentExceptionTarget = saveExceptionTarget;
activeLabelList = saveActiveLabelList;
hasExplicitReturn = saveHasExplicitReturn;
seenThisKeyword = node.kind === SyntaxKind.ArrowFunction ? saveSeenThisKeyword || seenThisKeyword : saveSeenThisKeyword;
}
else if (containerFlags & ContainerFlags.IsInterface) {
const saveSeenThisKeyword = seenThisKeyword;
seenThisKeyword = false;
bindChildren(node);
Debug.assertNotNode(node, isIdentifier); // ContainsThis cannot overlap with HasExtendedUnicodeEscape on Identifier
node.flags = seenThisKeyword ? node.flags | NodeFlags.ContainsThis : node.flags & ~NodeFlags.ContainsThis;
seenThisKeyword = saveSeenThisKeyword;
}
else {
bindChildren(node);
Expand Down Expand Up @@ -2873,6 +2880,9 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
}
// falls through
case SyntaxKind.ThisKeyword:
if (node.kind === SyntaxKind.ThisKeyword) {
seenThisKeyword = true;
}
// TODO: Why use `isExpression` here? both Identifier and ThisKeyword are expressions.
if (currentFlow && (isExpression(node) || parent.kind === SyntaxKind.ShorthandPropertyAssignment)) {
(node as Identifier | ThisExpression).flowNode = currentFlow;
Expand Down Expand Up @@ -3941,28 +3951,27 @@ export function getContainerFlags(node: Node): ContainerFlags {
// falls through
case SyntaxKind.Constructor:
case SyntaxKind.FunctionDeclaration:
case SyntaxKind.ClassStaticBlockDeclaration:
return ContainerFlags.IsContainer | ContainerFlags.IsControlFlowContainer | ContainerFlags.HasLocals | ContainerFlags.IsFunctionLike;
case SyntaxKind.MethodSignature:
case SyntaxKind.CallSignature:
case SyntaxKind.JSDocSignature:
case SyntaxKind.JSDocFunctionType:
case SyntaxKind.FunctionType:
case SyntaxKind.ConstructSignature:
case SyntaxKind.ConstructorType:
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@Andarist Andarist Aug 9, 2025

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Type-level AST nodes had ContainerFlags.IsControlFlowContainer here. This was messing up some of the changes I made since it was interfering with the implemented seenThisKeyword tracking. I don't see why those would be considered control flow containers and there are no tests proving it was needed.

Other changes in this function are basically of the same kind - I just removed ContainerFlags.IsControlFlowContainer from the type-level nodes.

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@gabritto gabritto Dec 8, 2025 •
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We seem to have added at least one of those on purpose:
#8941
So I'm wondering why it's not needed anymore. @ahejlsberg do you remember why this was needed?

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@Andarist Andarist Dec 8, 2025

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As to SyntaxKind.PropertyDeclaration - given the test from the referenced PR still works just OK, I'd assume that its needs are covered by arrow functions being treated as flow containers (arrows were used as property declaration initializers in that test).

case SyntaxKind.ClassStaticBlockDeclaration:
return ContainerFlags.IsContainer | ContainerFlags.IsControlFlowContainer | ContainerFlags.HasLocals | ContainerFlags.IsFunctionLike;
return ContainerFlags.IsContainer | ContainerFlags.HasLocals | ContainerFlags.IsFunctionLike;

case SyntaxKind.JSDocImportTag:
// treat as a container to prevent using an enclosing effective host, ensuring import bindings are scoped correctly
return ContainerFlags.IsContainer | ContainerFlags.IsControlFlowContainer | ContainerFlags.HasLocals;
return ContainerFlags.IsContainer | ContainerFlags.HasLocals;

case SyntaxKind.FunctionExpression:
case SyntaxKind.ArrowFunction:
return ContainerFlags.IsContainer | ContainerFlags.IsControlFlowContainer | ContainerFlags.HasLocals | ContainerFlags.IsFunctionLike | ContainerFlags.IsFunctionExpression;

case SyntaxKind.ModuleBlock:
return ContainerFlags.IsControlFlowContainer;
case SyntaxKind.PropertyDeclaration:
return (node as PropertyDeclaration).initializer ? ContainerFlags.IsControlFlowContainer : 0;

case SyntaxKind.CatchClause:
case SyntaxKind.ForStatement:
Expand Down
7 changes: 4 additions & 3 deletions src/compiler/checker.ts
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Expand Up @@ -21135,9 +21135,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
const { initializer } = node as JsxAttribute;
return !!initializer && isContextSensitive(initializer);
}
case SyntaxKind.JsxExpression: {
case SyntaxKind.JsxExpression:
case SyntaxKind.YieldExpression: {
// It is possible to that node.expression is undefined (e.g <div x={} />)
const { expression } = node as JsxExpression;
const { expression } = node as JsxExpression | YieldExpression;
return !!expression && isContextSensitive(expression);
}
}
Expand All @@ -21146,7 +21147,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
}

function isContextSensitiveFunctionLikeDeclaration(node: FunctionLikeDeclaration): boolean {
return hasContextSensitiveParameters(node) || hasContextSensitiveReturnExpression(node);
return hasContextSensitiveParameters(node) || hasContextSensitiveReturnExpression(node) || !!(getFunctionFlags(node) & FunctionFlags.Generator && node.body && forEachYieldExpression(node.body as Block, isContextSensitive));
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@Andarist Andarist Aug 9, 2025

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Previously generators were always context-sensitive as they can't even be arrow functions. At times, they are truly context-sensitive in cases like:

declare function test(
  gen: () => Generator<(arg: number) => string, void, void>,
): void;

test(function* () {
  yield (arg) => String(arg);
});

So I had to add this extra forEachYieldExpression to cover for this

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@gabritto gabritto Dec 8, 2025

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Just in terms of readability, maybe this would be better as its own function, then it can have a descriptive name like hasContextSensitiveYieldExpression and that example can be its documentation.

}

function hasContextSensitiveReturnExpression(node: FunctionLikeDeclaration) {
Expand Down
24 changes: 14 additions & 10 deletions src/compiler/utilities.ts
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Expand Up @@ -2864,19 +2864,24 @@ export function forEachReturnStatement<T>(body: Block | Statement, visitor: (stm
}
}

// Warning: This has the same semantics as the forEach family of functions,
// in that traversal terminates in the event that 'visitor' supplies a truthy value.
/** @internal */
export function forEachYieldExpression(body: Block, visitor: (expr: YieldExpression) => void): void {
export function forEachYieldExpression<T>(body: Block, visitor: (expr: YieldExpression) => T): T | undefined {
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this basically changes this function to work in the same way as forEachReturnStatement defined above

return traverse(body);

function traverse(node: Node): void {
function traverse(node: Node): T | undefined {
switch (node.kind) {
case SyntaxKind.YieldExpression:
visitor(node as YieldExpression);
const value = visitor(node as YieldExpression);
if (value) {
return value;
}
const operand = (node as YieldExpression).expression;
if (operand) {
traverse(operand);
if (!operand) {
return;
}
return;
return traverse(operand);
case SyntaxKind.EnumDeclaration:
case SyntaxKind.InterfaceDeclaration:
case SyntaxKind.ModuleDeclaration:
Expand All @@ -2889,14 +2894,13 @@ export function forEachYieldExpression(body: Block, visitor: (expr: YieldExpress
if (node.name && node.name.kind === SyntaxKind.ComputedPropertyName) {
// Note that we will not include methods/accessors of a class because they would require
// first descending into the class. This is by design.
traverse(node.name.expression);
return;
return traverse(node.name.expression);;
}
}
else if (!isPartOfTypeNode(node)) {
// This is the general case, which should include mostly expressions and statements.
// Also includes NodeArrays.
forEachChild(node, traverse);
return forEachChild(node, traverse);
}
}
}
Expand Down Expand Up @@ -10825,7 +10829,7 @@ export function hasContextSensitiveParameters(node: FunctionLikeDeclaration): bo
// an implicit 'this' parameter which is subject to contextual typing.
const parameter = firstOrUndefined(node.parameters);
if (!(parameter && parameterIsThisKeyword(parameter))) {
return true;
return !!(node.flags & NodeFlags.ContainsThis);
}
}
}
Expand Down
4 changes: 2 additions & 2 deletions tests/baselines/reference/circularlySimplifyingConditionalTypesNoCrash.types
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Expand Up @@ -68,8 +68,8 @@ declare var connect: Connect;
const myStoreConnect: Connect = function(
>myStoreConnect : Connect
> : ^^^^^^^
>function( mapStateToProps?: any, mapDispatchToProps?: any, mergeProps?: any, options: unknown = {},) { return connect( mapStateToProps, mapDispatchToProps, mergeProps, options, );} : <TStateProps, TOwnProps>(mapStateToProps?: any, mapDispatchToProps?: any, mergeProps?: any, options?: unknown) => InferableComponentEnhancerWithProps<TStateProps, Omit<P, Extract<keyof TStateProps, keyof P>> & TOwnProps>
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@Andarist Andarist Aug 9, 2025

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this just changes the inferred type to match the type inferred from the equivalent arrow function with type parameters, it's purely a result of making a thisless function context-insensitive

> : ^ ^^ ^^ ^^^ ^^ ^^^ ^^ ^^^ ^^ ^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>function( mapStateToProps?: any, mapDispatchToProps?: any, mergeProps?: any, options: unknown = {},) { return connect( mapStateToProps, mapDispatchToProps, mergeProps, options, );} : (mapStateToProps?: any, mapDispatchToProps?: any, mergeProps?: any, options?: unknown) => InferableComponentEnhancerWithProps<TStateProps, Omit<P, Extract<keyof TStateProps, keyof P>> & TOwnProps>
> : ^ ^^^ ^^ ^^^ ^^ ^^^ ^^ ^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

mapStateToProps?: any,
>mapStateToProps : any
Expand Down
8 changes: 4 additions & 4 deletions tests/baselines/reference/classCanExtendConstructorFunction.errors.txt
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@@ -1,14 +1,14 @@
first.js(23,9): error TS2554: Expected 1 arguments, but got 0.
first.js(31,5): error TS2416: Property 'load' in type 'Sql' is not assignable to the same property in base type 'Wagon'.
Type '(files: string[], format: "csv" | "json" | "xmlolololol") => void' is not assignable to type '(supplies?: any[]) => void'.
Type '(files: string[], format: "csv" | "json" | "xmlolololol") => void' is not assignable to type '(supplies?: any[] | undefined) => void'.
Target signature provides too few arguments. Expected 2 or more, but got 1.
first.js(47,24): error TS2507: Type '(numberEaten: number) => void' is not a constructor function type.
generic.js(19,19): error TS2554: Expected 1 arguments, but got 0.
generic.js(20,32): error TS2345: Argument of type 'number' is not assignable to parameter of type '{ claim: "ignorant" | "malicious"; }'.
second.ts(8,25): error TS2507: Type '(numberEaten: number) => void' is not a constructor function type.
second.ts(14,7): error TS2417: Class static side 'typeof Conestoga' incorrectly extends base class static side 'typeof Wagon'.
Types of property 'circle' are incompatible.
Type '(others: (typeof Wagon)[]) => number' is not assignable to type '(wagons?: Wagon[]) => number'.
Type '(others: (typeof Wagon)[]) => number' is not assignable to type '(wagons?: Wagon[] | undefined) => number'.
Types of parameters 'others' and 'wagons' are incompatible.
Type 'Wagon[]' is not assignable to type '(typeof Wagon)[]'.
Property 'circle' is missing in type 'Wagon' but required in type 'typeof Wagon'.
Expand Down Expand Up @@ -52,7 +52,7 @@ second.ts(17,15): error TS2345: Argument of type 'string' is not assignable to p
load(files, format) {
~~~~
!!! error TS2416: Property 'load' in type 'Sql' is not assignable to the same property in base type 'Wagon'.
!!! error TS2416: Type '(files: string[], format: "csv" | "json" | "xmlolololol") => void' is not assignable to type '(supplies?: any[]) => void'.
!!! error TS2416: Type '(files: string[], format: "csv" | "json" | "xmlolololol") => void' is not assignable to type '(supplies?: any[] | undefined) => void'.
!!! error TS2416: Target signature provides too few arguments. Expected 2 or more, but got 1.
if (format === "xmlolololol") {
throw new Error("please do not use XML. It was a joke.");
Expand Down Expand Up @@ -95,7 +95,7 @@ second.ts(17,15): error TS2345: Argument of type 'string' is not assignable to p
~~~~~~~~~
!!! error TS2417: Class static side 'typeof Conestoga' incorrectly extends base class static side 'typeof Wagon'.
!!! error TS2417: Types of property 'circle' are incompatible.
!!! error TS2417: Type '(others: (typeof Wagon)[]) => number' is not assignable to type '(wagons?: Wagon[]) => number'.
!!! error TS2417: Type '(others: (typeof Wagon)[]) => number' is not assignable to type '(wagons?: Wagon[] | undefined) => number'.
!!! error TS2417: Types of parameters 'others' and 'wagons' are incompatible.
!!! error TS2417: Type 'Wagon[]' is not assignable to type '(typeof Wagon)[]'.
!!! error TS2417: Property 'circle' is missing in type 'Wagon' but required in type 'typeof Wagon'.
Expand Down
4 changes: 2 additions & 2 deletions tests/baselines/reference/generatorTypeCheck62.types
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Expand Up @@ -117,8 +117,8 @@ export const Nothing2: Strategy<State> = strategy("Nothing", function*(state: St
export const Nothing3: Strategy<State> = strategy("Nothing", function* (state: State) {
>Nothing3 : Strategy<State>
> : ^^^^^^^^^^^^^^^
>strategy("Nothing", function* (state: State) { yield ; return state; // `return`/`TReturn` isn't supported by `strategy`, so this should error.}) : (a: State) => IterableIterator<State, void>
> : ^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>strategy("Nothing", function* (state: State) { yield ; return state; // `return`/`TReturn` isn't supported by `strategy`, so this should error.}) : (a: any) => IterableIterator<any, void>
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@Andarist Andarist Aug 9, 2025

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similarly here, this is a result of inferSignatureInstantiationForOverloadFailure no longer skipping the generator function on the basis it's context-sensitive (inferSignatureInstantiationForOverloadFailure uses CheckMode.SkipContextSensitive)

> : ^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>strategy : <T extends StrategicState>(stratName: string, gen: (a: T) => IterableIterator<T | undefined, void>) => (a: T) => IterableIterator<T | undefined, void>
> : ^ ^^^^^^^^^ ^^ ^^ ^^ ^^ ^^^^^
>"Nothing" : "Nothing"
Expand Down
4 changes: 2 additions & 2 deletions tests/baselines/reference/jsdocTemplateTag.errors.txt
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@@ -1,4 +1,4 @@
forgot.js(23,1): error TS2322: Type '(keyframes: any[]) => void' is not assignable to type '(keyframes: Keyframe[] | PropertyIndexedKeyframes, options?: number | KeyframeAnimationOptions) => Animation'.
forgot.js(23,1): error TS2322: Type '(keyframes: Array<any>) => void' is not assignable to type '(keyframes: Keyframe[] | PropertyIndexedKeyframes, options?: number | KeyframeAnimationOptions) => Animation'.
Type 'void' is not assignable to type 'Animation'.


Expand Down Expand Up @@ -27,6 +27,6 @@ forgot.js(23,1): error TS2322: Type '(keyframes: any[]) => void' is not assignab
*/
Element.prototype.animate = function(keyframes) {};
~~~~~~~~~~~~~~~~~~~~~~~~~
!!! error TS2322: Type '(keyframes: any[]) => void' is not assignable to type '(keyframes: Keyframe[] | PropertyIndexedKeyframes, options?: number | KeyframeAnimationOptions) => Animation'.
!!! error TS2322: Type '(keyframes: Array<any>) => void' is not assignable to type '(keyframes: Keyframe[] | PropertyIndexedKeyframes, options?: number | KeyframeAnimationOptions) => Animation'.
!!! error TS2322: Type 'void' is not assignable to type 'Animation'.

4 changes: 2 additions & 2 deletions tests/baselines/reference/redefineArray.errors.txt
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@@ -1,8 +1,8 @@
redefineArray.ts(1,1): error TS2741: Property 'isArray' is missing in type '<T>(n: number, s: string) => number' but required in type 'ArrayConstructor'.
redefineArray.ts(1,1): error TS2741: Property 'isArray' is missing in type '(n: number, s: string) => number' but required in type 'ArrayConstructor'.


==== redefineArray.ts (1 errors) ====
Array = function (n:number, s:string) {return n;};
~~~~~
!!! error TS2741: Property 'isArray' is missing in type '<T>(n: number, s: string) => number' but required in type 'ArrayConstructor'.
!!! error TS2741: Property 'isArray' is missing in type '(n: number, s: string) => number' but required in type 'ArrayConstructor'.
!!! related TS2728 lib.es5.d.ts:--:--: 'isArray' is declared here.
8 changes: 4 additions & 4 deletions tests/baselines/reference/redefineArray.types
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Expand Up @@ -2,12 +2,12 @@

=== redefineArray.ts ===
Array = function (n:number, s:string) {return n;};
>Array = function (n:number, s:string) {return n;} : <T>(n: number, s: string) => number
> : ^ ^^ ^^ ^^ ^^ ^^^^^^^^^^^
>Array = function (n:number, s:string) {return n;} : (n: number, s: string) => number
> : ^ ^^ ^^ ^^ ^^^^^^^^^^^
>Array : ArrayConstructor
> : ^^^^^^^^^^^^^^^^
>function (n:number, s:string) {return n;} : <T>(n: number, s: string) => number
> : ^ ^^ ^^ ^^ ^^ ^^^^^^^^^^^
>function (n:number, s:string) {return n;} : (n: number, s: string) => number
> : ^ ^^ ^^ ^^ ^^^^^^^^^^^
>n : number
> : ^^^^^^
>s : string
Expand Down
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