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Don't ignore all generic self tail calls when collecting the return type of a function #63999

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4cc66cc
Don't ignore all generic self tail calls when collecting the return t…
Andarist Aug 25, 2026
0a56620
Handle tuple spreads in recursive return inference
Andarist Aug 25, 2026
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11 changes: 7 additions & 4 deletions tsc/internal/checker/checker.go
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Original file line number Diff line number Diff line change
Expand Up @@ -20382,15 +20382,18 @@ func (c *Checker) checkAndAggregateReturnExpressionTypes(fn *ast.Node, checkMode
return false
}
expr = ast.SkipParentheses(expr)
// Bare calls to this same function don't contribute to inference
// and `return await` is also safe to unwrap here
// Bare calls to this same function that don't require a new generic instantiation
// don't contribute to inference, and `return await` is also safe to unwrap here.
if functionFlags&ast.FunctionFlagsAsync != 0 && ast.IsAwaitExpression(expr) {
expr = ast.SkipParentheses(expr.Expression())
}
if ast.IsCallExpression(expr) && ast.IsIdentifier(expr.Expression()) && c.checkExpressionCached(expr.Expression()).symbol == c.getMergedSymbol(fn.Symbol()) &&
(!ast.IsFunctionExpressionOrArrowFunction(fn.Symbol().ValueDeclaration) || c.isConstantReference(expr.Expression())) {
hasReturnOfTypeNever = true
return false
signature := c.getSignatureFromDeclaration(fn)
if len(signature.typeParameters) == 0 || len(expr.TypeArguments()) == 0 && c.isSignatureApplicable(expr, c.getEffectiveCallArguments(expr), signature, c.assignableRelation, checkMode, false /*reportErrors*/, nil /*diagnosticOutput*/) {
hasReturnOfTypeNever = true
return false
}
}
t := c.checkExpressionCachedEx(expr, checkMode & ^CheckModeSkipGenericFunctions)
if functionFlags&ast.FunctionFlagsAsync != 0 {
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60 changes: 60 additions & 0 deletions tsc/testdata/baselines/reference/compiler/genericRecursiveFunctionReturn.errors.txt
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@@ -0,0 +1,60 @@
genericRecursiveFunctionReturn.ts(2,7): error TS7023: 'f' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
genericRecursiveFunctionReturn.ts(12,10): error TS7023: 'g' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
genericRecursiveFunctionReturn.ts(34,10): error TS7023: 'nested' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
genericRecursiveFunctionReturn.ts(40,10): error TS7023: 'swap' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.


==== genericRecursiveFunctionReturn.ts (4 errors) ====
// https://github.com/microsoft/TypeScript/issues/63990
const f = <T>(a: T, p: number) => {
~
!!! error TS7023: 'f' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
if (!p) return a;
// this recursive call can't be ignored as that would lead to inferring T when the number return type is possible here
return f(p, p - 1);
};

const x = f("foo", 5);

declare const condition: boolean;

function g<T extends number>(value: T) {
~
!!! error TS7023: 'g' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
if (condition) return value;
// this recursive call can't be ignored as that would lead to inferring T when the number return type is possible here
return g<number>(value);
}

function h<T>(value: T, count: number) {
if (!count) return value;
// this recursive call can be ignored because its arguments are assignable to the current generic signature
return h(value, count - 1);
}

const y = h("foo", 1);

function tupleSpread<T>(value: T, count: number) {
if (!count) return value;
// tuple spreads are expanded into effective arguments before checking assignability
return tupleSpread(...[value, count - 1] as const);
}

const z = tupleSpread("foo", 1);

function nested<T>(value: T) {
~~~~~~
!!! error TS7023: 'nested' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
if (condition) return value;
// this recursive call can't be ignored as that would lead to inferring T when the T[] return type is possible here
return nested([value]);
}

function swap<T, U>(left: T, right: U, shouldSwap: boolean) {
~~~~
!!! error TS7023: 'swap' implicitly has return type 'any' because it does not have a return type annotation and is referenced directly or indirectly in one of its return expressions.
if (!shouldSwap) return [left, right] as const;
// this recursive call can't be ignored as that would lead to inferring readonly [T, U] when the readonly [U, T] return type is possible here
return swap(right, left, false);
}

91 changes: 91 additions & 0 deletions tsc/testdata/baselines/reference/compiler/genericRecursiveFunctionReturn.js
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//// [tests/cases/compiler/genericRecursiveFunctionReturn.ts] ////

//// [genericRecursiveFunctionReturn.ts]
// https://github.com/microsoft/TypeScript/issues/63990
const f = <T>(a: T, p: number) => {
if (!p) return a;
// this recursive call can't be ignored as that would lead to inferring T when the number return type is possible here
return f(p, p - 1);
};

const x = f("foo", 5);

declare const condition: boolean;

function g<T extends number>(value: T) {
if (condition) return value;
// this recursive call can't be ignored as that would lead to inferring T when the number return type is possible here
return g<number>(value);
}

function h<T>(value: T, count: number) {
if (!count) return value;
// this recursive call can be ignored because its arguments are assignable to the current generic signature
return h(value, count - 1);
}

const y = h("foo", 1);

function tupleSpread<T>(value: T, count: number) {
if (!count) return value;
// tuple spreads are expanded into effective arguments before checking assignability
return tupleSpread(...[value, count - 1] as const);
}

const z = tupleSpread("foo", 1);

function nested<T>(value: T) {
if (condition) return value;
// this recursive call can't be ignored as that would lead to inferring T when the T[] return type is possible here
return nested([value]);
}

function swap<T, U>(left: T, right: U, shouldSwap: boolean) {
if (!shouldSwap) return [left, right] as const;
// this recursive call can't be ignored as that would lead to inferring readonly [T, U] when the readonly [U, T] return type is possible here
return swap(right, left, false);
}


//// [genericRecursiveFunctionReturn.js]
"use strict";
// https://github.com/microsoft/TypeScript/issues/63990
const f = (a, p) => {
if (!p)
return a;
// this recursive call can't be ignored as that would lead to inferring T when the number return type is possible here
return f(p, p - 1);
};
const x = f("foo", 5);
function g(value) {
if (condition)
return value;
// this recursive call can't be ignored as that would lead to inferring T when the number return type is possible here
return g(value);
}
function h(value, count) {
if (!count)
return value;
// this recursive call can be ignored because its arguments are assignable to the current generic signature
return h(value, count - 1);
}
const y = h("foo", 1);
function tupleSpread(value, count) {
if (!count)
return value;
// tuple spreads are expanded into effective arguments before checking assignability
return tupleSpread(...[value, count - 1]);
}
const z = tupleSpread("foo", 1);
function nested(value) {
if (condition)
return value;
// this recursive call can't be ignored as that would lead to inferring T when the T[] return type is possible here
return nested([value]);
}
function swap(left, right, shouldSwap) {
if (!shouldSwap)
return [left, right];
// this recursive call can't be ignored as that would lead to inferring readonly [T, U] when the readonly [U, T] return type is possible here
return swap(right, left, false);
}
130 changes: 130 additions & 0 deletions tsc/testdata/baselines/reference/compiler/genericRecursiveFunctionReturn.symbols
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//// [tests/cases/compiler/genericRecursiveFunctionReturn.ts] ////

=== genericRecursiveFunctionReturn.ts ===
// https://github.com/microsoft/TypeScript/issues/63990
const f = <T>(a: T, p: number) => {
>f : Symbol(f, Decl(genericRecursiveFunctionReturn.ts, 1, 5))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 1, 11))
>a : Symbol(a, Decl(genericRecursiveFunctionReturn.ts, 1, 14))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 1, 11))
>p : Symbol(p, Decl(genericRecursiveFunctionReturn.ts, 1, 19))

if (!p) return a;
>p : Symbol(p, Decl(genericRecursiveFunctionReturn.ts, 1, 19))
>a : Symbol(a, Decl(genericRecursiveFunctionReturn.ts, 1, 14))

// this recursive call can't be ignored as that would lead to inferring T when the number return type is possible here
return f(p, p - 1);
>f : Symbol(f, Decl(genericRecursiveFunctionReturn.ts, 1, 5))
>p : Symbol(p, Decl(genericRecursiveFunctionReturn.ts, 1, 19))
>p : Symbol(p, Decl(genericRecursiveFunctionReturn.ts, 1, 19))

};

const x = f("foo", 5);
>x : Symbol(x, Decl(genericRecursiveFunctionReturn.ts, 7, 5))
>f : Symbol(f, Decl(genericRecursiveFunctionReturn.ts, 1, 5))

declare const condition: boolean;
>condition : Symbol(condition, Decl(genericRecursiveFunctionReturn.ts, 9, 13))

function g<T extends number>(value: T) {
>g : Symbol(g, Decl(genericRecursiveFunctionReturn.ts, 9, 33))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 11, 11))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 11, 29))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 11, 11))

if (condition) return value;
>condition : Symbol(condition, Decl(genericRecursiveFunctionReturn.ts, 9, 13))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 11, 29))

// this recursive call can't be ignored as that would lead to inferring T when the number return type is possible here
return g<number>(value);
>g : Symbol(g, Decl(genericRecursiveFunctionReturn.ts, 9, 33))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 11, 29))
}

function h<T>(value: T, count: number) {
>h : Symbol(h, Decl(genericRecursiveFunctionReturn.ts, 15, 1))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 17, 11))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 17, 14))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 17, 11))
>count : Symbol(count, Decl(genericRecursiveFunctionReturn.ts, 17, 23))

if (!count) return value;
>count : Symbol(count, Decl(genericRecursiveFunctionReturn.ts, 17, 23))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 17, 14))

// this recursive call can be ignored because its arguments are assignable to the current generic signature
return h(value, count - 1);
>h : Symbol(h, Decl(genericRecursiveFunctionReturn.ts, 15, 1))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 17, 14))
>count : Symbol(count, Decl(genericRecursiveFunctionReturn.ts, 17, 23))
}

const y = h("foo", 1);
>y : Symbol(y, Decl(genericRecursiveFunctionReturn.ts, 23, 5))
>h : Symbol(h, Decl(genericRecursiveFunctionReturn.ts, 15, 1))

function tupleSpread<T>(value: T, count: number) {
>tupleSpread : Symbol(tupleSpread, Decl(genericRecursiveFunctionReturn.ts, 23, 22))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 25, 21))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 25, 24))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 25, 21))
>count : Symbol(count, Decl(genericRecursiveFunctionReturn.ts, 25, 33))

if (!count) return value;
>count : Symbol(count, Decl(genericRecursiveFunctionReturn.ts, 25, 33))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 25, 24))

// tuple spreads are expanded into effective arguments before checking assignability
return tupleSpread(...[value, count - 1] as const);
>tupleSpread : Symbol(tupleSpread, Decl(genericRecursiveFunctionReturn.ts, 23, 22))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 25, 24))
>count : Symbol(count, Decl(genericRecursiveFunctionReturn.ts, 25, 33))
>const : Symbol(const)
}

const z = tupleSpread("foo", 1);
>z : Symbol(z, Decl(genericRecursiveFunctionReturn.ts, 31, 5))
>tupleSpread : Symbol(tupleSpread, Decl(genericRecursiveFunctionReturn.ts, 23, 22))

function nested<T>(value: T) {
>nested : Symbol(nested, Decl(genericRecursiveFunctionReturn.ts, 31, 32))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 33, 16))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 33, 19))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 33, 16))

if (condition) return value;
>condition : Symbol(condition, Decl(genericRecursiveFunctionReturn.ts, 9, 13))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 33, 19))

// this recursive call can't be ignored as that would lead to inferring T when the T[] return type is possible here
return nested([value]);
>nested : Symbol(nested, Decl(genericRecursiveFunctionReturn.ts, 31, 32))
>value : Symbol(value, Decl(genericRecursiveFunctionReturn.ts, 33, 19))
}

function swap<T, U>(left: T, right: U, shouldSwap: boolean) {
>swap : Symbol(swap, Decl(genericRecursiveFunctionReturn.ts, 37, 1))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 39, 14))
>U : Symbol(U, Decl(genericRecursiveFunctionReturn.ts, 39, 16))
>left : Symbol(left, Decl(genericRecursiveFunctionReturn.ts, 39, 20))
>T : Symbol(T, Decl(genericRecursiveFunctionReturn.ts, 39, 14))
>right : Symbol(right, Decl(genericRecursiveFunctionReturn.ts, 39, 28))
>U : Symbol(U, Decl(genericRecursiveFunctionReturn.ts, 39, 16))
>shouldSwap : Symbol(shouldSwap, Decl(genericRecursiveFunctionReturn.ts, 39, 38))

if (!shouldSwap) return [left, right] as const;
>shouldSwap : Symbol(shouldSwap, Decl(genericRecursiveFunctionReturn.ts, 39, 38))
>left : Symbol(left, Decl(genericRecursiveFunctionReturn.ts, 39, 20))
>right : Symbol(right, Decl(genericRecursiveFunctionReturn.ts, 39, 28))
>const : Symbol(const)

// this recursive call can't be ignored as that would lead to inferring readonly [T, U] when the readonly [U, T] return type is possible here
return swap(right, left, false);
>swap : Symbol(swap, Decl(genericRecursiveFunctionReturn.ts, 37, 1))
>right : Symbol(right, Decl(genericRecursiveFunctionReturn.ts, 39, 28))
>left : Symbol(left, Decl(genericRecursiveFunctionReturn.ts, 39, 20))
}

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