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Fix spread operator failing to distribute over union when type is inlined
Fixes #62812 When a union/intersection type like `number | string` appears in the true branch of a conditional type that has the same type as its check type, the type was incorrectly being narrowed with a substitution constraint. This caused issues when the union type was used as a type argument to a generic type - different occurrences of `number | string` in the code were being treated as the same entity when they should be independent. The fix adds a check to skip this narrowing for structural types (unions/intersections) that don't contain type variables. Named types (interfaces, classes, etc.) still get narrowed since different references to the same named type refer to the same entity.
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‎src/compiler/checker.ts‎

Lines changed: 8 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -17200,8 +17200,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
1720017200
covariant = !covariant;
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}
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// Always substitute on type parameters, regardless of variance, since even
17203-
// in contravariant positions, they may rely on substituted constraints to be valid
17204-
if ((covariant || type.flags & TypeFlags.TypeVariable) && parent.kind === SyntaxKind.ConditionalType && node === (parent as ConditionalTypeNode).trueType) {
17203+
// in contravariant positions, they may rely on substituted constraints to be valid.
17204+
// For union/intersection types that don't contain type variables, don't apply narrowing
17205+
// based on structural equality with the check type - different occurrences of
17206+
// `number | string` in the code are independent even if they resolve to the same
17207+
// canonical type. Named types (interfaces, etc.) should still be narrowed since
17208+
// different references to the same named type refer to the same entity.
17209+
const isStructuralTypeWithoutTypeVariables = !!(type.flags & TypeFlags.UnionOrIntersection) && !couldContainTypeVariables(type);
17210+
if (!isStructuralTypeWithoutTypeVariables && (covariant || type.flags & TypeFlags.TypeVariable) && parent.kind === SyntaxKind.ConditionalType && node === (parent as ConditionalTypeNode).trueType) {
1720517211
const constraint = getImpliedConstraint(type, (parent as ConditionalTypeNode).checkType, (parent as ConditionalTypeNode).extendsType);
1720617212
if (constraint) {
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constraints = append(constraints, constraint);

‎tests/baselines/reference/spreadTupleUnionDistribution.js‎

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//// [tests/cases/conformance/types/spread/spreadTupleUnionDistribution.ts] ////
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//// [spreadTupleUnionDistribution.ts]
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// Repro from #62812
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// Spread operator fails to distribute over union when recursive type call is inlined instead of aliased
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type CrossProduct<Union, Counter extends unknown[]> =
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Counter extends [infer Zero, ...infer Rest]
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? (Union extends infer Member
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? [Member, ...CrossProduct<Union, Rest>]
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: never)
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: [];
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14+
// Basic test - this works
15+
let test1: CrossProduct<number | string, [undefined]>; // [string] | [number]
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type Depth1 = CrossProduct<number | string, [undefined]> // [string] | [number]
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18+
// With alias - this should work and give full cross product
19+
let test2: (number | string extends infer Union ? (Union extends unknown ? [Union, ...Depth1]: never) : never);
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// Expected: [string, string] | [number, number] | [string, number] | [number, string]
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// With inlined type - this should also work but currently doesn't distribute properly
23+
let test3: (number | string extends infer Union ? (Union extends unknown ? [Union, ...CrossProduct<number | string, [undefined]>]: never) : never);
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// Expected: [string, string] | [number, number] | [string, number] | [number, string]
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// Actual (bug): [string, string] | [number, number]
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// With literal union - this works
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let test4: (number | string extends infer Union ? (Union extends unknown ? [Union, ...([string] | [number])]: never) : never);
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// Expected: [string, string] | [number, number] | [string, number] | [number, string]
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// Test that the types are actually correct by checking assignability
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type Expected = [string, string] | [number, number] | [string, number] | [number, string];
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// These should all be true (no error)
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type Test1Check = Expected extends typeof test2 ? true : false;
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type Test2Check = typeof test2 extends Expected ? true : false;
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// If the bug is fixed, these will also be true (no error)
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type Test3Check = Expected extends typeof test3 ? true : false;
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type Test4Check = typeof test3 extends Expected ? true : false;
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type Test5Check = Expected extends typeof test4 ? true : false;
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type Test6Check = typeof test4 extends Expected ? true : false;
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// Force an error if checks fail
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const _check1: Test1Check = true;
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const _check2: Test2Check = true;
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const _check3: Test3Check = true; // This will error if bug exists
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const _check4: Test4Check = true; // This will error if bug exists
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const _check5: Test5Check = true;
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const _check6: Test6Check = true;
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//// [spreadTupleUnionDistribution.js]
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"use strict";
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// Repro from #62812
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// Spread operator fails to distribute over union when recursive type call is inlined instead of aliased
58+
// Basic test - this works
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var test1; // [string] | [number]
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// With alias - this should work and give full cross product
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var test2;
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// Expected: [string, string] | [number, number] | [string, number] | [number, string]
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// With inlined type - this should also work but currently doesn't distribute properly
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var test3;
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// Expected: [string, string] | [number, number] | [string, number] | [number, string]
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// Actual (bug): [string, string] | [number, number]
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// With literal union - this works
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var test4;
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// Force an error if checks fail
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var _check1 = true;
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var _check2 = true;
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var _check3 = true; // This will error if bug exists
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var _check4 = true; // This will error if bug exists
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var _check5 = true;
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var _check6 = true;
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//// [spreadTupleUnionDistribution.d.ts]
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type CrossProduct<Union, Counter extends unknown[]> = Counter extends [infer Zero, ...infer Rest] ? (Union extends infer Member ? [Member, ...CrossProduct<Union, Rest>] : never) : [];
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declare let test1: CrossProduct<number | string, [undefined]>;
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type Depth1 = CrossProduct<number | string, [undefined]>;
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declare let test2: (number | string extends infer Union ? (Union extends unknown ? [Union, ...Depth1] : never) : never);
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declare let test3: (number | string extends infer Union ? (Union extends unknown ? [Union, ...CrossProduct<number | string, [undefined]>] : never) : never);
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declare let test4: (number | string extends infer Union ? (Union extends unknown ? [Union, ...([string] | [number])] : never) : never);
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type Expected = [string, string] | [number, number] | [string, number] | [number, string];
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type Test1Check = Expected extends typeof test2 ? true : false;
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type Test2Check = typeof test2 extends Expected ? true : false;
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type Test3Check = Expected extends typeof test3 ? true : false;
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type Test4Check = typeof test3 extends Expected ? true : false;
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type Test5Check = Expected extends typeof test4 ? true : false;
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type Test6Check = typeof test4 extends Expected ? true : false;
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declare const _check1: Test1Check;
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declare const _check2: Test2Check;
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declare const _check3: Test3Check;
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declare const _check4: Test4Check;
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declare const _check5: Test5Check;
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declare const _check6: Test6Check;

‎tests/baselines/reference/spreadTupleUnionDistribution.symbols‎

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//// [tests/cases/conformance/types/spread/spreadTupleUnionDistribution.ts] ////
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=== spreadTupleUnionDistribution.ts ===
4+
// Repro from #62812
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// Spread operator fails to distribute over union when recursive type call is inlined instead of aliased
6+
7+
type CrossProduct<Union, Counter extends unknown[]> =
8+
>CrossProduct : Symbol(CrossProduct, Decl(spreadTupleUnionDistribution.ts, 0, 0))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 3, 18))
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>Counter : Symbol(Counter, Decl(spreadTupleUnionDistribution.ts, 3, 24))
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Counter extends [infer Zero, ...infer Rest]
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>Counter : Symbol(Counter, Decl(spreadTupleUnionDistribution.ts, 3, 24))
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>Zero : Symbol(Zero, Decl(spreadTupleUnionDistribution.ts, 4, 26))
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>Rest : Symbol(Rest, Decl(spreadTupleUnionDistribution.ts, 4, 41))
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? (Union extends infer Member
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 3, 18))
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>Member : Symbol(Member, Decl(spreadTupleUnionDistribution.ts, 5, 26))
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? [Member, ...CrossProduct<Union, Rest>]
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>Member : Symbol(Member, Decl(spreadTupleUnionDistribution.ts, 5, 26))
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>CrossProduct : Symbol(CrossProduct, Decl(spreadTupleUnionDistribution.ts, 0, 0))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 3, 18))
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>Rest : Symbol(Rest, Decl(spreadTupleUnionDistribution.ts, 4, 41))
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: never)
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: [];
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// Basic test - this works
31+
let test1: CrossProduct<number | string, [undefined]>; // [string] | [number]
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>test1 : Symbol(test1, Decl(spreadTupleUnionDistribution.ts, 11, 3))
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>CrossProduct : Symbol(CrossProduct, Decl(spreadTupleUnionDistribution.ts, 0, 0))
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type Depth1 = CrossProduct<number | string, [undefined]> // [string] | [number]
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>Depth1 : Symbol(Depth1, Decl(spreadTupleUnionDistribution.ts, 11, 54))
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>CrossProduct : Symbol(CrossProduct, Decl(spreadTupleUnionDistribution.ts, 0, 0))
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// With alias - this should work and give full cross product
40+
let test2: (number | string extends infer Union ? (Union extends unknown ? [Union, ...Depth1]: never) : never);
41+
>test2 : Symbol(test2, Decl(spreadTupleUnionDistribution.ts, 15, 3))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 15, 41))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 15, 41))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 15, 41))
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>Depth1 : Symbol(Depth1, Decl(spreadTupleUnionDistribution.ts, 11, 54))
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47+
// Expected: [string, string] | [number, number] | [string, number] | [number, string]
48+
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// With inlined type - this should also work but currently doesn't distribute properly
50+
let test3: (number | string extends infer Union ? (Union extends unknown ? [Union, ...CrossProduct<number | string, [undefined]>]: never) : never);
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>test3 : Symbol(test3, Decl(spreadTupleUnionDistribution.ts, 19, 3))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 19, 41))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 19, 41))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 19, 41))
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>CrossProduct : Symbol(CrossProduct, Decl(spreadTupleUnionDistribution.ts, 0, 0))
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57+
// Expected: [string, string] | [number, number] | [string, number] | [number, string]
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// Actual (bug): [string, string] | [number, number]
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60+
// With literal union - this works
61+
let test4: (number | string extends infer Union ? (Union extends unknown ? [Union, ...([string] | [number])]: never) : never);
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>test4 : Symbol(test4, Decl(spreadTupleUnionDistribution.ts, 24, 3))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 24, 41))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 24, 41))
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>Union : Symbol(Union, Decl(spreadTupleUnionDistribution.ts, 24, 41))
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// Expected: [string, string] | [number, number] | [string, number] | [number, string]
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// Test that the types are actually correct by checking assignability
70+
type Expected = [string, string] | [number, number] | [string, number] | [number, string];
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>Expected : Symbol(Expected, Decl(spreadTupleUnionDistribution.ts, 24, 126))
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// These should all be true (no error)
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type Test1Check = Expected extends typeof test2 ? true : false;
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>Test1Check : Symbol(Test1Check, Decl(spreadTupleUnionDistribution.ts, 28, 90))
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>Expected : Symbol(Expected, Decl(spreadTupleUnionDistribution.ts, 24, 126))
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>test2 : Symbol(test2, Decl(spreadTupleUnionDistribution.ts, 15, 3))
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type Test2Check = typeof test2 extends Expected ? true : false;
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>Test2Check : Symbol(Test2Check, Decl(spreadTupleUnionDistribution.ts, 31, 63))
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>test2 : Symbol(test2, Decl(spreadTupleUnionDistribution.ts, 15, 3))
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>Expected : Symbol(Expected, Decl(spreadTupleUnionDistribution.ts, 24, 126))
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// If the bug is fixed, these will also be true (no error)
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type Test3Check = Expected extends typeof test3 ? true : false;
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>Test3Check : Symbol(Test3Check, Decl(spreadTupleUnionDistribution.ts, 32, 63))
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>Expected : Symbol(Expected, Decl(spreadTupleUnionDistribution.ts, 24, 126))
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>test3 : Symbol(test3, Decl(spreadTupleUnionDistribution.ts, 19, 3))
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type Test4Check = typeof test3 extends Expected ? true : false;
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>Test4Check : Symbol(Test4Check, Decl(spreadTupleUnionDistribution.ts, 35, 63))
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>test3 : Symbol(test3, Decl(spreadTupleUnionDistribution.ts, 19, 3))
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>Expected : Symbol(Expected, Decl(spreadTupleUnionDistribution.ts, 24, 126))
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type Test5Check = Expected extends typeof test4 ? true : false;
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>Test5Check : Symbol(Test5Check, Decl(spreadTupleUnionDistribution.ts, 36, 63))
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>Expected : Symbol(Expected, Decl(spreadTupleUnionDistribution.ts, 24, 126))
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>test4 : Symbol(test4, Decl(spreadTupleUnionDistribution.ts, 24, 3))
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type Test6Check = typeof test4 extends Expected ? true : false;
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>Test6Check : Symbol(Test6Check, Decl(spreadTupleUnionDistribution.ts, 38, 63))
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>test4 : Symbol(test4, Decl(spreadTupleUnionDistribution.ts, 24, 3))
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>Expected : Symbol(Expected, Decl(spreadTupleUnionDistribution.ts, 24, 126))
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// Force an error if checks fail
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const _check1: Test1Check = true;
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>_check1 : Symbol(_check1, Decl(spreadTupleUnionDistribution.ts, 42, 5))
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>Test1Check : Symbol(Test1Check, Decl(spreadTupleUnionDistribution.ts, 28, 90))
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const _check2: Test2Check = true;
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>_check2 : Symbol(_check2, Decl(spreadTupleUnionDistribution.ts, 43, 5))
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>Test2Check : Symbol(Test2Check, Decl(spreadTupleUnionDistribution.ts, 31, 63))
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const _check3: Test3Check = true; // This will error if bug exists
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>_check3 : Symbol(_check3, Decl(spreadTupleUnionDistribution.ts, 44, 5))
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>Test3Check : Symbol(Test3Check, Decl(spreadTupleUnionDistribution.ts, 32, 63))
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const _check4: Test4Check = true; // This will error if bug exists
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>_check4 : Symbol(_check4, Decl(spreadTupleUnionDistribution.ts, 45, 5))
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>Test4Check : Symbol(Test4Check, Decl(spreadTupleUnionDistribution.ts, 35, 63))
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const _check5: Test5Check = true;
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>_check5 : Symbol(_check5, Decl(spreadTupleUnionDistribution.ts, 46, 5))
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>Test5Check : Symbol(Test5Check, Decl(spreadTupleUnionDistribution.ts, 36, 63))
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const _check6: Test6Check = true;
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>_check6 : Symbol(_check6, Decl(spreadTupleUnionDistribution.ts, 47, 5))
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>Test6Check : Symbol(Test6Check, Decl(spreadTupleUnionDistribution.ts, 38, 63))
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‎tests/baselines/reference/spreadTupleUnionDistribution.types‎

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//// [tests/cases/conformance/types/spread/spreadTupleUnionDistribution.ts] ////
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3+
=== spreadTupleUnionDistribution.ts ===
4+
// Repro from #62812
5+
// Spread operator fails to distribute over union when recursive type call is inlined instead of aliased
6+
7+
type CrossProduct<Union, Counter extends unknown[]> =
8+
>CrossProduct : CrossProduct<Union, Counter>
9+
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
10+
11+
Counter extends [infer Zero, ...infer Rest]
12+
? (Union extends infer Member
13+
? [Member, ...CrossProduct<Union, Rest>]
14+
: never)
15+
: [];
16+
17+
// Basic test - this works
18+
let test1: CrossProduct<number | string, [undefined]>; // [string] | [number]
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>test1 : [string] | [number]
20+
> : ^^^^^^^^^^^^^^^^^^^
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22+
type Depth1 = CrossProduct<number | string, [undefined]> // [string] | [number]
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>Depth1 : [string] | [number]
24+
> : ^^^^^^^^^^^^^^^^^^^
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26+
// With alias - this should work and give full cross product
27+
let test2: (number | string extends infer Union ? (Union extends unknown ? [Union, ...Depth1]: never) : never);
28+
>test2 : [string, string] | [string, number] | [number, string] | [number, number]
29+
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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31+
// Expected: [string, string] | [number, number] | [string, number] | [number, string]
32+
33+
// With inlined type - this should also work but currently doesn't distribute properly
34+
let test3: (number | string extends infer Union ? (Union extends unknown ? [Union, ...CrossProduct<number | string, [undefined]>]: never) : never);
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>test3 : [string, string] | [string, number] | [number, string] | [number, number]
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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// Expected: [string, string] | [number, number] | [string, number] | [number, string]
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// Actual (bug): [string, string] | [number, number]
40+
41+
// With literal union - this works
42+
let test4: (number | string extends infer Union ? (Union extends unknown ? [Union, ...([string] | [number])]: never) : never);
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>test4 : [string, string] | [string, number] | [number, string] | [number, number]
44+
> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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// Expected: [string, string] | [number, number] | [string, number] | [number, string]
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// Test that the types are actually correct by checking assignability
49+
type Expected = [string, string] | [number, number] | [string, number] | [number, string];
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>Expected : Expected
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> : ^^^^^^^^
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// These should all be true (no error)
54+
type Test1Check = Expected extends typeof test2 ? true : false;
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>Test1Check : true
56+
> : ^^^^
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>test2 : [string, string] | [string, number] | [number, string] | [number, number]
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>true : true
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> : ^^^^
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>false : false
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> : ^^^^^
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type Test2Check = typeof test2 extends Expected ? true : false;
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>Test2Check : true
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> : ^^^^
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>test2 : [string, string] | [string, number] | [number, string] | [number, number]
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>true : true
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> : ^^^^
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>false : false
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> : ^^^^^
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// If the bug is fixed, these will also be true (no error)
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type Test3Check = Expected extends typeof test3 ? true : false;
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>Test3Check : true
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> : ^^^^
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>test3 : [string, string] | [string, number] | [number, string] | [number, number]
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>true : true
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> : ^^^^
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>false : false
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> : ^^^^^
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type Test4Check = typeof test3 extends Expected ? true : false;
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>Test4Check : true
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> : ^^^^
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>test3 : [string, string] | [string, number] | [number, string] | [number, number]
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>true : true
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> : ^^^^
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>false : false
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> : ^^^^^
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type Test5Check = Expected extends typeof test4 ? true : false;
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>Test5Check : true
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> : ^^^^
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>test4 : [string, string] | [string, number] | [number, string] | [number, number]
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>true : true
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> : ^^^^
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>false : false
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> : ^^^^^
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type Test6Check = typeof test4 extends Expected ? true : false;
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>Test6Check : true
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> : ^^^^
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>test4 : [string, string] | [string, number] | [number, string] | [number, number]
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> : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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>true : true
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> : ^^^^
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>false : false
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> : ^^^^^
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// Force an error if checks fail
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const _check1: Test1Check = true;
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>_check1 : true
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> : ^^^^
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>true : true
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> : ^^^^
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const _check2: Test2Check = true;
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>_check2 : true
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> : ^^^^
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>true : true
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> : ^^^^
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const _check3: Test3Check = true; // This will error if bug exists
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>_check3 : true
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> : ^^^^
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>true : true
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> : ^^^^
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const _check4: Test4Check = true; // This will error if bug exists
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>_check4 : true
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> : ^^^^
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>true : true
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> : ^^^^
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const _check5: Test5Check = true;
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>_check5 : true
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> : ^^^^
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>true : true
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> : ^^^^
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const _check6: Test6Check = true;
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>_check6 : true
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> : ^^^^
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>true : true
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> : ^^^^
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