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Normalize variadic tuples with reduced never element type to never #59866

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Normalize variadic tuples with reduced never element type to never #59866

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3 changes: 3 additions & 0 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -17455,6 +17455,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
// Spread variadic elements with tuple types into the resulting tuple.
forEach(elements, (t, n) => addElement(t, type.target.elementFlags[n], type.target.labeledElementDeclarations?.[n]));
}
else if (getReducedApparentType(type).flags & TypeFlags.Never) {
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Without this the code hits the last branch:

                else {
                    // Treat everything else as an array type and create a rest element.
                    addElement(isArrayLikeType(type) && getIndexTypeOfType(type, numberType) || errorType, ElementFlags.Rest, target.labeledElementDeclarations?.[i]);
                }

But a never type (and a reduced never type) don't have index types. So that getIndexTypeOfType returns undefined and the compiler ends up normalizing this to an anyType.

Note that getIndexTypeOfType leads to calling getReducedApparentType so this effectively just calls it a little bit more eagerly.

This also matches how getIndexedAccessTypeOrUndefined + getPropertyTypeForIndexType handle case like this:

type Test = ([] & [string])[number];
//   ^? type Test = never

Those functions also call getReducedApparentType and when the object type has TypeFlags.Never they return the object type itself. The small difference here is that I decided to return neverType. This seems like a better product of this normalization... but perhaps this should also return the type? The tests wouldn't mind this change (I checked).

An extra note: this could also be checked closer to the top of createNormalizedTupleType (where checkCrossProductUnion gets called) but it seemed me this extra branch here is easier to follow~.

return neverType;
}
else {
// Treat everything else as an array type and create a rest element.
addElement(isArrayLikeType(type) && getIndexTypeOfType(type, numberType) || errorType, ElementFlags.Rest, target.labeledElementDeclarations?.[i]);
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6 changes: 6 additions & 0 deletions tests/baselines/reference/variadicTuples1.errors.txt
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Expand Up @@ -542,4 +542,10 @@ variadicTuples1.ts(411,7): error TS2322: Type '[boolean, false]' is not assignab
type ToStringLength2<T extends any[]> = `${[...T]['length']}`;

type AnyArr = [...any];

type NeverSpread1 = [...never];
type IdentitySpread<T extends readonly unknown[]> = [...T];
type NeverSpread2 = IdentitySpread<never>;
type ReducedNeverSpread1 = IdentitySpread<[] & [string]>;
type ReducedNeverSpread2 = IdentitySpread<[string | boolean, boolean, ...number[]] & ['foo', string, 44]>;

11 changes: 11 additions & 0 deletions tests/baselines/reference/variadicTuples1.js
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Expand Up @@ -423,6 +423,12 @@ type ToStringLength1<T extends any[]> = `${T['length']}`;
type ToStringLength2<T extends any[]> = `${[...T]['length']}`;

type AnyArr = [...any];

type NeverSpread1 = [...never];
type IdentitySpread<T extends readonly unknown[]> = [...T];
type NeverSpread2 = IdentitySpread<never>;
type ReducedNeverSpread1 = IdentitySpread<[] & [string]>;
type ReducedNeverSpread2 = IdentitySpread<[string | boolean, boolean, ...number[]] & ['foo', string, 44]>;


//// [variadicTuples1.js]
Expand Down Expand Up @@ -833,6 +839,11 @@ type U3 = [...[string, number], boolean];
type ToStringLength1<T extends any[]> = `${T['length']}`;
type ToStringLength2<T extends any[]> = `${[...T]['length']}`;
type AnyArr = [...any];
type NeverSpread1 = [...never];
type IdentitySpread<T extends readonly unknown[]> = [...T];
type NeverSpread2 = IdentitySpread<never>;
type ReducedNeverSpread1 = IdentitySpread<[] & [string]>;
type ReducedNeverSpread2 = IdentitySpread<[string | boolean, boolean, ...number[]] & ['foo', string, 44]>;


!!!! File variadicTuples1.d.ts differs from original emit in noCheck emit
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20 changes: 20 additions & 0 deletions tests/baselines/reference/variadicTuples1.symbols
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Expand Up @@ -1419,3 +1419,23 @@ type ToStringLength2<T extends any[]> = `${[...T]['length']}`;
type AnyArr = [...any];
>AnyArr : Symbol(AnyArr, Decl(variadicTuples1.ts, 419, 62))

type NeverSpread1 = [...never];
>NeverSpread1 : Symbol(NeverSpread1, Decl(variadicTuples1.ts, 421, 23))

type IdentitySpread<T extends readonly unknown[]> = [...T];
>IdentitySpread : Symbol(IdentitySpread, Decl(variadicTuples1.ts, 423, 31))
>T : Symbol(T, Decl(variadicTuples1.ts, 424, 20))
>T : Symbol(T, Decl(variadicTuples1.ts, 424, 20))

type NeverSpread2 = IdentitySpread<never>;
>NeverSpread2 : Symbol(NeverSpread2, Decl(variadicTuples1.ts, 424, 59))
>IdentitySpread : Symbol(IdentitySpread, Decl(variadicTuples1.ts, 423, 31))

type ReducedNeverSpread1 = IdentitySpread<[] & [string]>;
>ReducedNeverSpread1 : Symbol(ReducedNeverSpread1, Decl(variadicTuples1.ts, 425, 42))
>IdentitySpread : Symbol(IdentitySpread, Decl(variadicTuples1.ts, 423, 31))

type ReducedNeverSpread2 = IdentitySpread<[string | boolean, boolean, ...number[]] & ['foo', string, 44]>;
>ReducedNeverSpread2 : Symbol(ReducedNeverSpread2, Decl(variadicTuples1.ts, 426, 57))
>IdentitySpread : Symbol(IdentitySpread, Decl(variadicTuples1.ts, 423, 31))

22 changes: 21 additions & 1 deletion tests/baselines/reference/variadicTuples1.types
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@@ -1,7 +1,7 @@
//// [tests/cases/conformance/types/tuple/variadicTuples1.ts] ////

=== Performance Stats ===
Type Count: 1,000
Type Count: 1,000 -> 2,500
Instantiation count: 2,500

=== variadicTuples1.ts ===
Expand Down Expand Up @@ -2304,3 +2304,23 @@ type AnyArr = [...any];
>AnyArr : any[]
> : ^^^^^

type NeverSpread1 = [...never];
>NeverSpread1 : never
> : ^^^^^

type IdentitySpread<T extends readonly unknown[]> = [...T];
>IdentitySpread : [...T]
> : ^^^^^^

type NeverSpread2 = IdentitySpread<never>;
>NeverSpread2 : never
> : ^^^^^

type ReducedNeverSpread1 = IdentitySpread<[] & [string]>;
>ReducedNeverSpread1 : never
> : ^^^^^

type ReducedNeverSpread2 = IdentitySpread<[string | boolean, boolean, ...number[]] & ['foo', string, 44]>;
>ReducedNeverSpread2 : never
> : ^^^^^

6 changes: 6 additions & 0 deletions tests/cases/conformance/types/tuple/variadicTuples1.ts
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Expand Up @@ -423,3 +423,9 @@ type ToStringLength1<T extends any[]> = `${T['length']}`;
type ToStringLength2<T extends any[]> = `${[...T]['length']}`;

type AnyArr = [...any];

type NeverSpread1 = [...never];
type IdentitySpread<T extends readonly unknown[]> = [...T];
type NeverSpread2 = IdentitySpread<never>;
type ReducedNeverSpread1 = IdentitySpread<[] & [string]>;
type ReducedNeverSpread2 = IdentitySpread<[string | boolean, boolean, ...number[]] & ['foo', string, 44]>;