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Simplify relationship check for conditional type on target side #46429

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Oct 22, 2021
Merged

Simplify relationship check for conditional type on target side #46429

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9ea685b
Simplify relationship check for conditional type on target side
ahejlsberg Oct 19, 2021
75ab347
Accept new baselines
ahejlsberg Oct 19, 2021
3012d35
Better support for non-distribution-dependent types
ahejlsberg Oct 21, 2021
45a0113
Accept new API baselines
ahejlsberg Oct 21, 2021
dc9da80
Accept new baselines
ahejlsberg Oct 21, 2021
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41 changes: 7 additions & 34 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -15315,13 +15315,6 @@ namespace ts {
return type[cache] = type;
}

function isConditionalTypeAlwaysTrueDisregardingInferTypes(type: ConditionalType) {
const extendsInferParamMapper = type.root.inferTypeParameters && createTypeMapper(type.root.inferTypeParameters, map(type.root.inferTypeParameters, () => wildcardType));
const checkType = type.checkType;
const extendsType = type.extendsType;
return isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(instantiateType(extendsType, extendsInferParamMapper)));
}

function getSimplifiedConditionalType(type: ConditionalType, writing: boolean) {
const checkType = type.checkType;
const extendsType = type.extendsType;
Expand Down Expand Up @@ -19030,35 +19023,15 @@ namespace ts {
resetErrorInfo(saveErrorInfo);
return Ternary.Maybe;
}
const c = target as ConditionalType;
// Check if the conditional is always true or always false but still deferred for distribution purposes
const skipTrue = !isTypeAssignableTo(getPermissiveInstantiation(c.checkType), getPermissiveInstantiation(c.extendsType));
const skipFalse = !skipTrue && isConditionalTypeAlwaysTrueDisregardingInferTypes(c);

// Instantiate with a replacement mapper if the conditional is distributive, replacing the check type with a clone of itself,
// this way {x: string | number, y: string | number} -> (T extends T ? { x: T, y: T } : never) appropriately _fails_ when
// T = string | number (since that will end up distributing and producing `{x: string, y: string} | {x: number, y: number}`,
// to which `{x: string | number, y: string | number}` isn't assignable)
let distributionMapper: TypeMapper | undefined;
const checkVar = getActualTypeVariable(c.root.checkType);
if (c.root.isDistributive && checkVar.flags & TypeFlags.TypeParameter) {
const newParam = cloneTypeParameter(checkVar);
distributionMapper = prependTypeMapping(checkVar, newParam, c.mapper);
newParam.mapper = distributionMapper;
}

// TODO: Find a nice way to include potential conditional type breakdowns in error output, if they seem good (they usually don't)
const expanding = isDeeplyNestedType(target, targetStack, targetDepth);
let localResult: Ternary | undefined = expanding ? Ternary.Maybe : undefined;
if (skipTrue || expanding || (localResult = isRelatedTo(source, distributionMapper ? instantiateType(getTypeFromTypeNode(c.root.node.trueType), distributionMapper) : getTrueTypeFromConditionalType(c), RecursionFlags.Target, /*reportErrors*/ false))) {
if (!skipFalse && !expanding) {
localResult = (localResult || Ternary.Maybe) & isRelatedTo(source, distributionMapper ? instantiateType(getTypeFromTypeNode(c.root.node.falseType), distributionMapper) : getFalseTypeFromConditionalType(c), RecursionFlags.Target, /*reportErrors*/ false);
if (!(target as ConditionalType).root.inferTypeParameters) {
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Something @weswigham mentioned in the design meeting is that this also has to gate out distributive conditionals.

Suggested change
if (!(target as ConditionalType).root.inferTypeParameters) {
if (!(target as ConditionalType).root.inferTypeParameters && !(target as ConditionalType).root.isDistributive) {

It would be good to have a test case here if nothing else fails.

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Specifically, this is because let's say you're relating {x: U, y: U} where U extends T to a T extends Whatever ? { x: T, y: T } : { x: unknown, y: unknown }, you have to return false because the conditional is going to distribute over T - so let's say T = string | number and U = "a" | 0, that's {x: "a" | 0, y: "a" | 0} related to {x: string, y: string} | {x: number, y: number}, which is clearly false.

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(And this kind of relation is precisely what happens during variance analysis of something like T extends Whatever ? { x: T, y: T } : { x: unknown, y: unknown }, so we can't afford to get it wrong, otherwise we'll taint the variance-based non-generic results with incorrect results)

if (result = isRelatedTo(source, getTrueTypeFromConditionalType(target as ConditionalType), RecursionFlags.Target, /*reportErrors*/ false)) {
result &= isRelatedTo(source, getFalseTypeFromConditionalType(target as ConditionalType), RecursionFlags.Target, /*reportErrors*/ false);
if (result) {
resetErrorInfo(saveErrorInfo);
return result;
}
}
}
if (localResult) {
resetErrorInfo(saveErrorInfo);
return localResult;
}
}
else if (target.flags & TypeFlags.TemplateLiteral) {
if (source.flags & TypeFlags.TemplateLiteral) {
Expand Down
15 changes: 14 additions & 1 deletion tests/baselines/reference/conditionalTypeAssignabilityWhenDeferred.errors.txt
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Original file line number Diff line number Diff line change
@@ -1,5 +1,10 @@
tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(12,20): error TS2345: Argument of type '"value"' is not assignable to parameter of type 'XX extends XX ? "value" : never'.
tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(28,20): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(29,21): error TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.
tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(33,22): error TS2345: Argument of type 'T | null' is not assignable to parameter of type 'null extends T | null ? any : never'.
Type 'null' is not assignable to type 'null extends T | null ? any : never'.
tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(34,23): error TS2345: Argument of type 'T | null' is not assignable to parameter of type '[null] extends [T | null] ? any : never'.
Type 'null' is not assignable to type '[null] extends [T | null] ? any : never'.
tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(39,3): error TS2322: Type '{ x: T; y: T; }' is not assignable to type 'T extends T ? { x: T; y: T; } : never'.
tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(46,3): error TS2322: Type 'string' is not assignable to type 'Foo<T>'.
tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(63,9): error TS2322: Type '{ a: number; b: number; }' is not assignable to type '[T] extends [[infer U]] ? U : { b: number; }'.
Expand All @@ -10,7 +15,7 @@ tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(116,3): error T
Type '(arg: any) => any' is not assignable to type 'InferBecauseWhyNotDistributive<Q>'.


==== tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts (8 errors) ====
==== tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts (11 errors) ====
export type FilterPropsByType<T, TT> = {
[K in keyof T]: T[K] extends TT ? K : never
}[keyof T];
Expand All @@ -23,6 +28,8 @@ tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(116,3): error T

export function func<XX extends string>(x: XX, tipos: { value: XX }[]) {
select(x, tipos, "value");
~~~~~~~
!!! error TS2345: Argument of type '"value"' is not assignable to parameter of type 'XX extends XX ? "value" : never'.
}

declare function onlyNullablePlease<T extends null extends T ? any : never>(
Expand All @@ -48,7 +55,13 @@ tests/cases/compiler/conditionalTypeAssignabilityWhenDeferred.ts(116,3): error T
function f<T>(t: T) {
var x: T | null = Math.random() > 0.5 ? null : t;
onlyNullablePlease(x); // should work
~
!!! error TS2345: Argument of type 'T | null' is not assignable to parameter of type 'null extends T | null ? any : never'.
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This one is interesting - is this actually a distributive conditional type? Why is it deferred?

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We always defer if either of the check and extends types contains generics. This is somewhat of a blunt instrument, and we could possibly do better in cases like this.

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And by "contains generics" I mean we defer when isGenericType returns true for either of the check and extends types.

!!! error TS2345: Type 'null' is not assignable to type 'null extends T | null ? any : never'.
onlyNullablePlease2(x); // should work
~
!!! error TS2345: Argument of type 'T | null' is not assignable to parameter of type '[null] extends [T | null] ? any : never'.
!!! error TS2345: Type 'null' is not assignable to type '[null] extends [T | null] ? any : never'.
}

function f2<T>(t1: { x: T; y: T }, t2: T extends T ? { x: T; y: T } : never) {
Expand Down