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Trust variance checking during identity checking #30903

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Trust variance checking during identity checking #30903

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61f0736
Optimize variance checking during identity checking
weswigham Apr 12, 2019
0923a7c
Accept good (!??!) baseline changes
weswigham Apr 12, 2019
637d0b6
Walk back initial change a bit to prevent baseline changes
weswigham Apr 12, 2019
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7 changes: 5 additions & 2 deletions src/compiler/checker.ts
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Expand Up @@ -10250,15 +10250,15 @@ namespace ts {

function getConditionalTypeWorker(root: ConditionalRoot, mapper: TypeMapper | undefined, checkType: Type, extendsType: Type, trueType: Type, falseType: Type) {
// Simplifications for types of the form `T extends U ? T : never` and `T extends U ? never : T`.
if (falseType.flags & TypeFlags.Never && getActualTypeVariable(trueType) === getActualTypeVariable(checkType)) {
if (falseType.flags & TypeFlags.Never && isTypeIdenticalTo(getActualTypeVariable(trueType), getActualTypeVariable(checkType))) {
if (checkType.flags & TypeFlags.Any || isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(extendsType))) { // Always true
return trueType;
}
else if (isIntersectionEmpty(checkType, extendsType)) { // Always false
return neverType;
}
}
else if (trueType.flags & TypeFlags.Never && getActualTypeVariable(falseType) === getActualTypeVariable(checkType)) {
else if (trueType.flags & TypeFlags.Never && isTypeIdenticalTo(getActualTypeVariable(falseType), getActualTypeVariable(checkType))) {
if (!(checkType.flags & TypeFlags.Any) && isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(extendsType))) { // Always true
return neverType;
}
Expand Down Expand Up @@ -13082,6 +13082,9 @@ namespace ts {
if (result = typeArgumentsRelatedTo(sourceTypeArguments, targetTypeArguments, variances, reportErrors)) {
return result;
}
if (relation === identityRelation) {
return result;
}
const allowStructuralFallback = (targetTypeArguments && hasCovariantVoidArgument(targetTypeArguments, variances)) || isNonGeneric(source) || isNonGeneric(target);
varianceCheckFailed = !allowStructuralFallback;
// The type arguments did not relate appropriately, but it may be because we have no variance
Expand Down