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Check for definitely false condition first

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This commit is contained in:
Anders Hejlsberg
2018-01-26 13:03:17 -08:00
parent e4502121f0
commit fa6c4b79dd
1 changed files with 5 additions and 4 deletions
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9
src/compiler/checker.ts
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@@ -8104,6 +8104,11 @@ namespace ts {
function getConditionalType(checkType: Type, baseExtendsType: Type, baseTrueType: Type, baseFalseType: Type, inferTypeParameters: TypeParameter[], target: ConditionalType, mapper: TypeMapper, aliasSymbol?: Symbol, baseAliasTypeArguments?: Type[]): Type {
// Instantiate extends type without instantiating any 'infer T' type parameters
const extendsType = instantiateType(baseExtendsType, mapper);
// Return falseType for a definitely false extends check
if (!isTypeAssignableTo(instantiateType(checkType, anyMapper), instantiateType(extendsType, constraintMapper))) {
return instantiateType(baseFalseType, mapper);
}
// The check could be true for some instantiation
let combinedMapper: TypeMapper;
if (inferTypeParameters) {
const inferences = map(inferTypeParameters, createInferenceInfo);
@@ -8125,10 +8130,6 @@ namespace ts {
if (isTypeAssignableTo(checkType, inferredExtendsType)) {
return instantiateType(baseTrueType, combinedMapper || mapper);
}
// Return falseType for a definitely false extends check
if (!isTypeAssignableTo(instantiateType(checkType, anyMapper), instantiateType(extendsType, constraintMapper))) {
return instantiateType(baseFalseType, mapper);
}
// Return a deferred type for a check that is neither definitely true nor definitely false
const erasedCheckType = getActualTypeParameter(checkType);
const trueType = instantiateType(baseTrueType, mapper);