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Add secondary subtype check and tests

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This commit is contained in:
Dan Vanderkam
2024-02-21 16:16:15 -05:00
parent e2684f1289
commit 101df932d5
6 changed files with 223 additions and 4 deletions
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13
src/compiler/checker.ts
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@@ -37440,14 +37440,19 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
// Refining "x: boolean" to "x is true" or "x is false" isn't useful.
return;
}
const trueType = checkIfExpressionRefinesParameter(expr, param, initType);
const trueType = checkIfExpressionRefinesParameter(expr, param, initType, /*forceFullCheck*/ false);
if (trueType) {
return [i, trueType];
// A type predicate would be valid if the function were called with param of type initType.
// The predicate must also be valid for all subtypes of initType. In particular, it must be valid when called with param of type trueType.
const trueSubtype = checkIfExpressionRefinesParameter(expr, param, trueType, /*forceFullCheck*/ true);
if (trueSubtype) {
return [i, trueType];
}
}
});
}
function checkIfExpressionRefinesParameter(expr: Expression, param: ParameterDeclaration, initType: Type): Type | undefined {
function checkIfExpressionRefinesParameter(expr: Expression, param: ParameterDeclaration, initType: Type, forceFullCheck: boolean): Type | undefined {
const antecedent = (expr as Expression & { flowNode?: FlowNode; }).flowNode ?? { flags: FlowFlags.Start };
const trueCondition: FlowCondition = {
flags: FlowFlags.TrueCondition,
@@ -37456,7 +37461,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
};
const trueType = getFlowTypeOfReference(param.name, initType, initType, func, trueCondition);
if (trueType === initType) return undefined;
if (trueType === initType && !forceFullCheck) return undefined;
// "x is T" means that x is T if and only if it returns true. If it returns false then x is not T.
// However, TS may not be able to represent "not T", in which case we can be more lax.