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First approximation at better error reporting with literal types.

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Daniel Rosenwasser 2017-05-12 00:44:39 -07:00
parent a12ec1af43
commit ab8555f6fa
1 changed files with 29 additions and 0 deletions
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29
src/compiler/checker.ts
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@ -8564,6 +8564,35 @@ namespace ts {
}
function reportRelationError(message: DiagnosticMessage, source: Type, target: Type) {
// Much of the time, a user will write something simple like
//
// let x: boolean | number
// x = Math.random() < 0.5 ? "hello" : Symbol.iterator
//
// In such cases, we'd rather not give an error like
//
// Type '"hello" | symbol' is not assignable to type 'number | boolean'
//
// The fact that "hello" had a string literal type is, at best, confusing for newcomers,
// and noise for experienced users. Here we can do an extra check in the error case to
// see whether or not substituting in the base types could simplify what we display to the user.
replaceLiterals: {
if (isTypeOfKind(source, TypeFlags.Literal)) {
const baseSource = getBaseTypeOfLiteralType(source);
if (isTypeRelatedTo(baseSource, target, relation)) {
break replaceLiterals;
}
source = baseSource;
}
if (isTypeOfKind(target, TypeFlags.Literal)) {
const baseTarget = getBaseTypeOfLiteralType(target);
if (isTypeRelatedTo(source, baseTarget, relation)) {
break replaceLiterals;
}
target = baseTarget;
}
}
let sourceType = typeToString(source);
let targetType = typeToString(target);
if (sourceType === targetType) {