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To me it looks like tsc is able to infer the correct type in the first case as it tries hard to find a type that matches both possible return values. In the other two cases which return only one string it doesn't search and assumes the return value is a plain string, even though the intended type of the return value should indicate it is not.
Search Terms: Type Promise is not assignable to type promise inconsistent
Code
type DooDad = 'SOMETHING' | 'ELSE' ;
class Interesting {
public compiles = () : Promise<DooDad> => {
return Promise.resolve().then(() => {
if (1 < 2) {
return 'SOMETHING';
}
return 'ELSE';
});
};
public doesnt = () : Promise<DooDad> => {
// TS2322: Type Promise<string> is not assignable to type Promise<DooDad>
// Type 'string' is not assignable to type 'DooDad'
return Promise.resolve().then(() => {
//return Promise.resolve().then(() : DooDad => { // specifying return type here fixes the issue
return 'ELSE';
});
};
public slightlyDifferentErrorMessage = () : Promise<DooDad> => {
// TS2322: Type Promise<string> is not assignable to type Promise<DooDad>
return Promise.resolve().then(() => {
//return Promise.resolve().then(() : DooDad => { // specifying return type here fixes the issue
if (1 < 2) {
return 'SOMETHING';
}
return 'SOMETHING';
});
};
}
Expected behavior: All three functions compile and DooDad is inferred in all three cases ( alternatively, it is inferred in no case; this inconsistency is the worst case scenario IMO )
Actual behavior: First function compiles, the other two fail with TS2322
Playground Link: here
liamor and fatcerberus
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BugA bug in TypeScriptA bug in TypeScriptFixedA PR has been merged for this issueA PR has been merged for this issue