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Promise of type { } ignored when nested #13008
Description
TypeScript Version: 2.1.4
Code
function getDate(): Promise<Date> {
return Promise.resolve(new Date());
}
function getFoo(): Promise<{}> {
return Promise.resolve({});
}
function doFoo() : Promise<number> {
// Error: Type 'Promise<Date>' is not assignable to type 'Promise<number>'.
return (
getDate()
.then(() => {
return (
getFoo()
.then(() => {
return 10;
})
);
})
);
}Expected behavior:
No compilation error
Actual behavior:
Compiler error: Type 'Promise<Date>' is not assignable to type 'Promise<number>'.
Futher Notes
Changing getFoo to return any other value, e.g. function getFoo(): Promise<void> fixes the compilation error.
Activity
aluanhaddad commentedon Dec 18, 2016
It looks like the order of the overloads of
.theninlib.es2015.promise.d.tsare wrong.I could be wrong but my understanding is that they need to be declared in decreasing order of specificity. When I change this, it fixes the error.
Here is a more minimal repro (note that no error is reported but that the type of
resultis wrong).NoelAbrahams commentedon Dec 18, 2016
Aluan Haddad (@aluanhaddad), I might even suspect that the non-generic overload of
thenis redundant and should be removed.aluanhaddad commentedon Dec 18, 2016
Noel Abrahams (@NoelAbrahams)
Yes I think you are right. Regardless the overload order should be fixed because it is always going to pick the wrong one for cases where
Tis assignable toTResultas is the case whenTResultis{}for anyT.