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Bug: Boolean type literals misbehaving #10432
Description
TypeScript Version: nightly (2.1.0-dev.20160818)
Code
I lifted this from Anders's final example in #9407 (comment).
type Result<T> = { success: true, value: T } | { success: false };
function foo(): Result<number> {
if (someTest()) {
return { success: true, value: 42 };
}
else {
return { success: false };
}
}
function someTest(): Boolean { return true; }Expected behavior:
Type-checks OK
Actual behavior:
Type error:
result.ts(5,16): error TS2322: Type '{ success: boolean; value: number; }' is not assignable to type 'Result<number>'.
Type '{ success: boolean; value: number; }' is not assignable to type '{ success: false; }'.
Types of property 'success' are incompatible.
Type 'boolean' is not assignable to type 'false'.
result.ts(8,16): error TS2322: Type '{ success: boolean; }' is not assignable to type 'Result<number>'.
Type '{ success: boolean; }' is not assignable to type '{ success: false; }'.
Types of property 'success' are incompatible.
Type 'boolean' is not assignable to type 'false'.
If I add manual type annotations (true as true, false as false) to the success assignments it works:
function foo(): Result<number> {
if (someTest()) {
return { success: true as true, value: 42 };
}
else {
return { success: false as false };
}
}The bug only affects Boolean literals. If I use enum, number, or string type literals it works - for example:
type Result<T> = { success: 1, value: T } | { success: 0 };
function foo(): Result<number> {
if (someTest()) {
return { success: 1, value: 42 };
}
else {
return { success: 0 };
}
}
Activity
bchenSyd commentedon Mar 10, 2017
not fully fixed....
RyanCavanaugh commentedon Mar 10, 2017
@bochen2014 I don't see how we could fix that case. In general we don't assume that functions won't modify a type within its own domain, and the
truevalue gets quickly widened tobooleanhere because it's not used in a position where it's known to not be mutated.bchenSyd commentedon Mar 11, 2017
hi Ryan, thanks for your reply.
Just so you know that I've walked around the issue by
which does the same as Object.assign