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non-null assertion on TypeParameter changes string | undefined to String #21360
Description
TypeScript Version: 2.7.0-dev.20180123
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Code
declare function foo(p: string): void;
function fn<T extends string | undefined, U extends string>(one: T,two: U) {
let three = Boolean() ? one : two;
foo(one!);
foo(two!);
foo(three!); // error on this line
}Expected behavior:
No type error.
Actual behavior:
Argument of type 'string | String' is not assignable to parameter of type 'string'.
Type 'String' is not assignable to type 'string'.
'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible.
This is caused by #20995. To be fair, without that change the above was not valid at all.
Playground Link:
Related Issues:
Maybe related to #21317
Activity
ahejlsberg commentedon Jan 24, 2018
Wesley Wigham (@weswigham)
weswigham commentedon Jan 24, 2018
getApparentTypeForLocation(which is how we enabled this, similarly to how we've made guards onthis.foo.barwork whenthis.foois an indexed access type) usesgetAparrentType. That replaces primitives with their corresponding interfaces; I don't think we (ever) actually want do do that here (it just didn't matter when it was only ever used for the left hand side of a property access), so it's probably appropriate to factor out the constraint-mapping behaviors and perform only that here.