Type any overriding all other type definitions in a union type

[bradzacher]

TypeScript Version: 2.5.2

Code

function testOne(arg : string | number | { foo: string } | any) { }
function testTwo(arg : 'foo' | 'bar' | string) { }

Expected behavior:
testOne will report that it allows types string | number | { foo: string } | any.
testTwo will report that it allows types 'foo' | 'bar' | string

Actual behavior:
testOne reports that it allows type any.
testTwo reports that it allows type string.

---

Sometimes you want to allow a variable to be a given type, a given structure, whilst also providing a general fallback any typing.
The problem is that typescript seems to collapse the types together into the most generic types possible.

Because of how typescript reports the variable types, editors like VSCode report the function arguments as any and string respectively, thus preventing any granular definitions/autocompletes.

I believe this could be resolved by something like an exclusive or operator (such as #14094), as long as the logic is coded such that any is always the last type to be checked.

[mhegazy]

a union with any will always be any. we relay on this in the implementation to optimize union type comparisons.
for cases like this we recommend you use overloads.

function testTwo(arg : 'foo' | 'bar');
function testTwo(string);

[bradzacher]

That would work for functions, but what about for things like object properties?

The reason thar I've run into this is because of the way react types its styles attribute. There are a lot of scenarios where the CSS spec allows exact keywords, as well as any value. Because typescript collapses the type to any, you lose autocomplete on the exact string values.

[mhegazy]

The reason thar I've run into this is because of the way react types its styles attribute. There are a lot of scenarios where the CSS spec allows exact keywords, as well as any value. Because typescript collapses the type to any, you lose autocomplete on the exact string values.

I understand. but as i said we rely on some of these optimizations to be able to do comparisons between union types in a performant fashion.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.