Consistent inferences when inferring to template literal type

[ahejlsberg]

This PR makes a minor adjustment to template literal inference to make results more consistent:

type Foo<T> = T extends `*${infer S}*` ? S : never;

type T1 = Foo<any>;      // never (previously string)
type T2 = Foo<string>;   // never (previously string)
type T3 = Foo<'abc'>;    // never
type T4 = Foo<'*abc*'>;  // 'abc'

Inference now consistently infers never for all placeholder type variables when the source string isn't a literal type that matches the pattern.

[DanielRosenwasser]

@typescript-bot pack this

[typescript-bot]

Heya @DanielRosenwasser, I've started to run the tarball bundle task on this PR at a3a2db2. You can monitor the build here.

[DanielRosenwasser]

Just checking, it doesn't matter whether the template has any literal content, right? For example, could I write the following and have it work (apart from any and never)?

type IsStringLiteral<T> = T extends `${infer U}` ? U extends never ? false : true : false;

[DanielRosenwasser]

I think the behavior here is surprising, but is probably desirable.