TypeScript 2.5 "unused type parameter" breaks existing code

[benny-medflyt]

TypeScript Version: 2.5.2

Code

type Foo<s> = object;

This worked fine in TypeScript 2.4, but causes the following error in 2.5.2:

error TS6133: 's' is declared but never used.

This change in TypeScript is breaking massive amounts of code that use phantom type parameters. And there doesn't seem to be any workaround.

[RyanCavanaugh]

You can turn off no unused variables, or name them with a leading underscore (Foo<_s> will not cause an error).

[benny-medflyt]

I tried adding a leading underscore:

type Foo<_s> = object;

Still causes the same error ('s' is declared but never used.)

Switching off noUnusedLocals does work, but at the great expense of loosing that feature

[gcnew]

I'm interested in what you use phantom type parameters for, as they are not enforced by the compiler. If a type parameter is not referenced in the type's body, it's not considered during checking.

[benny-medflyt]

I'm interested in what you use phantom type parameters for, as they are not enforced by the compiler. If a type parameter is not referenced in the type's body, it's not considered during checking.

You are right, it turns out that the code in the style above was actually from some older experiments with phantom types, that we don't actually use.

The way we actually use phantom types with correct type checking is by adding a dummy field:

export class Q<s> {
    protected dummy: [Q<s>, s];
}

Relevant: #202 #5855

[RyanCavanaugh]

Logged #18420