Mapped properties in interfaces have undefined behavior #18299

TypeScript Version: Playground version

Code

Linked playground

type Foo = {
    [P in 'a' | 'b']: any;
}

interface Bar {
    [P in 'a' | 'b']: any;
}

Expected behavior:

One of these two options:

1. No errors at all, and roughly equivalent semantics.
2. A parse error, in case this should specifically be disallowed (to align with classes, since [foo in bar] is required to be a valid class property/method name).

Actual behavior:

A mess of type errors because [P in T]: any is parsed in interfaces as a computed symbol property with key P in T (i.e. a boolean). In kind, I get three sets of type errors:

1. For P:

   • A computed property name in an interface must directly refer to a built-in symbol.
   • Cannot find name 'P'.
   • The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.

2. For 'a' | 'b':

   • A computed property name in an interface must directly refer to a built-in symbol.
   • The right-hand side of an 'in' expression must be of type 'any', an object type or a type parameter.
   • The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.