Cannot map over intersection of string with string literal (e.g., "a" & string) #23651

TypeScript Version: 2.9.0-dev.20180424 but reproducible as far back as I tested (2.7)

Search Terms: intersection, mapped, empty, string, literal

Code

type X = { [K in "a" & string]: 1 };

Expected behavior:
X should be equivalent to { a: 1 }.

Actual behavior:
X is {}.

Playground Link: link

Related Issues:
maybe #12114 for mapped types in general
maybe #23592 for making me notice this
maybe #16386 because I always think everything would be fixed by absorption
maybe #9410 although back then these things were treated as never everywhere

The recent #23592 change allowing mapping over number and symbol keys caused some of my existing code to break, and the obvious fix for some of them was to restrict those instances where I really expected strings from keyof T to (keyof T) & string. But that doesn't work. It looks like "a" & string gets treated like never inside of a mapped type key set, but like "a" everywhere else. Absorption would fix this, but barring that, maybe some less aggressive simplification inside mapped types?

Workaround: Since TS2.8 it is possible to use Extract<T, U> instead of T & U in some instances. In this case it can reduce the intersection before the mapping: type X = { [K in Extract<'a',string>]: 1 } is equivalent to { a: 1 } as expected.