TypeScript: infer extends within union fails to compile if it's at the top level

[satohshi]

Issue

The following code fails to compile with error Expected "?" but found end of file:

type InferUnion<T> = T extends { a: infer U extends number } | infer U extends number
    ? U
    : never

The issue seems to be with infer U extends at the top level of a union, as the following compiles fine:

type InferUnion<T> = T extends { a: infer U extends number } | { b: infer U extends number }
    ? U
    : never

Reproduction

esbuild playground