Enable single return statement type inference in slow types

[lucacasonato]

Isolated declarations now allows this: https://www.typescriptlang.org/play/?isolatedDeclarations=true&ts=5.5.0-beta#code/KYDwDg9gTgLgBAMwK4DsDGMCWEVwIYDOAJgBQCUcA3gFBx1xTAxJS4CMA3NQL5A

[dsherret]

Don't we already do this?

[lucacasonato]

No, only for arrow functions that have an expression rather than a block as a body