Issue Description:

Here's the test case:

import SwiftUI
func f<Doodad>(_ type: Doodad.Type) {
    let kp = \EnvironmentValues.[doodadFor: type]
} 

When I compile this on Catalina beta 3 (10.15 Beta (19A501i)) with the swiftc from Xcode 11 beta 3 (Version 11.0 beta 3 (11M362v)), the compiler hangs:

:; time xcrun swiftc -c hang.swift
^C

real    0m15.842s
user    0m15.542s
sys 0m0.270s 

I tried to simplify the test case by removing the use of SwiftUI as follows:

extension Int {
    subscript<Doodad>(doodadFor type: Doodad.Type) -> Void { () }
}
func f<Doodad>(_ type: Doodad.Type) {
    let kp = \Int.[doodadFor: type]
} 

But this doesn't make swiftc hang. Instead it emits an error and exits:

:; time swiftc -c Doodad.swift 
Doodad.swift:8:19: error: subscript index of type 'Doodad.Type' in a key path must be Hashable
    let kp = \Int.[doodadFor: type]
                  ^

real    0m0.219s
user    0m0.065s
sys 0m0.145s 

This leads me to believe that the hang is triggered in some way by Doodad.Type's lack of conformance to Hashable, but I don't know what's special about EnvironmentValues.

Note also that in the hanging case, I don't actually have to define subscript(doodadFor🙂 to make swiftc hang. Defining it doesn't lead swiftc to terminate.