Open Radar

 
Summary:
This one is just… well, you’ll see.


Steps to Reproduce:
1. In two different files, write this code:

File one:

protocol P { typealias R }
struct S<U : P> {
	let t: U.R
}
func f<T>(t: T) -> S<Q<T>>? {
	return nil
}


File two:

struct Q<T> : P { typealias R = T; let t: T }


Expected Results:
Not a crash.


Actual Results:
A crash.


Regression:
Lots of cases where this doesn’t repro:
- change S to a class
- put f and Q in the same file
- remove t from S (i.e. don’t have a constant/variable of the associated type)
- replace t with a func returning e.g. U.R?
- remove f
- change t’s type to U instead of U.R

On the other hand it doesn’t matter if t’s type is U.R, () -> U.R, U.R -> (), or optional variants of any of those. 


Notes:
This is significantly reduced from how I encountered it, which is in the Swift version of my testing framework Lagrangian.

Project to follow.