[SR-15742] Unreachable "these types are always canonical" executed

[ahoppen]

Previous ID	SR-15742
Radar	rdar://87684281
Original Reporter	@ahoppen
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, CompilerCrash, FoundByStressTester
Assignee	None
Priority	Medium

md5: ddd1211125b9d74d76168f30e67db9b1

Issue Description:

The following test case fails with an llvm_unreachable these types are always canonical being executed.

// RUN: %sourcekitd-test -req=cursor -pos=5:36 %s -- %s

func fetch() {
  sryMap{ return try processResponse() }
  .napError{ parseError(reason: $0.abc()) }
}

func sryMap<String>(_ transform: () throws -> String) -> SryMap<String> {
  fatalError()
}

func processResponse() throws -> String { return "" }

func parseError(reason: String) {}

protocol MyError {}
extension MyError {
  func abc() -> String { return "" }
}

protocol MyProto {
  associatedtype Failure
}
extension MyProto {
  func napError(_ transform: (Self.Failure) -> Void) {}
}

struct SryMap<Output> : MyProto {
    typealias Failure = MyError
}

Bisecting determined that this was caused by #40666 (specifically commit ee331a8).

[hborla]

Here's the fix: #40925