Hang interacting with which-func mode

[dabrahams]

Open this file, turn on which-func mode, C-n to the last line, and hit return. It will hang

protocol P { func f() }
extension P { func g() { f() } }
struct X<T> : P {
    func f() { print("unconstrained") }
}

struct X<T> : where T : BinaryInteger {
    func f() { print("constrained") }
}

The backtrace I get is:

Backtrace.txt

[taku0]

Sorry, I missed this issue for long. Fixed by 04c8a621e9844ef3f7d53c41b046f71aadb1a9d2. Thank you for reporting the issue.