Open Radar

 
Summary:
When an associated type is constrained to a protocol and the type parameters of a method or constructor attempt to constrain the associated type to be equal to a type parameter constraining to the same protocol, the compiler gives spurious errors.


Steps to Reproduce:
1. Compile this:
protocol P {
	typealias T: P
}

struct S<A: P> {
	init<Q: P where Q.T == A>(_ q: Q) {}
}



Expected Results:
I expected it to compile. Q.T is constrained to conform to P by the protocol’s definition; A is constrained to conform to P by the struct’s definition; what’s the problem?


Actual Results:
Spurious errors:
./same-type.swift:9:22: error: same-type constraint type 'A' does not conform to required protocol 'P'


Regression:
These errors do not occur if P.T does not itself have any constraints.
You can work around this by constructing S with a free function, which is unidiomatic, but actually compiles.
A static method on S works in this case, but not in others, e.g. a CollectionOf type. (Still investigating that.)


Notes:
This bites me surprisingly frequently—any time I’m trying to do something interesting with CollectionType, for example.