[SR-7250] Cannot conform to protocol in extension constrained to that protocol

[hamishknight]

Previous ID	SR-7250
Radar	None
Original Reporter	@hamishknight
Type	Bug
Status	Closed
Resolution	Done

Environment

Swift version 4.2-dev (LLVM cef3fe6d75, Clang bdc970d89c, Swift 5b60ace)
Target: x86_64-apple-darwin17.4.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	@hamishknight
Priority	Medium

md5: f1c1f35674f7cf782c53c23ed4e7f453

Issue Description:

The following doesn't compile:

protocol P {
  var i: Int { get }
}

extension P {
  var i: Int { return 0 }
}

struct S<T> : P {} // error: Type 'S<T>' does not conform to protocol 'P'

extension S where T : P {
  var i: Int { return 0 }
}

However it only doesn't compile while the last extension is constrained to T : P, if we constrain it to some other protocol (or not at all), the code compiles fine:

// This is fine.
protocol P1 {}
extension S where T : P1 {
  var i: Int { return 0 }
}

[hamishknight]

#15412