[SR-9595] Unexpected error when type checking constrained associated types

[rxwei]

Previous ID	SR-9595
Radar	rdar://problem/75329340
Original Reporter	@rxwei
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	@slavapestov
Priority	Medium

md5: 566220f9d77423bbecc919b4912e7ab1

relates to:

• TF-213 [AD] Fix Differentiable protocol generic signature bug

Issue Description:

I expect the following to compile:

protocol Q {
  associatedtype Assoc
}

protocol P {
  associatedtype T : P, Q where T.Assoc : FloatingPoint, T == U.U
  associatedtype U : P, Q where U.Assoc : FloatingPoint, U == T.U
}

... but the compiler produces a ton of warnings that don't really make sense.

sr9595.swift:6:22: warning: redundant conformance constraint 'Self.T': 'P'
  associatedtype T : P, Q where T.Assoc : FloatingPoint, T == U.U
                     ^
sr9595.swift:7:22: note: conformance constraint 'Self.T.T': 'P' implied here
  associatedtype U : P, Q where U.Assoc : FloatingPoint, U == T.U
                     ^
sr9595.swift:6:25: warning: redundant conformance constraint 'Self.T': 'Q'
  associatedtype T : P, Q where T.Assoc : FloatingPoint, T == U.U
                        ^
sr9595.swift:7:22: note: conformance constraint 'Self.T.T': 'Q' implied here
  associatedtype U : P, Q where U.Assoc : FloatingPoint, U == T.U
                     ^
sr9595.swift:6:41: warning: redundant conformance constraint 'Self.T.Assoc': 'FloatingPoint'
  associatedtype T : P, Q where T.Assoc : FloatingPoint, T == U.U
                                        ^
sr9595.swift:7:22: note: conformance constraint 'Self.T.T.Assoc': 'FloatingPoint' implied here
  associatedtype U : P, Q where U.Assoc : FloatingPoint, U == T.U
                     ^
sr9595.swift:7:25: warning: redundant conformance constraint 'Self.U': 'Q'
  associatedtype U : P, Q where U.Assoc : FloatingPoint, U == T.U
                        ^
sr9595.swift:7:22: note: conformance constraint 'Self.U': 'Q' implied here
  associatedtype U : P, Q where U.Assoc : FloatingPoint, U == T.U
                     ^
sr9595.swift:7:41: warning: redundant conformance constraint 'Self.U.Assoc': 'FloatingPoint'
  associatedtype U : P, Q where U.Assoc : FloatingPoint, U == T.U
                                        ^
sr9595.swift:7:22: note: conformance constraint 'Self.U.Assoc': 'FloatingPoint' implied here
  associatedtype U : P, Q where U.Assoc : FloatingPoint, U == T.U
                     ^

[belkadan]

cc @DougGregor, @slavapestov

[dan-zheng]

@DougGregor: you said that this is a generic signature minimization bug.
Could you please give some pointers towards debugging/fixing this issue in lib/AST/GenericSignatureBuilder.cpp? That would be much appreciated.

[slavapestov]

@swift-ci create

[slavapestov]

Nice test case!

[slavapestov]

#42113