[SR-13018] Type constraints are not honored when invoking method

[mbrandonw]

Previous ID	SR-13018
Radar	rdar://problem/64953119
Original Reporter	@mbrandonw
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

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

md5: 2c03afa2034e8d179cd73102ecf6a551

Issue Description:

Consider the following method:

struct T<A> {
  func run<C>(_ c: C) where C: Collection, C.SubSequence == A {
    print(c[...] as! A)
  }
}

This should restrict the collection passed into `run` to be such that its subsequence matches the generic A.

For example, these following compile just fine:

// These should compile ✅
T<Substring>().run("")
T<Data>().run(Data())

However, there seem to be some situations where we can pass in a collection that does not match this constraint and yet it still compiles, and hence crashes at runtime:

// These should not but do⁉️
T<Substring>().run(Data())
T<Data>().run("")

[CodaFi]

Ah, this is an impressive soundness hole. We're straight-up forming a generic signature with the wrong constraints.

Generic signature: <A, C where A == A.SubSequence, C : Collection>

@swift-ci create

[slavapestov]

Here is a reduction:

protocol P {
  associatedtype A : P where A.A == A
}

func foo<X, Y>(_: X, _: Y) where X : P, Y : P, Y.A == X {}

[slavapestov]

#42113