[SR-15926] Generic type put into wrong position

[davezarzycki]

Previous ID	SR-15926
Radar	None
Original Reporter	@davezarzycki
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: e36e5f9935917b445f49367065ced788

Issue Description:

The bottom two lines should be essentially the same to Sema but aren't in practice. If you look closely at the diagnostics output and debug constraints of the former, then you'll see that "Float" is inferred into the wrong position, which seems very wrong. Specifically:

unresolved_specialize_expr type='MyType<Float, $T2>.Type' location=/tmp/g.swift:10:53 range=[/tmp/g.swift:10:38 - line:10:59]

If the test looks familiar it is because it is derived from the bottom of test/decl/typealias/protocol.swift:

struct MyType<TyA, TyB> {
  var a : TyA, b : TyB
}
class GenericClass<T> {
  typealias TA<U> = MyType<T, U>
}
let _: GenericClass<Int>.TA<Float> = GenericClass.TA<Float>(a: 1, b: 4.0)
let _: GenericClass<Int>.TA<Float> = GenericClass<_>.TA<Float>(a: 1, b: 4.0)