[SR-1876] Overloaded generic methods missing default parameters don't select most specific function

[swift-ci]

Previous ID	SR-1876
Radar	rdar://problem/27033947
Original Reporter	sswezeystk (JIRA User)
Type	Bug

Environment

Xcode 8 Beta 1

Additional Detail from JIRA

Votes	2
Component/s	Compiler
Labels	Bug, 3.0Regression, TypeChecker
Assignee	@slavapestov
Priority	Medium

md5: 36965762b7cc9100f831c2eb26c79e70

Issue Description:

When there are several functions with similar signatures, but differing generic constraints, omitting default parameters in the function call results in the wrong function being called. This works correctly with Swift 2.2 in Xcode 7.3

import Foundation

protocol A {
    init(int: Int)
}

extension Int: A {
    init(int: Int) {
        self.init(int)
    }
}
extension String: A {
    init(int: Int) {
        self.init(int.description)
    }
}

protocol B {
    init(string: String)
}

extension String: B {
    init(string: String) {
        self.init(string)
    }
}

func d<T where T: A, T: B>(key key: String, new: Bool = true) -> Array<T> {
    return [T(int:1), T(string:"b")]
}

func d<T where T: A>(key key: String, new: Bool = true) -> Array<T> {
    return [T(int: 3), T(int: 4)]
}

func d<T>(key key: String) -> Array<T> {
    return Array<T>()
}

let a: Array<Int> = d(key: "adf")
let aa: Array<Int> = d(key: "adf", new: false)
let b: Array<String> = d(key: "adfa")
let bb: Array<String> = d(key: "adfa", new: false)
let c: Array<Double> = d(key: "dfa")

print(a, aa, a == aa) // should be true
print(b, bb, b == bb) // should be true
print(c)

[swift-ci]

Comment by Andrii Chernenko (JIRA)

Somewhat simpler example that reproduces the same issue in Xcode 8.1:

protocol P1 {}
protocol P2 {}
class A: P1, P2 {}

func foo<T>(_: T.Type) where T: P1 {
  print("overload for P1")
}

func foo<T>(_: T.Type, a: Optional<Any> = nil) where T: P1 & P2 {
  print("overload for P1 and P2")
}

foo(A.self)

this prints

overload for P1 

instead of the expected

overload for P1 and P2

[swift-ci]

Comment by Chibundu Anwuna (JIRA)

@slavapestov what's the status of this bug as problem still exists in Xcode 11 with Swift 5.1.3 and Xcode 12 with Swift 5.3. However I tested it with Swift 4.2 and it works.