[SR-4206] Override checking does not properly enforce requirements

[swift-ci]

Previous ID	SR-4206
Radar	rdar://problem/23626260
Original Reporter	Pr0Ger (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

Swift Ver. 3.0.2

Additional Detail from JIRA

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

md5: 09af3d026b3ca16963a2c72d8d00ef63

is duplicated by:

• SR-4986 Overriding of a method with a constrained generic placeholder is incorrectly permitted
• SR-7573 Overridden class function with generic constraint causes unsafe behavior
• SR-10076 [Swift 5] Generic parameter cannot get correct type with generic override constraint
• SR-10198 Cannot override more than one superclass declaration
• SR-10603 Overloading Overriden Method By Generic Constraint-ness Causes Compiler Error
• SR-11904 EXC_BAD_ACCESS when calling overridden generic function as parent's one.
• SR-11098 swiftc segfault when compiling in release mode

relates to:

• SR-10582 EXC_BAD_ACCESS with protocol
• SR-10603 Overloading Overriden Method By Generic Constraint-ness Causes Compiler Error

Issue Description:

protocol Parent {}

protocol A: Parent {}
protocol B: Parent {}

struct Foo: A {}

class ClassA {
    func foo<T: A>(param: T) {
        print("A")
    }
}

class ClassB: ClassA {
    override func foo<T: A>(param: T) {
        print("A")
    }

    override func foo<T: B>(param: T) {
        print("B")
    }

}

let foo = Foo()
let classB = ClassB()
classB.foo(param: foo)

Expected result: "A" will be printed
Actual result: "B" is printed

If we remove foo<T: A> overload compiler will complain argument type 'Foo' does not conform to expected type 'B'.

[belkadan]

This is "fixed" in Swift 3.1:

<stdin>:19:19: error: 'foo(param:)' has already been overridden
    override func foo<T: B>(param: T) {
                  ^
<stdin>:15:19: note: 'foo(param:)' previously overridden here
    override func foo<T: A>(param: T) {
                  ^

but if you take out the first override the compiler incorrectly accepts the code:

protocol Parent {}

protocol A: Parent {}
protocol B: Parent {}

class ClassA {
    func foo<T: A>(param: T) {
        print("A")
    }
}

class ClassB: ClassA {
    override func foo<T: B>(param: T) {
        print("B")
    }
}

[belkadan]

@swift-ci create

[slavapestov]

#24484