[SR-5931] incorrect error message "'UnsafePointer<UInt8>' is not convertible to 'UnsafePointer<_>'"

[swift-ci]

Previous ID	SR-5931
Radar	None
Original Reporter	comex (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug, DiagnosticsQoI, TypeChecker
Assignee	None
Priority	Medium

md5: 5879afa706162516f67d24b564cacb9a

Issue Description:

The following code:

import Foundation
func appendFirstFiveBytes(of source: Data, to target: inout Data) {
    source.withUnsafeBytes { (ptr: UnsafePointer<UInt8>) in
        target.append(ptr, 5)
    }
}

…is erroneous only because the call to `target.append` is missing a `count:` label. But instead of complaining about that, swiftc produces two useless and misleading errors:

a.swift:3:30: error: 'UnsafePointer<UInt8>' is not convertible to 'UnsafePointer<_>'
    source.withUnsafeBytes { (ptr: UnsafePointer<UInt8>) in
                             ^
a.swift:4:9: error: escaping closures can only capture inout parameters explicitly by value
        target.append(ptr, 5)
        ^

Similarly, if `count:` is added but `inout` is removed:

import Foundation
func appendFirstFiveBytes(of source: Data, to target: Data) {
    source.withUnsafeBytes { (ptr: UnsafePointer<UInt8>) in
        target.append(ptr, count: 5)
    }
}

swiftc produces the "not convertible" error instead of complaining that `target` is immutable:

a.swift:3:30: error: 'UnsafePointer<UInt8>' is not convertible to 'UnsafePointer<_>'
    source.withUnsafeBytes { (ptr: UnsafePointer<UInt8>) in                   

In both cases, removing the type annotation causes the correct error to be displayed, but in the first case the meaningless "escaping closures" error is still present:

a.swift:4:22: error: missing argument label 'count:' in call
        target.append(ptr, 5)
                     ^
                           count: 
a.swift:4:9: error: escaping closures can only capture inout parameters explicitly by value
        target.append(ptr, 5)

a.swift:4:9: error: cannot use mutating member on immutable value: 'target' is a 'let' constant
        target.append(ptr, count: 5)
        ^~~~~~

Tested on latest nightly.

(Probably the same as SR-2297, which I just closed because the original test case works correctly now.)