[SR-14484] odd "error: no exact matches in call to initializer"

[weissi]

Previous ID	SR-14484
Radar	rdar://problem/77794156
Original Reporter	@weissi
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

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

md5: 76471e1930f4bfd0901075d1dc28402d

Issue Description:

This Swift program compiles and works:

func truncate<Input: FixedWidthInteger & UnsignedInteger & BinaryInteger,
              Output: FixedWidthInteger & SignedInteger & BinaryInteger>(_ input: Input, outputType: Output.Type = Output.self) -> Output {
    precondition(Output.Magnitude.self == Input.self)
    return Output(truncatingIfNeeded: input)
}

print(truncate(UInt(4), outputType: Int.self))

but if I add where Output.Magnitude == Input to the type constraints, then the compilation fails with

test.swift:1:22: warning: redundant conformance constraint 'Input': 'FixedWidthInteger'
func truncate<Input: FixedWidthInteger & UnsignedInteger & BinaryInteger,
                     ^
test.swift:2:31: note: conformance constraint 'Input': 'FixedWidthInteger' implied here
                      Output: FixedWidthInteger & SignedInteger & BinaryInteger>(_ input: Input, outputType: Output.Type = Output.self) -> Output
                              ^
test.swift:1:42: warning: redundant conformance constraint 'Input': 'UnsignedInteger'
func truncate<Input: FixedWidthInteger & UnsignedInteger & BinaryInteger,
                                         ^
test.swift:2:31: note: conformance constraint 'Input': 'UnsignedInteger' implied here
                      Output: FixedWidthInteger & SignedInteger & BinaryInteger>(_ input: Input, outputType: Output.Type = Output.self) -> Output
                              ^
test.swift:4:12: error: no exact matches in call to initializer 
    return Output(truncatingIfNeeded: input)
           ^
Swift.BinaryInteger:6:5: note: candidate requires that 'Input' conform to 'BinaryInteger' (requirement specified as 'T' == 'BinaryInteger')
    init<T>(truncatingIfNeeded source: T) where T : BinaryInteger
    ^
Swift.FixedWidthInteger:6:23: note: candidate requires that 'Input' conform to 'BinaryInteger' (requirement specified as 'T' == 'BinaryInteger')
    @inlinable public init<T>(truncatingIfNeeded source: T) where T : BinaryInteger

which is odd because there are matches for that initialiser (see the runtime precondition check).

The failing program is

func truncate<Input: FixedWidthInteger & UnsignedInteger & BinaryInteger,
              Output: FixedWidthInteger & SignedInteger & BinaryInteger>(_ input: Input, outputType: Output.Type = Output.self) -> Output 
              where Output.Magnitude == Input {
    precondition(Output.Magnitude.self == Input.self)
    return Output(truncatingIfNeeded: input)
}

print(truncate(UInt(4), outputType: Int.self))

The above error is from 5.3 / 5.4, on main the compiler actually crashes: SR-14485

[weissi]

@swift-ci generate

[slavapestov]

@swift-ci create

[slavapestov]

#42113