[SR-4246] Generic Parameter is Missing in Vague Error Message

[palimondo]

Previous ID	SR-4246
Radar	None
Original Reporter	@palimondo
Type	Bug

Attachment: Download

Environment

Apple Swift version 3.0.2 (swiftlang-800.0.63 clang-800.0.42.1)
Target: x86_64-apple-macosx10.9

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, DiagnosticsQoI, TypeChecker
Assignee	None
Priority	Medium

md5: 1bda1dd32a2bee653ddb7117169bec0e

Issue Description:

Following code:

public struct ComposedSequence<Element, State> {
    var _state: State
    var _next: (inout State) -> Element?
}

func composePrefix<BaseIterator : IteratorProtocol>(_ prefix: Int, _ iterator: BaseIterator) -> ComposedSequence<BaseIterator.Element, (count: Int, iterator: BaseIterator)> {

    func _prefix(state: inout (count: Int, iterator: BaseIterator)) -> BaseIterator.Element? {
        return nil
    }
    
    // The first member in tuple is deliberatly of wrong type (String instead of Int) to force error
    return ComposedSequence(_state: ("prefix", iterator), _next: _prefix)
}

produces following compiler error (scroll right for the interesting part):

GenericParameterMissingInVagueErrorMessage.swift:13:66: error: cannot convert value of type '(inout (count: Int, iterator: BaseIterator)) -> BaseIterator.Element?' to expected argument type '(inout (String, _)) -> _?'
        return ComposedSequence(_state: ("prefix", iterator), _next: _prefix)
                                                                     ^~~~~~~

Notice how the generic parameter BaseIterator is missing from the reported expected type:
(inout (String, _)) -> _?

That type should be (inout (String, BaseIterator)) -> BaseIterator.Element?

[belkadan]

I think this is deliberate: it's trying to show you just the part that went wrong. @rudkx?

[rudkx]

I'm not sure it's so much by design as by accident. As soon as something goes wrong in type checking we bail out leaving "unresolved" types (things that we have not yet inferred at that point) in the expression being type checked, and these get printed as "_".

[palimondo]

So, I should be reading this error message as: Type '(inout (count: Int, ... doesn’t match '(inout (String, ...? Hmm… That makes some kind of sense, but is not what I expected.

Do you want me to close this bug, or is there a way to improve this diagnostic?

[rudkx]

Let's leave it open for now and consider whether there is an improvement we can make here.

[palimondo]

I think I have another instance, where this issue came up, and this makes the early bail behavior quite harmful in diagnosing the problem…

In #46830 @belkadan showed me how to work around the issue there, to create correctly specialized instance of a struct by explicitly specifying the generic arguments when constructing the return value. This is quite a hairy case, where the struct I return should be specialized with a named tuple, but when I return the instance I have specified the tuple with correct types but omitted the names:

public struct ComposedSequence<Element, State> {
    var _state: State
    var _next: (inout State) -> Element?
    
    static func prefix<I : IteratorProtocol>(_ prefix: Int, _ iterator: I) -> ComposedSequence<I.Element, (count: Int, iterator: I)> {

        func _prefix(state: inout (count: Int, iterator: I)) -> I.Element? { return nil }

        return ComposedSequence<I.Element, (Int, I)>((prefix, iterator), _prefix)
    }
}

Correctly I should return ComposedSequence<I.Element, (count: Int, iterator: I)>(..., but the error message is really not helping me to figure this out:

Cannot convert value of type '(inout (count: Int, iterator: I)) -> I.Element?' to expected argument type '(inout _) -> _?’

[gregomni]

Swift 5.10, the produced error messages are now very good.

Original code now produces error:

sequence.swift:13:37: error: cannot convert return expression of type '(String, BaseIterator)' to return type '(count: Int, iterator: BaseIterator)'
    return ComposedSequence(_state: ("prefix", iterator), _next: _prefix)
                                    ^

And follow up code in the last comment produces error:

sequence2.swift:9:53: error: missing argument labels '_state:_next:' in call
        return ComposedSequence<I.Element, (Int, I)>((prefix, iterator), _prefix)
                                                    ^
                                                     _state:             _next: 

[AnthonyLatsis]

sequence.swift:13:37: error: cannot convert return expression of type '(String, BaseIterator)' to return type '(count: Int, iterator: BaseIterator)'
    return ComposedSequence(_state: ("prefix", iterator), _next: _prefix)
                                    ^

This message is suboptimal because it refers to the argument as the return expression. Although, the message regressed in 5.6, and the original problem was completely different, so here’s the sequel: #74463.