[generics/error message] ?typeid shown in error message for missing type constraint #6725

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1. cartermp

   added

   Area-Compiler

   Bug

   on May 14, 2019
2. cartermp

   added this to the Backlog milestone on May 14, 2019
3. cartermp commented on May 14, 2019

   

   cartermp

   on May 14, 2019

   Contributor

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   Labeling as a bug due to not wanting to leak the type ID here. The missing constraint is correct though.
4. smoothdeveloper commented on May 16, 2019

   

   smoothdeveloper

   on May 16, 2019

   ContributorAuthor

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   probable location of the bug, or the callsite which gives some display settings as input parameter:

   fsharp/src/fsharp/NicePrint.fs

   Lines 782 to 804 in 2046609

   	and layoutConstraintsWithInfo denv env cxs =
   	// Internally member constraints get attached to each type variable in their support.
   	// This means we get too many constraints being printed.
   	// So we normalize the constraints to eliminate duplicate member constraints
   	let cxs =
   	cxs
   	|> ListSet.setify (fun (_, cx1) (_, cx2) ->
   	match cx1, cx2 with
   	| TyparConstraint.MayResolveMember(traitInfo1, _),
   	TyparConstraint.MayResolveMember(traitInfo2, _) -> traitsAEquiv denv.g TypeEquivEnv.Empty traitInfo1 traitInfo2
   	| _ -> false)
   	let cxsL = List.collect (layoutConstraintWithInfo denv env) cxs
   	match cxsL with
   	| [] -> emptyL
   	| _ ->
   	if denv.abbreviateAdditionalConstraints then
   	wordL (tagKeyword "when") ^^ wordL(tagText "<constraints>")
   	elif denv.shortConstraints then
   	leftL (tagPunctuation "(") ^^ wordL (tagKeyword "requires") ^^ sepListL (wordL (tagKeyword "and")) cxsL ^^ rightL (tagPunctuation ")")
   	else
   	wordL (tagKeyword "when") ^^ sepListL (wordL (tagKeyword "and")) cxsL
5. smoothdeveloper

   added a commit that references this issue on Jan 31, 2020

   proposed work around the flaky error message until dotnet#6725 has a fix

   ...

   f2bb3b8
6. KevinRansom

   added a commit that references this issue on Feb 19, 2020

   improve compiler error message for failed overload resolution (#6596)

   ...

   acd7cfd
7. baronfel

   added a commit that references this issue on Feb 19, 2020

   improve compiler error message for failed overload resolution (#6596)

   ...

   4842127
8. dsyme

   added

   Impact-Low(Internal MS Team use only) Describes an issue with limited impact on existing code.(Internal MS Team use only) Describes an issue with limited impact on existing code.

   on Aug 31, 2020
9. cartermp

   modified the milestones: Backlog, Backlog - Lower Priority on Aug 31, 2020
10. cartermp

    modified the milestones: Backlog - Lower Priority, Backlog on Oct 28, 2020
11. nosami

    added a commit that references this issue on Feb 23, 2021

    improve compiler error message for failed overload resolution (dotnet…

    ...

    b9d9536
12. dsyme commented on Nov 5, 2021

    

    dsyme

    on Nov 5, 2021

    Contributor

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    Duplicate of #385
13. dsyme

    marked this as a duplicate of Stubborn warning FS0193 on valid code #385 on Nov 5, 2021
14. dsyme

    closed this as completedon Nov 5, 2021
15. abelbraaksma commented on May 16, 2024

    

    abelbraaksma

    on May 16, 2024

    Contributor

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    This was closed as duplicate, but the resolution of the duplicate did not resolve this one. Reopening, as the typeid-error is still there:
    

    Related: #3508
16. abelbraaksma

    reopened this on May 16, 2024
17. github-actions commented on Oct 3, 2026

    

    github-actionsbot

    on Oct 3, 2026 – with GitHub Actions

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    🤖 This is an automated response from Repo Assist.

    Verified that the underlying diagnostic bug still reproduces at repo HEAD 8512f9acbd9f757b75440b1ca6afe33fec321208 (commit dated 2026-10-02, about 13 hours old at the start of this run).

    The freshly built FSI reports version 15.2.200.0 for F# 11.2, FSharp.Core 11.0.0.0, on .NET 11.0.0-rc.1.26420.103.

    For the original static member M<'a, 'b>(a: 'a, b: 'b) = a + b repro, the current compiler no longer prints a numbered identifier such as ^?8018, but it still exposes an unresolved placeholder in the public diagnostic:

    error FS0043: A type parameter is missing a constraint 'when (^a or ^b) : (static member (+) : ^a * ^b -> '``?``)'
    

    So the numeric suffix has disappeared, but the result type is still rendered as the internal-looking `'``?``` rather than a meaningful type variable.

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