[ShryukGrandhi]

Description

When a generic member needs a constraint it doesn't declare, FS0043 printed the constraint as-is. Unsolved inference variables then showed up as internal placeholders. For

type C() =
    static member M<'a,'b>(a: 'a, b: 'b) = a + b

the compiler reported

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

In some of the duplicate diagnostics for the same expression, the support types were also printed as ^``?```. Older compilers showed the stamp instead (^?8018`).

richTextOfTyparConstraint, used only for this diagnostic, now goes through a new PrintTypes.prettyLayoutOfTyparConstraint. It attaches the constraint to a copy of the typar and runs PrettyTypes.PrettifyType, the same approach prettyLayoutOfTrait already uses. If a member constraint's support doesn't include the typar, the typar isn't printed, so the copy gets a placeholder name. That way it can't take a name from the support types and turn ^a into ^a0. The message now reads

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

Fixes #6725

Checklist

• Test cases added
• Performance benchmarks added in case of performance changes
• Release notes entry updated

Testing

• Added Issue 6725 - missing constraint message does not show unnamed inference variables to ErrorMessages/DiagnosticRegressionTests.fs. It fails on main (the messages contain '``?``` and ^``?```) and passes with this change.
• Ran the ComponentTests classes that check "missing a constraint" text or SRTP/constraint printing: DiagnosticRegressionTests, MigratedTypeCheckTests (neg06/neg10/neg45/neg112 baselines), RecordSpreadsTests, TypeAbbreviations, CheckingOfImplementationFiles, TypeInference, TypeConstraints, IWSAM/SRTP. 486 passed. The 34 failures are all IL-baseline tests that couldn't start the x64-only ildasm package on my arm64 Mac (Bad CPU type in executable), so they're unrelated to this change.

[github-actions]

✅ Release notes checked

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✅ Found changes and release notes in following paths:

Warning

No PR link found in some release notes, please consider adding it.

Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.200.md	No current pull request URL (#20928) found, please consider adding it