FS0073 internal error: Undefined or unsolved type variable: 'C, intermittent in Release #20812

The compiler fails now and then with error FS0073: internal error: Undefined or unsolved type variable: 'C while it compiles a test project in Release. The next build of the same sources succeeds. 6 of 66 builds failed in my measurements, and none of 72 builds failed with --parallelcompilation-.

The error is reported at a lambda in a member of a generic abstract class. The lambda captures a constructor parameter, a record of functions over the type parameter of the class, and is passed to a method of the base class:

[<AbstractClass>]
type SeqLikeAccessContract<'C> (ops : SeqOps<'C>) =
    inherit SeqContractBase<'C> (ops)

    [<TestMethod>]
    member this.``head throws for an empty collection as FSharp.Core does`` () =
        this.ThrowsLikeFSharpCore (
            (fun () -> Array.head Array.empty<int>),
            (fun () -> List.head List.empty<int>),
            (fun () -> ops.Head (ops.OfSeq Inputs.Empty)), // SeqLikeAccessContract.fs(81,14)
            "head of an empty collection must throw"
        )

Repro steps

FSharp.Collections.Immutable-fs0073-repro.zip

1. Install the .NET SDK 10.0.402 and PowerShell 7. The failure was seen on Windows, Ubuntu and macOS.
2. Unpack the attached FSharp.Collections.Immutable-fs0073-repro.zip. It holds the tree of fsprojects/FSharp.Collections.Immutable@58af8f1, reduced to what the build needs, and repro.ps1.
3. Run pwsh ./repro.ps1 in the unpacked folder. It builds the library once and compiles the test project in Release again and again (dotnet build -c Release -f net10.0 --no-dependencies --no-incremental) until FS0073 appears, 60 builds at most. About 9 builds in 100 fail; the first failure came after 23 and after 10 builds in two runs.

Expected behavior

Every build succeeds.

Actual behavior

10/80: FAILED (21,3 s) ...\tests\FSharp.Collections.Immutable.Tests\Contracts\SeqLike\SeqLikeAccessContract.fs(81,14): error FS0073: internal error: Undefined or unsolved type variable: 'C [...\FSharp.Collections.Immutable.Tests.fsproj::TargetFramework=net10.0]

The compiler gives no stack trace. The same error showed up on the GitHub Actions runners windows-latest, ubuntu-latest and macos-latest, for both target frameworks (net8.0 and net10.0), in the same file.

Known workarounds

Compile without parallel compilation: <ParallelCompilation>false</ParallelCompilation> in the project, or pwsh ./repro.ps1 -ExtraArguments '-p:ParallelCompilation=false'. None of 72 builds failed. The compiler warns with FS0075 ("The command-line option '--parallelcompilation' is for test purposes only").

Related information

• Windows 11 with 32 logical processors. The failure also appears when the process is restricted to 4 logical processors (3 of 24 builds), and on the Linux and macOS runners of GitHub Actions.
• .NET SDK 10.0.402 (FSharp.Compiler.Service 43.12.402), FSharp.Core 10.1.301.
• Release configuration, net8.0 and net10.0, LangVersion 10.0, nullable reference types on.
• Without the switch 6 of 66 builds failed (2 of 12, 1 of 30 and 3 of 24 on 4 processors). With --parallelcompilation- none of 72 failed.