Fix: FCS Crash With ConstraintSolver In Valid F# File

Nov 25, 2025 by Alex Johnson 54 views

FCS Crash with ConstraintSolverMissingConstraint on Valid F# File

Experiencing crashes with the F# Compiler Service (FCS) due to a ConstraintSolverMissingConstraint error can be frustrating, especially when your code appears to be valid. This article delves into a specific scenario where this issue arises, provides a detailed explanation, and offers potential solutions or workarounds.

The Issue: ConstraintSolverMissingConstraint

The core problem is a crash within FCS, incorrectly flagging a missing constraint related to generic type parameters. In the provided example, the issue surfaces when accessing the ImmediateSubexpressions property within a small, valid F# library. The compiler seems to believe that a generic type parameter, 'appEvent in this case, requires comparison capabilities, even when it shouldn't.

namespace Foo

type Bar<'appEvent> =
    | Wibble of 'appEvent

This problem was initially encountered while running an Ionide F# analyzer, highlighting its impact on real-world development workflows. Understanding the root cause and potential solutions is crucial for F# developers facing similar challenges.

Reproducing the Crash: Step-by-Step Guide

To replicate this issue, follow these steps:

Create a Test File (TestThing.fs):

namespace WoofWare.Zoomies.Test

open System
open System.IO
open FSharp.Compiler.CodeAnalysis
open FSharp.Compiler.Symbols
open FSharp.Compiler.Text
open FsUnitTyped
open NUnit.Framework

[<TestFixture>]
module TestWorldFreezer =

    [<Test>]
    let ``demo of bug`` () = task {
        let s = """namespace Foo

type Bar<'appEvent> = | Wibble of 'appEvent"""

        let outFile = Path.Combine (Path.GetTempPath (), Guid.NewGuid().ToString () + ".fsx")
        try
            File.WriteAllText (outFile, s)

            let checker = FSharpChecker.Create (keepAssemblyContents=true)
            let! options, _diags = checker.GetProjectOptionsFromScript (outFile, SourceText.ofString s)

            let! t, u = checker.ParseAndCheckFileInProject (outFile, 0, SourceText.ofString s, options)
            t.Diagnostics.Length |> shouldEqual 0
            let v =
                match u with
                | FSharpCheckFileAnswer.Succeeded x -> x
                | FSharpCheckFileAnswer.Aborted -> failwith "bad"

            let decl =
                let rec go (decl : FSharpImplementationFileDeclaration) =
                    match decl with
                    | FSharpImplementationFileDeclaration.Entity (_, declarations) ->
                        declarations |> List.iter go
                    | FSharpImplementationFileDeclaration.MemberOrFunctionOrValue (_, _, e) ->
                        e.ImmediateSubExpressions
                        |> ignore
                    | _ -> failwith "no"

                go (v.ImplementationFile.Value.Declarations |> List.exactlyOne)

            return ()
        finally
            try
                File.Delete outFile
            with | _ -> ()
    }
```

Create a Project File (.csproj):

<Project Sdk="Microsoft.NET.Sdk">

<PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <IsPackable>false</IsPackable>
    <OutputType>Exe</OutputType>
</PropertyGroup>

<ItemGroup>
    <Compile Include="TestThing.fs" />
</ItemGroup>

<ItemGroup>
    <PackageReference Include="FsCheck" Version="3.3.1" />
    <PackageReference Include="FsUnit" Version="7.1.1"/>
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1"/>
    <PackageReference Include="FSharp.Compiler.Service" Version="[43.10.100]" />
    <PackageReference Include="FSharp.Core" Version="10.0.100" />
    <PackageReference Include="NUnit" Version="4.3.2" />
    <PackageReference Include="NUnit3TestAdapter" Version="5.0.0" />
</ItemGroup>
</Project>

Run the Test: Execute the test using your preferred testing framework (e.g., NUnit).

Expected vs. Actual Behavior

Expected Behavior: The test should pass, indicating successful traversal of each node in the Typed Abstract Syntax Tree (TAST).

Actual Behavior: The test will fail, showcasing the ConstraintSolverMissingConstraint error. The error message provides detailed information about the context of the crash, including the involved types and constraints.

FSharp.Compiler.DiagnosticsLogger+ReportedError : The exception has been reported. This internal exception should now be caught at an error recovery point on the stack. Original message: ConstraintSolverMissingConstraint
  ({ includeStaticParametersInTypeNames = false
     openTopPathsSorted =
      Internal.Utilities.Library.InterruptibleLazy`1[Microsoft.FSharp.Collections.FSharpList`1[Microsoft.FSharp.Collections.FSharpList`1[System.String]]]
     openTopPathsRaw = []
     shortTypeNames = false
     suppressNestedTypes = false
     maxMembers = None
     showObsoleteMembers = false
     showHiddenMembers = false
     showTyparBinding = false
     showInferenceTyparAnnotations = false
     suppressInlineKeyword = true
     suppressMutableKeyword = false
     showMemberContainers = false
     shortConstraints = false
     useColonForReturnType = false
     showAttributes = false
     showCsharpCodeAnalysisAttributes = false
     showOverrides = true
     showStaticallyResolvedTyparAnnotations = true
     showNullnessAnnotations = None
     abbreviateAdditionalConstraints = false
     showTyparDefaultConstraints = false
     showDocumentation = false
     shrinkOverloads = true
     printVerboseSignatures = false
     escapeKeywordNames = false
     g = <TcGlobals>
     contextAccessibility = public
     generatedValueLayout = <fun:Empty@3244>
     genericParameterStyle = Implicit }, appEvent,
   SupportsComparison (4,6--4,12), (4,6--4,12), (4,6--4,12)))
   at FSharp.Compiler.DiagnosticsLogger.DiagnosticsLoggerExtensions.DiagnosticsLogger.Error[T](DiagnosticsLogger x, Exception exn) in D:\a\_work\1\s\src\fsharp\src\Compiler\Facilities\DiagnosticsLogger.fs:line 475
   at FSharp.Compiler.DiagnosticsLogger.CommitOperationResult[T](OperationResult`1 res) in D:\a\_work\1\s\src\fsharp\src\Compiler\Facilities\DiagnosticsLogger.fs:line 664
   at FSharp.Compiler.Symbols.FSharpExprConvert.GetWitnessArgs(SymbolEnv cenv, ExprTranslationEnv env, ValRef vref, Range m, FSharpList`1 tps, FSharpList`1 tyargs) in D:\a\_work\1\s\src\fsharp\src\Compiler\Symbols\Exprs.fs:line 519
   at FSharp.Compiler.Symbols.FSharpExprConvert.ConvModuleValueOrMemberUseLinear(SymbolEnv cenv, ExprTranslationEnv env, Expr expr, ValRef vref, ValUseFlag vFlags, FSharpList`1 tyargs, FSharpList`1 curriedArgs, FSharpFunc`2 contF) in D:\a\_work\1\s\src\fsharp\src\Compiler\Symbols\Exprs.fs:line 505
   at FSharp.Compiler.Symbols.FSharpExprConvert.ConvExprOnDemand@1339.Invoke(Unit unitVar0) in D:\a\_work\1\s\src\fsharp\src\Compiler\Symbols\Exprs.fs:line 1339
   at FSharp.Compiler.Symbols.FSharpExpr.get_E() in D:\a\_work\1\s\src\fsharp\src\Compiler\Symbols\Exprs.fs:line 153
   at FSharp.Compiler.Symbols.FSharpExpr.get_ImmediateSubExpressions() in D:\a\_work\1\s\src\fsharp\src\Compiler\Symbols\Exprs.fs:line 157

Root Cause Analysis

The error message ConstraintSolverMissingConstraint indicates that the F# compiler's constraint solver is unable to find a necessary constraint for the generic type parameter 'appEvent. Specifically, it's looking for a SupportsComparison constraint, implying that the compiler believes it needs to compare values of this type. However, in the provided code snippet, there's no explicit operation that necessitates comparison.

The issue likely stems from how the F# compiler infers constraints based on the usage of generic type parameters. In certain scenarios, the compiler might conservatively assume a need for comparison, even if it's not immediately apparent.

Potential Solutions and Workarounds

Currently, there are no known workarounds for this specific issue. However, here are some general strategies that might help in similar situations:

Explicitly Add Constraints: You can try adding explicit constraints to the generic type parameter. In this case, if comparison is indeed needed, you could add a when clause to the type definition:

type Bar<'appEvent when 'appEvent : comparison> = | Wibble of 'appEvent ```

However, if comparison isn't actually required, this won't resolve the underlying issue.

Simplify the Code: Sometimes, complex code structures can confuse the compiler's type inference. Try simplifying the code around the point where the error occurs to see if that resolves the issue.
Report as a Compiler Bug: Since this appears to be an incorrect constraint inference by the compiler, it's crucial to report it as a bug to the F# language maintainers. This allows them to investigate the issue and implement a fix in a future release.
Downgrade Compiler Version: As a temporary workaround, you might try using an older version of the F# compiler. However, this should be done with caution, as it might introduce other compatibility issues.

Conclusion

The ConstraintSolverMissingConstraint error in FCS can be a challenging issue to diagnose and resolve. By understanding the error message, reproducing the issue, and analyzing the potential causes, developers can take steps towards finding a solution. Reporting such issues to the F# community is vital for improving the compiler and ensuring a smoother development experience.

For further information on F# compiler internals and reporting issues, consider exploring the official F# GitHub repository: FSharp Compiler SDK. This is a valuable resource for staying up-to-date with the latest developments and contributing to the F# ecosystem.