"error FS0192: internal error: duplicate entry in type index table" with simple code

Compiling the following snippet in a dotnet interactive session (either in Visual Studio Code or with the command line dotnet fsi) fails with an error similar to error FS0192: internal error: duplicate entry 'f@6' in type index table (with an entry name that is not constant):

open System

let f (start: DateTime) (stop: DateTime) (input: (DateTime * 'a) list) =
    [
        for i in start.Ticks .. stop.Ticks ->
            input |> List.where (fun (k, v) -> true)
    ]

I wasn't able to isolate the problem further.

This is also failing in a Polyglot Notebook with error input.fsx (1,1)-(1,1) interactive error internal error: Duplicate type name within an assembly....

Known workarounds

I couldn't find any.

Related information

Provide any related information (optional):

I'm running Windows 11, and I just updated my .NET SDK version just in case:

> dotnet --info
.NET SDK:
 Version:           10.0.101
 Commit:            fad253f51b
 Workload version:  10.0.100-manifests.9f71effe
 MSBuild version:   18.0.6+fad253f51

[...]

workaround is to materialize the range:

let range = [ start.Ticks .. stop.Ticks ]
for i in range -> ...

As for the fix:
Claude Opus with some guidance found the error and wrote a working solution. Remarkable.

The cause is duplicating body expression in mkOptimizedRangeLoop and the fix is to copy the expression in mkCond to not emit lambda with same stamp twice.

I have no time atm to make a proper PR but this change to mkOptimizedRangeLoop does the trick:

        | RangeCount.PossiblyOversize calc ->
            calc (fun count wouldOvf ->
                buildLoop count (fun mkBody ->
                    // mkBody creates expressions that may contain lambdas with unique stamps.
                    // We need to copy the expression for the second branch to avoid duplicate type names.
                    let mkBodyCopied idxVar loopVar = copyExpr g CloneAll (mkBody idxVar loopVar)
                    mkCond
                        DebugPointAtBinding.NoneAtInvisible
                        mIn
                        g.unit_ty
                        wouldOvf
                        (mkCountUpInclusive mkBody (tyOfExpr g count))
                        (mkCompGenLetIn mIn (nameof count) (tyOfExpr g count) count (fun (_, count) -> mkCountUpExclusive mkBodyCopied count))))

FWIW, another workaround until this is fixed should be to set the language version to 8 (or lower). That will make the compiler use the older way of lowering ranges + comprehensions.

That was very quick! Thanks!

Fix dotnet#19156 (dotnet#19157)

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"error FS0192: internal error: duplicate entry in type index table" with simple code #19156

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"error FS0192: internal error: duplicate entry in type index table" with simple code #19156

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majocha commented on Dec 13, 2025

brianrourkeboll commented on Dec 13, 2025

mlaily commented on Dec 15, 2025

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