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Disallow recursive structs with lifted type parameters - #19031

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T-Gro merged 4 commits into
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brianrourkeboll:recursive-generic-struct-types
Nov 3, 2025
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Disallow recursive structs with lifted type parameters#19031
T-Gro merged 4 commits into
dotnet:mainfrom
brianrourkeboll:recursive-generic-struct-types

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@brianrourkeboll

@brianrourkebollbrianrourkeboll commented Oct 25, 2025•
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Description

Fixes#18993.
Fixes#6069.

  • Emit "error FS0954: This type definition involves an immediate cyclic reference through a struct field or inheritance relation" for generic struct types that recursively reference themselves but (infinitely) lift their generic type parameter.

I don't know if "lifting" is the correct term for this; let me know if there is a more correct term or phrase that I should use to describe this in the comment and the tests. Maybe nested app ty/type application?

Checklist

  • Test cases added.
  • Release notes entry updated.

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github-actionsBot commented Oct 25, 2025•
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❗ Release notes required

✅ Found changes and release notes in following paths:

Change pathRelease notes pathDescription
src/Compilerdocs/release-notes/.FSharp.Compiler.Service/11.0.0.md

brianrourkeboll
brianrourkeboll commented Oct 25, 2025
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Comment threadsrc/Compiler/Checking/CheckDeclarations.fs
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brianrourkeboll marked this pull request as ready for review October 25, 2025 17:23
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brianrourkeboll requested a review from a team as a code ownerOctober 25, 2025 17:23
@edgarfgp

edgarfgp commented Oct 27, 2025

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I think this PR might also fix #6069 ?

@brianrourkeboll

brianrourkeboll commented Oct 28, 2025

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I think this PR might also fix #6069 ?

Hmm yes, probably. I will add a test/tests for mutually-recursive structs.

@github-project-automationgithub-project-automationBot moved this from New to In Progress in F# Compiler and ToolingNov 3, 2025
@T-Gro
T-Gro merged commit 9670f60 into dotnet:mainNov 3, 2025
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brianrourkeboll deleted the recursive-generic-struct-types branch November 3, 2025 21:06
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Compilation fails to terminate for polymorphic recursion on value types Infinite loop on expanding cycle for struct types

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