This PR adds regression tests documenting two consumer-side failures reported inFSharpPlus issue #613 that occur with F# 9.

Overview

The tests reproduce real-world FSharpPlus patterns that currently fail to compile in F# 9, serving as regression tests to track when these issues are resolved in the compiler.

Test Scenarios

1. monad.plus SRTP Pattern (Currently Failing)

Tests the use of statically resolved type parameters (SRTP) for ad-hoc polymorphism with overloaded static members:

typeMonadPlusClass=static member inlineMPlus(x:option<'a>,y:option<'a>)=...static member inlineMPlus(x:list<'a>,y:list<'a>)=...let inlinemplus(x:^M)(y:^M):^M=((^MonadPlusClassor^M):(static memberMPlus:^M*^M->^M)(x, y))letresult:int option= mplus(Some1)(Some2)// Fails with type constraint mismatch

Current Error:Type constraint mismatch when applying the default type 'obj' for a type inference variable. None of the types 'obj, int option' support the operator 'MPlus'

2. Custom ResultTBuilder (Currently Passing)

Tests defining a custom computation expression builder for Result types:

typeResultTBuilder()=member inline_.Return(x:'T):Result<'T,'Error>= Ok xmember inline_.Bind(m,f)=...// ... other membersletresultT= ResultTBuilder()letcompute x y=    resultT{let!a= Ok xlet!b= Ok yreturn a+ b}

This scenario currently works correctly in F# 9.

Test Configuration

Each test runs with three compiler configurations:

• --langversion:8.0 - F# 8 baseline
• --langversion:preview - F# 9 preview
• --langversion:preview --checknulls+ - F# 9 preview with nullable reference types

Implementation Details

• UsesFSharpScript interactive API for scripting evaluation
• Tests are structured as xUnitTheory tests withInlineData parameters
• Located intests/FSharp.Compiler.ComponentTests/Language/FSharpPlusRegressionTests.fs
• Tests assert compilation success, documenting expected behavior

Test Results

Test run summary:- monad.plus usage (3 scenarios): ❌ All failing - documents SRTP regression- ResultTBuilder usage (3 scenarios): ✅ All passing - pattern works correctly

The failing monad.plus tests will serve as indicators when the SRTP resolution issue is fixed in the F# compiler.

Running the Tests

# Run all FSharpPlus regression testsdotnettest tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj \    --filter"FullyQualifiedName~FSharpPlusRegressionTests"# Run specific testdotnettest tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj \    --filter"FullyQualifiedName~monad.plus"

Related Issues

• FSharpPlus issue:F# Regressions affecting F#+ fsprojects/FSharpPlus#613

This pull request was created as a result of the following prompt from Copilot chat.

Add regression tests for FSharpPlus consumer scenarios affected by F# 9. These tests should reproduce two consumer-side failures described in the last comment offsprojects/FSharpPlus#613: (1) usage of monad.plus, and (2) defining a custom ResultTBuilder. For each, write a Theory test with InlineData that runs against three compiler settings: (a) --langversion:8, (b) --langversion:preview, (c) --langversion:preview with --checknulls+ and --checknulls-. The tests should use the FSharpScript interactive scripting API, and should assert compilation success (i.e., as if the code should work), even though they are expected to fail with current F# 9 builds. Include collapsible markdown instructions in the PR body, detailing exactly what, where, and how is being tested.

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