Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Instructions for Adding--typecheck-only Support to F# Interactive Scripts

Problem Statement

The--typecheck-only flag already exists for F# project compilation but is not supported for.fsx script files in F# Interactive (FSI). Currently, there's no way to type-check scripts without executing them through the FSI command line. This feature would allow developers to validate script syntax and types without running potentially side-effect-producing code.

Implementation Steps

1. Add Command Line Option

Add the--typecheck-only option to the FSI command line parser. Insert a newCompilerOption in the advanced options section:

CompilerOption("typecheck-only","", OptionUnit(fun()-> tcConfigB.typeCheckOnly<-true), None, Some("Type-check only, don't execute"))

This should be added alongside other advanced options likeexec,gui,quiet, etc.

2. Modify ProcessInputs Function

The core implementation goes in theProcessInputs function. In2 , add a check afterCheckClosedInputSet and beforeProcessTypedImpl:

lettcState,topCustomAttrs,declaredImpls,tcEnvAtEndOfLastInput=    lock tcLockObject(fun _->        CheckClosedInputSet(            ctok,(fun()-> diagnosticsLogger.CheckForRealErrorsIgnoringWarnings),            tcConfig,            tcImports,            tcGlobals,            Some prefixPath,            tcState,            eagerFormat,            inputs))// Add this check after CheckClosedInputSetif tcConfig.typeCheckOnlythen    raise StopProcessingletcodegenResults,optEnv,fragName=    ProcessTypedImpl(...)

3. Exception Handling

TheStopProcessing exception is already handled . This infrastructure will properly catch the exception and stop processing without executing the script.

Testing Implementation

Test Location and Structure

All tests should be added to theFSharp.Compiler.ComponentTests project.

Create a new test file:
tests/FSharp.Compiler.ComponentTests/Scripting/TypeCheckOnlyTests.fs

Test Implementation

moduleFSharp.Compiler.ComponentTests.Scripting.TypeCheckOnlyTestsopenXunitopenFSharp.TestopenFSharp.Test.Compiler[<Fact>]let``typecheck-only flag works for valid script``()=    Fsx"""let x = 42printfn "This should not execute""""|> withOptions["--typecheck-only"]|> compile|> shouldSucceed[<Fact>]let``typecheck-only flag catches type errors``()=    Fsx"""let x: int = "string"  // Type error"""|> withOptions["--typecheck-only"]|> compile|> shouldFail|> withDiagnostics[(Error1, Line2, Col14, Line2, Col22,"This expression was expected to have type\n    'int'\nbut here has type\n    'string'")][<Fact>]let``typecheck-only flag prevents execution side effects``()=    Fsx"""System.IO.File.WriteAllText("test-file.txt", "should not be created")let x = 42"""|> withOptions["--typecheck-only"]|> compile|> shouldSucceed// Verify file was not created (test would need additional verification logic)

Project File Update

Add the new test file:

<CompileInclude="Scripting/TypeCheckOnlyTests.fs" />

Test Utilities

The ComponentTests project references Test utilities , which provides testing utilities likeFsx,withOptions,compile,shouldSucceed, andshouldFail.

Key Implementation Notes

1. The--typecheck-only flag already exists in the core F# compiler configuration (TcConfigBuilder), so you're primarily adding FSI-specific handling.

2. TheProcessInputs function is the correct location for this check because it occurs after parsing and type-checking but before code generation and execution.

3. TheStopProcessing exception mechanism is already established in FSI for handling compilation-stopping conditions.

4. All new tests should use the ComponentTests project following modern F# testing practices.

This implementation will allow users to runfsi --typecheck-only script.fsx to validate script correctness without execution.

Fixes#18686.

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