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Specialize FS0027 for function/method parameters #19866

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Specialize FS0027 for function/method parameters #19866

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f91e28e
Add failing tests for FS0027 on function parameters (issue #15803)
May 29, 2026
8bb0b3f
Specialize FS0027 for function/method parameters (#15803)
May 29, 2026
e03da54
Format and validate FS0027 parameter message fix (#15803)
May 29, 2026
b7cc31a
Add PR reference to release notes entry for #15803
May 29, 2026
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1 change: 1 addition & 0 deletions docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
### Fixed

* Diagnostic FS0027 now emits a parameter-specific message (suggesting a `let mutable x = x` shadow or `byref<_>`) instead of the illegal `let mutable x = expression` shadow when the assignment target is a function or method parameter. ([Issue #15803](https://github.com/dotnet/fsharp/issues/15803), [PR #19866](https://github.com/dotnet/fsharp/pull/19866))
* Reject non-function bindings for single-case and partial active pattern names with FS1209, matching the existing multi-case behavior. ([PR #19763](https://github.com/dotnet/fsharp/pull/19763))
* Fix FS0421 "The address of the variable cannot be used at this point" incorrectly raised for the discard pattern `let _ = &expr` when `let x = &expr` compiles. ([Issue #18841](https://github.com/dotnet/fsharp/issues/18841), [PR #19811](https://github.com/dotnet/fsharp/pull/19811))
* Honor `--nowarn` and `--warnaserror` for warnings emitted during command-line option parsing ([Issue #19576](https://github.com/dotnet/fsharp/issues/19576), [PR #19776](https://github.com/dotnet/fsharp/pull/19776))
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8 changes: 8 additions & 0 deletions src/Compiler/Checking/CheckIncrementalClasses.fs
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Expand Up @@ -161,6 +161,14 @@ let TcImplicitCtorInfo_Phase2A(cenv: cenv, env, tpenv, tcref: TyconRef, vis, att

// Put them in order
let ctorArgs = List.map (fun v -> NameMap.find v vspecs) ctorArgNames

// Mark constructor arguments as "displayed as parameters" so that diagnostics
// (e.g. FS0027 on assignment to a non-mutable parameter) can distinguish them
// from ordinary local bindings.
for v in ctorArgs do
if v.ArgReprInfoForDisplay.IsNone then
v.SetArgReprInfoForDisplay (Some (ValReprInfo.InferArgReprInfo v))

let safeThisValOpt = MakeAndPublishSafeThisVal cenv env thisIdOpt thisTy

// NOTE: the type scheme here is not complete!!! The ctorTy is more or less
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12 changes: 11 additions & 1 deletion src/Compiler/Driver/CompilerDiagnostics.fs
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Expand Up @@ -574,6 +574,7 @@ module OldStyleMessages =
let RuleNeverMatchedE () = Message("RuleNeverMatched", "")
let EnumMatchIncomplete1E () = Message("EnumMatchIncomplete1", "")
let ValNotMutableE () = Message("ValNotMutable", "%s")
let ValNotMutableParameterE () = Message("ValNotMutableParameter", "%s%s%s")
let ValNotLocalE () = Message("ValNotLocal", "")
let Obsolete1E () = Message("Obsolete1", "")
let Obsolete2E () = Message("Obsolete2", "%s")
Expand Down Expand Up @@ -1838,7 +1839,16 @@ type Exception with

| PatternMatchCompilation.RuleNeverMatched _ -> os.AppendString(RuleNeverMatchedE().Format)

| ValNotMutable(_, vref, _) -> os.AppendString(ValNotMutableE().Format(vref.DisplayName))
| ValNotMutable(_, vref, _) ->
let name = vref.DisplayName

let msg =
if vref.Deref.ArgReprInfoForDisplay.IsSome then
ValNotMutableParameterE().Format name name name
else
ValNotMutableE().Format name

os.AppendString msg

| ValNotLocal _ -> os.AppendString(ValNotLocalE().Format)

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3 changes: 3 additions & 0 deletions src/Compiler/FSStrings.resx
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Expand Up @@ -1041,6 +1041,9 @@
<data name="ValNotMutable" xml:space="preserve">
<value>This value is not mutable. Consider using the mutable keyword, e.g. 'let mutable {0} = expression'.</value>
</data>
<data name="ValNotMutableParameter" xml:space="preserve">
<value>The parameter '{0}' is not mutable. Either shadow it with a mutable binding (e.g. 'let mutable {1} = {2}'), or change its type to 'byref&lt;_&gt;'.</value>
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@brianrourkeboll brianrourkeboll Jun 1, 2026

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Hmm. In the original example from #15803,

let f x =
    x <- 42

…if we assume that the user just wants x to have a new value in the remainder of the function scope, then shadowing via a regular, non-mutable let-binding would suffice:

let f x =
    let x = 42
    // Code that uses the new value of x.

Suggesting shadowing with let mutable only really makes sense if they want it to actually behave as a mutable value in the rest of the function scope.

</data>
<data name="ValNotLocal" xml:space="preserve">
<value>This value is not local</value>
</data>
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5 changes: 5 additions & 0 deletions src/Compiler/xlf/FSStrings.cs.xlf
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5 changes: 5 additions & 0 deletions src/Compiler/xlf/FSStrings.de.xlf
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5 changes: 5 additions & 0 deletions src/Compiler/xlf/FSStrings.es.xlf
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5 changes: 5 additions & 0 deletions src/Compiler/xlf/FSStrings.fr.xlf
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5 changes: 5 additions & 0 deletions src/Compiler/xlf/FSStrings.it.xlf
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5 changes: 5 additions & 0 deletions src/Compiler/xlf/FSStrings.ja.xlf
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