Warn if function shadows union case

src/Compiler/Checking/CheckExpressions.fs

@@ -10758,7 +10758,17 @@ and TcNonrecBindingTyparDecls cenv env tpenv bind =
     let (NormalizedBinding(_, _, _, _, _, _, synTyparDecls, _, _, _, _, _)) = bind
     TcBindingTyparDecls true cenv env tpenv synTyparDecls
 
-and TcNonRecursiveBinding declKind cenv env tpenv ty binding =
+and TcNonRecursiveBinding declKind (cenv: cenv) env tpenv ty binding =
+    // Check for unintended shadowing
+    if cenv.g.langVersion.SupportsFeature (LanguageFeature.WarnIfFunctionShadowsUnionCase) then
+        match binding with
+        | SynBinding(headPat = SynPat.LongIdent(longDotId = SynLongIdent(id = [ident]); range = headPatRange)) ->
+            match env.eNameResEnv.ePatItems.TryFind ident.idText with
+            | Some (Item.UnionCase(_, false)) ->
+                warning(Error(FSComp.SR.tcWarnIfFunctionShadowsUnionCase(), headPatRange))
+            | _ -> ()
+        | _ -> ()
+
     let binding = BindingNormalization.NormalizeBinding ValOrMemberBinding cenv env binding
     let explicitTyparInfo, tpenv = TcNonrecBindingTyparDecls cenv env tpenv binding
     TcNormalizedBinding declKind cenv env tpenv ty None NoSafeInitInfo ([], explicitTyparInfo) binding

src/Compiler/FSComp.txt

@@ -1588,6 +1588,7 @@ featureChkNotTailRecursive,"Raises warnings if a member or function has the 'Tai
 featureWhileBang,"'while!' expression"
 featureExtendedFixedBindings,"extended fixed bindings for byref and GetPinnableReference"
 featurePreferStringGetPinnableReference,"prefer String.GetPinnableReference in fixed bindings"
+featureWarnIfFunctionShadowsUnionCase,"Raises a warning when a function definition shadows a union case"
 3354,tcNotAFunctionButIndexerNamedIndexingNotYetEnabled,"This value supports indexing, e.g. '%s.[index]'. The syntax '%s[index]' requires /langversion:preview. See https://aka.ms/fsharp-index-notation."
 3354,tcNotAFunctionButIndexerIndexingNotYetEnabled,"This expression supports indexing, e.g. 'expr.[index]'. The syntax 'expr[index]' requires /langversion:preview. See https://aka.ms/fsharp-index-notation."
 3355,tcNotAnIndexerNamedIndexingNotYetEnabled,"The value '%s' is not a function and does not support index notation."
@@ -1724,3 +1725,4 @@ featureUnmanagedConstraintCsharpInterop,"Interop between C#'s and F#'s unmanaged
 3578,chkCopyUpdateSyntaxInAnonRecords,"This expression is an anonymous record, use {{|...|}} instead of {{...}}."
 3579,alwaysUseTypedStringInterpolation,"Interpolated string contains untyped identifiers. Adding typed format specifiers is recommended."
 3580,tcUnexpectedFunTypeInUnionCaseField,"Unexpected function type in union case field definition. If you intend the field to be a function, consider wrapping the function signature with parens, e.g. | Case of a -> b into | Case of (a -> b)."
+3582,tcWarnIfFunctionShadowsUnionCase,"This is a function definition that shadows a union case. If this is what you want, ignore or suppress this warning. If you want it to be a union case deconstruction, add parantheses."

src/Compiler/Facilities/DiagnosticsLogger.fs

@@ -13,6 +13,8 @@ open System.Threading
 open Internal.Utilities.Library
 open Internal.Utilities.Library.Extras
 
+#nowarn "3582" // Remove this line when the Error function is removed as planned below
+
 /// Represents the style being used to format errors
 [<RequireQualifiedAccess>]
 type DiagnosticStyle =

src/Compiler/Facilities/LanguageFeatures.fs

@@ -79,6 +79,7 @@ type LanguageFeature =
     | WhileBang
     | ExtendedFixedBindings
     | PreferStringGetPinnableReference
+    | WarnIfFunctionShadowsUnionCase
 
 /// LanguageVersion management
 type LanguageVersion(versionText) =
@@ -185,6 +186,7 @@ type LanguageVersion(versionText) =
                 // F# preview
                 LanguageFeature.FromEndSlicing, previewVersion
                 LanguageFeature.UnmanagedConstraintCsharpInterop, previewVersion
+                LanguageFeature.WarnIfFunctionShadowsUnionCase, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -321,6 +323,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.WhileBang -> FSComp.SR.featureWhileBang ()
         | LanguageFeature.ExtendedFixedBindings -> FSComp.SR.featureExtendedFixedBindings ()
         | LanguageFeature.PreferStringGetPinnableReference -> FSComp.SR.featurePreferStringGetPinnableReference ()
+        | LanguageFeature.WarnIfFunctionShadowsUnionCase -> FSComp.SR.featureWarnIfFunctionShadowsUnionCase ()
 
     /// Get a version string associated with the given feature.
     static member GetFeatureVersionString feature =

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -69,6 +69,7 @@ type LanguageFeature =
     | WhileBang
     | ExtendedFixedBindings
     | PreferStringGetPinnableReference
+    | WarnIfFunctionShadowsUnionCase
 
 /// LanguageVersion management
 type LanguageVersion =

src/Compiler/Service/ServiceLexing.fs

@@ -21,6 +21,8 @@ open FSharp.Compiler.Text
 open FSharp.Compiler.Text.Position
 open FSharp.Compiler.Text.Range
 
+#nowarn "3582" // Don't complain about shadowing the token cases by their tags
+
 module FSharpTokenTag =
 
     let Identifier = tagOfToken (IDENT "a")