Default-off warning if function shadows union case

src/Compiler/Checking/CheckExpressions.fs

@@ -10759,6 +10759,15 @@ and TcNonrecBindingTyparDecls cenv env tpenv bind =
     TcBindingTyparDecls true cenv env tpenv synTyparDecls
 
 and TcNonRecursiveBinding declKind cenv env tpenv ty binding =
+    // Check for unintended shadowing
+    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.tcInfoIfFunctionShadowsUnionCase(), 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/Driver/CompilerDiagnostics.fs

@@ -388,6 +388,7 @@ type PhasedDiagnostic with
         | 3395 -> false // tcImplicitConversionUsedForMethodArg - off by default
         | 3559 -> false // typrelNeverRefinedAwayFromTop - off by default
         | 3579 -> false // alwaysUseTypedStringInterpolation - off by default
+        | 3582 -> false // infoIfFunctionShadowsUnionCase - off by default
         | _ ->
             match x.Exception with
             | DiagnosticEnabledWithLanguageFeature (_, _, _, enabled) -> enabled

src/Compiler/FSComp.txt

@@ -1724,3 +1724,4 @@ 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,tcInfoIfFunctionShadowsUnionCase,"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 parentheses."