Improve error reporting for abstract members when used in classes

docs/release-notes/.FSharp.Compiler.Service/8.0.400.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Improve error reporting for abstract members when used in classes. ([PR #17063](https://github.com/dotnet/fsharp/pull/17063))
 * Make typechecking of indexed setters with tuples on the right more consistent. ([Issue #16987](https://github.com/dotnet/fsharp/issues/16987), [PR #17017](https://github.com/dotnet/fsharp/pull/17017))
 * Static abstract method on classes no longer yields internal error. ([Issue #17044](https://github.com/dotnet/fsharp/issues/17044), [PR #17055](https://github.com/dotnet/fsharp/pull/17055))
 * Disallow calling abstract methods directly on interfaces. ([Issue #14012](https://github.com/dotnet/fsharp/issues/14012), [Issue #16299](https://github.com/dotnet/fsharp/issues/16299), [PR #17021](https://github.com/dotnet/fsharp/pull/17021))

src/Compiler/Checking/MethodOverrides.fs

@@ -831,7 +831,7 @@ module DispatchSlotChecking =
                     let allCorrect = CheckDispatchSlotsAreImplemented (denv, infoReader, m, nenv, sink, tcaug.tcaug_abstract, false, reqdTy, dispatchSlots, availPriorOverrides, overrides)
 
                     // Tell the user to mark the thing abstract if it was missing implementations
-                    if not allCorrect && not tcaug.tcaug_abstract && not (isInterfaceTy g reqdTy) then 
+                    if not allCorrect && not tcaug.tcaug_abstract && (isClassTy g reqdTy) then
                         errorR(TypeIsImplicitlyAbstract(m))
 
                     let overridesToCheck = 

src/Compiler/FSStrings.resx

@@ -220,7 +220,7 @@
     <value>A coercion from the value type \n    {0}    \nto the type \n    {1}    \nwill involve boxing. Consider using 'box' instead</value>
   </data>
   <data name="TypeIsImplicitlyAbstract" xml:space="preserve">
-    <value>This type is 'abstract' since some abstract members have not been given an implementation. If this is intentional then add the '[&lt;AbstractClass&gt;]' attribute to your type.</value>
+    <value>Non-abstract classes cannot contain abstract members. Either provide a default member implementation or add the '[&lt;AbstractClass&gt;]' attribute to your type.</value>
   </data>
   <data name="NonRigidTypar1" xml:space="preserve">
     <value>This construct causes code to be less generic than indicated by its type annotations. The type variable implied by the use of a '#', '_' or other type annotation at or near '{0}' has been constrained to be type '{1}'.</value>

tests/FSharp.Compiler.ComponentTests/Conformance/GeneratedEqualityHashingComparison/Basic/Basic.fs

@@ -62,15 +62,12 @@ module Basic =
             (Error 344, Line 13, Col 6, Line 13, Col 7, "The struct, record or union type 'R' has an explicit implementation of 'Object.GetHashCode' or 'Object.Equals'. You must apply the 'CustomEquality' attribute to the type")
             (Warning 345, Line 13, Col 6, Line 13, Col 7, "The struct, record or union type 'R' has an explicit implementation of 'Object.GetHashCode'. Consider implementing a matching override for 'Object.Equals(obj)'")
             (Error 359, Line 13, Col 6, Line 13, Col 7, "More than one override implements 'GetHashCode: unit -> int'")
-            (Error 54, Line 13, Col 6, Line 13, Col 7, "This type is 'abstract' since some abstract members have not been given an implementation. If this is intentional then add the '[<AbstractClass>]' attribute to your type.")
             (Error 344, Line 17, Col 6, Line 17, Col 7, "The struct, record or union type 'U' has an explicit implementation of 'Object.GetHashCode' or 'Object.Equals'. You must apply the 'CustomEquality' attribute to the type")
             (Warning 345, Line 17, Col 6, Line 17, Col 7, "The struct, record or union type 'U' has an explicit implementation of 'Object.GetHashCode'. Consider implementing a matching override for 'Object.Equals(obj)'")
             (Error 359, Line 17, Col 6, Line 17, Col 7, "More than one override implements 'GetHashCode: unit -> int'")
-            (Error 54, Line 17, Col 6, Line 17, Col 7, "This type is 'abstract' since some abstract members have not been given an implementation. If this is intentional then add the '[<AbstractClass>]' attribute to your type.")
             (Error 344, Line 21, Col 6, Line 21, Col 7, "The struct, record or union type 'S' has an explicit implementation of 'Object.GetHashCode' or 'Object.Equals'. You must apply the 'CustomEquality' attribute to the type")
             (Warning 345, Line 21, Col 6, Line 21, Col 7, "The struct, record or union type 'S' has an explicit implementation of 'Object.GetHashCode'. Consider implementing a matching override for 'Object.Equals(obj)'")
             (Error 359, Line 21, Col 6, Line 21, Col 7, "More than one override implements 'GetHashCode: unit -> int'")
-            (Error 54, Line 21, Col 6, Line 21, Col 7, "This type is 'abstract' since some abstract members have not been given an implementation. If this is intentional then add the '[<AbstractClass>]' attribute to your type.")
            ]
 
     // SOURCE=E_ExceptionsNoComparison.fs SCFLAGS="--test:ErrorRanges"          # E_ExceptionsNoComparison.fs

tests/FSharp.Compiler.ComponentTests/Conformance/Types/UnionTypes/UnionStructTypes.fs

@@ -752,6 +752,18 @@ type GenericStructDu<'T> = EmptyFirst | SingleVal of f:'T | DoubleVal of f2:'T *
         """
         |> compile
         |> shouldSucceed
+
+    [<Fact>]
+    let ``Error when declaring an abstract member in union struct type`` () =
+        Fsx """
+[<Struct>]
+type U = 
+  | A | B
+  abstract M : unit -> unit
+       """
+        |> typecheck 
+        |> shouldFail
+        |> withSingleDiagnostic (Error 912, Line 5, Col 3, Line 5, Col 28, "This declaration element is not permitted in an augmentation")
 
     [<Fact>]
     let ``Regression 16282 DefaultAugment false on a struct union with fields`` ()  =

tests/FSharp.Compiler.ComponentTests/Conformance/Types/UnionTypes/UnionTypes.fs

@@ -709,3 +709,15 @@ if (sprintf "%%A" ABC.ab) <> "[A; B]" then failwith (sprintf "Failed: printing '
         |> asExe
         |> compileAndRun
         |> shouldSucceed
+
+    [<Fact>]
+    let ``Error when declaring an abstract member in union type`` () =
+        Fsx """
+type U = 
+  | A | B
+  abstract M : unit -> unit
+       """
+        |> typecheck 
+        |> shouldFail
+        |>  withSingleDiagnostic (Error 912, Line 4, Col 3, Line 4, Col 28, "This declaration element is not permitted in an augmentation")
+