Improve error reporting for abstract members when used in classes (#17063)

* Improve error reporting for abstract members when used in classes

* Revert "Improve error reporting for abstract members when used in classes"

This reverts commit 49f4722.

* Improve error reporting for abstract members when used in classes

* Update tests

* more tests

* update tests

* release notes

* more tests

* Update src/Compiler/FSStrings.resx

Co-authored-by: Tomas Grosup <tomasgrosup@microsoft.com>

* Update error messages

* Update src/Compiler/FSStrings.resx

Co-authored-by: Petr <psfinaki@users.noreply.github.com>

* Another one

---------

Co-authored-by: Tomas Grosup <tomasgrosup@microsoft.com>
Co-authored-by: Petr <psfinaki@users.noreply.github.com>

Author: 3 people

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>