Improve error reporting: ambiguous override method in object expression

docs/release-notes/.FSharp.Compiler.Service/8.0.300.md

@@ -1,6 +1,7 @@
 ### Fixed
 
 * Fix a false positive of the `[<TailCall>]` analysis in combination with `yield!`. ([PR #16933](https://github.com/dotnet/fsharp/pull/16933))
+* Improve error reporting: ambiguous override method in object expression. ([PR #16985](https://github.com/dotnet/fsharp/pull/16985))
 * Don't blow the stack when traversing deeply nested sequential expressions. ([PR #16882](https://github.com/dotnet/fsharp/pull/16882))
 * Fix wrong range start of INTERP_STRING_END. ([PR #16774](https://github.com/dotnet/fsharp/pull/16774), [PR #16785](https://github.com/dotnet/fsharp/pull/16785))
 * Fix missing warning for recursive calls in list comprehensions. ([PR #16652](https://github.com/dotnet/fsharp/pull/16652))

src/Compiler/Checking/MethodOverrides.fs

@@ -416,7 +416,7 @@ module DispatchSlotChecking =
                                     fail(Error(FSComp.SR.typrelMemberDoesNotHaveCorrectNumberOfTypeParameters(FormatOverride denv overrideBy, FormatMethInfoSig g amap m denv dispatchSlot), overrideBy.Range))
                                 elif not (IsTyparKindMatch compiledSig overrideBy) then
                                     fail(Error(FSComp.SR.typrelMemberDoesNotHaveCorrectKindsOfGenericParameters(FormatOverride denv overrideBy, FormatMethInfoSig g amap m denv dispatchSlot), overrideBy.Range))
-                                else 
+                                else
                                     fail(Error(FSComp.SR.typrelMemberCannotImplement(FormatOverride denv overrideBy, NicePrint.stringOfMethInfo infoReader m denv dispatchSlot, FormatMethInfoSig g amap m denv dispatchSlot), overrideBy.Range))
                         | overrideBy :: _ -> 
                             errorR(Error(FSComp.SR.typrelOverloadNotFound(FormatMethInfoSig g amap m denv dispatchSlot, FormatMethInfoSig g amap m denv dispatchSlot), overrideBy.Range))
@@ -427,8 +427,8 @@ module DispatchSlotChecking =
                         else
                             // Error will be reported below in CheckOverridesAreAllUsedOnce 
                             ()
-                    | _ -> 
-                        fail(Error(FSComp.SR.typrelOverrideWasAmbiguous(FormatMethInfoSig g amap m denv dispatchSlot), m))
+                    | ambiguousOverride :: _ ->
+                        fail(Error(FSComp.SR.typrelOverrideWasAmbiguous(FormatMethInfoSig g amap ambiguousOverride.Range denv dispatchSlot), ambiguousOverride.Range))
             | _ -> fail(Error(FSComp.SR.typrelMoreThenOneOverride(FormatMethInfoSig g amap m denv dispatchSlot), m))
 
         if missingOverloadImplementation.Count > 0 then

tests/FSharp.Compiler.ComponentTests/Conformance/Expressions/ObjectExpressions/ObjectExpressions.fs

@@ -330,4 +330,32 @@ consoleLogger.Log("Hello World")
         """
          |> withLangVersion80
          |> compileExeAndRun
-         |> shouldSucceed
+         |> shouldSucceed
+
+    [<Fact>]
+    let ``Error reporting ambiguous override method in object expression`` () =
+        Fsx """
+type IExample =
+    abstract member Overloaded : string -> bool
+    abstract member Overloaded : int -> bool
+
+let failingExample x =
+    { new IExample with
+        member __.Overloaded (_ : string) = x
+        member __.Overloaded (_ : int)    = x }
+        """
+        |> typecheck
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 3213, Line 8, Col 19, Line 8, Col 29, "The member 'Overloaded: string -> 'a' matches multiple overloads of the same method.
+Please restrict it to one of the following:
+   Overloaded: int -> bool
+   Overloaded: string -> bool.")
+            (Error 3213, Line 9, Col 19, Line 9, Col 29, "The member 'Overloaded: int -> 'a' matches multiple overloads of the same method.
+Please restrict it to one of the following:
+   Overloaded: int -> bool
+   Overloaded: string -> bool.")
+            (Error 358, Line 8, Col 19, Line 8, Col 29, "The override for 'Overloaded: int -> bool' was ambiguous")
+            (Error 358, Line 8, Col 19, Line 8, Col 29, "The override for 'Overloaded: string -> bool' was ambiguous")
+            (Error 783, Line 7, Col 11, Line 7, Col 19, "At least one override did not correctly implement its corresponding abstract member")
+        ]

tests/fsharp/Compiler/Language/DefaultInterfaceMemberTests.fs

@@ -938,7 +938,7 @@ type Test2 () =
 
         CompilerAssert.CompileWithErrors(fsCmpl, [|
             (FSharpDiagnosticSeverity.Error, 3350, (10, 15, 10, 22), "Feature 'default interface member consumption' is not available in F# 4.6. Please use language version " + targetVersion + " or greater.")
-            (FSharpDiagnosticSeverity.Error, 358, (10, 15, 10, 22), "The override for 'M<'U> : 'U * int -> unit' was ambiguous")
+            (FSharpDiagnosticSeverity.Error, 358, (14, 19, 14, 20), "The override for 'M<'U> : 'U * int -> unit' was ambiguous")
         |])
 
 #else