Improve error reporting: override method does not match abstract member - references #1430 (#1950)

Author: forki

src/fsharp/FSComp.txt

@@ -680,7 +680,7 @@ tcUnnamedArgumentsDoNotFormPrefix,"The unnamed arguments do not form a prefix of
 853,tcAbstractMembersIllegalInAugmentation,"Abstract members are not permitted in an augmentation - they must be defined as part of the type itself"
 854,tcMethodOverridesIllegalHere,"Method overrides and interface implementations are not permitted here"
 855,tcNoMemberFoundForOverride,"No abstract or interface member was found that corresponds to this override"
-856,tcOverrideArityMismatch,"This override takes a different number of arguments to the corresponding abstract member"
+856,tcOverrideArityMismatch,"This override takes a different number of arguments to the corresponding abstract member. The following abstract members were found:%s"
 857,tcDefaultImplementationAlreadyExists,"This method already has a default implementation"
 858,tcDefaultAmbiguous,"The method implemented by this default is ambiguous"
 859,tcNoPropertyFoundForOverride,"No abstract property was found that corresponds to this override"

src/fsharp/TypeChecker.fs

@@ -10597,7 +10597,7 @@ and ComputeIsComplete enclosingDeclaredTypars declaredTypars ty =
 /// Determine if a uniquely-identified-abstract-slot exists for an override member (or interface member implementation) based on the information available 
 /// at the syntactic definition of the member (i.e. prior to type inference). If so, we know the expected signature of the override, and the full slotsig 
 /// it implements. Apply the inferred slotsig. 
-and ApplyAbstractSlotInference cenv (envinner:TcEnv) (bindingTy,m,synTyparDecls,declaredTypars,memberId,tcrefObjTy,renaming,_objTy,optIntfSlotTy,valSynData,memberFlags,attribs) = 
+and ApplyAbstractSlotInference (cenv:cenv) (envinner:TcEnv) (bindingTy,m,synTyparDecls,declaredTypars,memberId,tcrefObjTy,renaming,_objTy,optIntfSlotTy,valSynData,memberFlags,attribs) = 
 
     let ad = envinner.eAccessRights
     let typToSearchForAbstractMembers = 
@@ -10627,11 +10627,17 @@ and ApplyAbstractSlotInference cenv (envinner:TcEnv) (bindingTy,m,synTyparDecls,
                      errorR(Error(FSComp.SR.tcNoMemberFoundForOverride(),memberId.idRange))
                      []
 
-                 | _ -> 
+                 | slots -> 
                      match dispatchSlotsArityMatch with 
                      | meths when meths |> makeUniqueBySig |> List.length = 1 -> meths
                      | [] -> 
-                         errorR(Error(FSComp.SR.tcOverrideArityMismatch(),memberId.idRange))
+                         let details =
+                             slots
+                             |> List.map (NicePrint.stringOfMethInfo cenv.amap m envinner.DisplayEnv)
+                             |> Seq.map (sprintf "%s   %s" System.Environment.NewLine)
+                             |> String.concat ""
+
+                         errorR(Error(FSComp.SR.tcOverrideArityMismatch(details),memberId.idRange))
                          []
                      | _ -> [] // check that method to override is sealed is located at CheckOverridesAreAllUsedOnce (typrelns.fs)
                       // We hit this case when it is ambiguous which abstract method is being implemented. 

tests/fsharpqa/Source/Warnings/OverrideErrors.fs

@@ -0,0 +1,22 @@
+// #Warnings
+//<Expects status="Error" span="(12,16)" id="FS0856">This override takes a different number of arguments to the corresponding abstract member. The following abstract members were found:</Expects>
+//<Expects>abstract member Base.Member : int * string -> string</Expects>
+//<Expects status="Error" span="(20,24)" id="FS0001">This expression was expected to have type</Expects>
+
+type Base() =
+    abstract member Member: int * string -> string
+    default x.Member (i, s) = s
+
+type Derived1() =
+    inherit Base()
+    override x.Member() = 5
+
+type Derived2() =
+    inherit Base()
+    override x.Member (i : int) = "Hello"
+
+type Derived3() =
+    inherit Base()
+    override x.Member (s : string, i : int) = sprintf "Hello %s" s
+    
+exit 0

tests/fsharpqa/Source/Warnings/env.lst

@@ -9,6 +9,7 @@
 	SOURCE=ValidCommaInRecCtor.fs # ValidCommaInRecCtor.fs
 	SOURCE=FS0988AtEndOfFile.fs # FS0988AtEndOfFile.fs
 	SOURCE=WrongArity.fs # WrongArity.fs
+	SOURCE=OverrideErrors.fs # OverrideErrors.fs
 	SOURCE=AccessOfTypeAbbreviation.fs # AccessOfTypeAbbreviation.fs
 	SOURCE=AccessOfTypeAbbreviation2.fs # AccessOfTypeAbbreviation2.fs
 	SOURCE=AccessOfTypeAbbreviation3.fs # AccessOfTypeAbbreviation3.fs