Better error message when abstract member impl is wrong

src/fsharp/FSComp.txt

@@ -22,6 +22,7 @@ derefInsteadOfNot,"The '!' operator is used to dereference a ref cell. Consider
 buildUnexpectedTypeArgs,"The non-generic type '%s' does not expect any type arguments, but here is given %d type argument(s)"
 returnUsedInsteadOfReturnBang,"Consider using 'return!' instead of 'return'."
 yieldUsedInsteadOfYieldBang,"Consider using 'yield!' instead of 'yield'."
+tupleRequiredInAbstractMethod,"\nA tuple type is required for one or more arguments. Consider wrapping the given arguments in additional parentheses or review the definition of the interface."
 203,buildInvalidWarningNumber,"Invalid warning number '%s'"
 204,buildInvalidVersionString,"Invalid version string '%s'"
 205,buildInvalidVersionFile,"Invalid version file '%s'"
@@ -592,8 +593,8 @@ tcExpressionWithIfRequiresParenthesis,"This list or array expression includes an
 766,tcObjectExpressionsCanOnlyOverrideAbstractOrVirtual,"Only overrides of abstract and virtual members may be specified in object expressions"
 767,tcNoAbstractOrVirtualMemberFound,"The member '%s' does not correspond to any abstract or virtual method available to override or implement.%s"
 767,tcMemberFoundIsNotAbstractOrVirtual,"The type %s contains the member '%s' but it is not a virtual or abstract method that is available to override or implement.%s"
-768,tcArgumentArityMismatch,"The member '%s' does not accept the correct number of arguments, %d arguments are expected"
-769,tcArgumentArityMismatchOneOverload,"The member '%s' does not accept the correct number of arguments. One overload accepts %d arguments."
+768,tcArgumentArityMismatch,"The member '%s' does not accept the correct number of arguments. %d argument(s) are expected, but %d were given. The required signature is '%s'.%s"
+769,tcArgumentArityMismatchOneOverload,"The member '%s' does not accept the correct number of arguments. One overload accepts %d arguments, but %d were given. The required signature is '%s'.%s"
 770,tcSimpleMethodNameRequired,"A simple method name is required here"
 771,tcPredefinedTypeCannotBeUsedAsSuperType,"The types System.ValueType, System.Enum, System.Delegate, System.MulticastDelegate and System.Array cannot be used as super types in an object expression or class"
 772,tcNewMustBeUsedWithNamedType,"'new' must be used with a named type"

src/fsharp/TypeChecker.fs

@@ -6210,11 +6210,23 @@ and GetNameAndArityOfObjExprBinding _cenv _env b =
         lookPat pat
 
 
-and FreshenObjExprAbstractSlot cenv (_env: TcEnv) (implty:TType) virtNameAndArityPairs (bind,bindAttribs,bindName,absSlots:(_ * MethInfo) list) = 
-    let (NormalizedBinding (_,_,_,_,_,_,synTyparDecls,_,_,_,mBinding,_)) = bind 
+and FreshenObjExprAbstractSlot cenv (env: TcEnv) (implty:TType) virtNameAndArityPairs (bind,bindAttribs,bindName,absSlots:(_ * MethInfo) list) = 
+    let (NormalizedBinding (_,_,_,_,_,_,synTyparDecls,_,_,_,mBinding,_)) = bind
     match absSlots with 
     | [] when not (CompileAsEvent cenv.g bindAttribs) -> 
         let absSlotsByName = List.filter (fst >> fst >> (=) bindName) virtNameAndArityPairs
+        let getSignature absSlot = (NicePrint.stringOfMethInfo cenv.amap mBinding env.DisplayEnv absSlot).Replace("abstract ","")
+        let getDetails (absSlot:MethInfo) = 
+            if absSlot.GetParamTypes(cenv.amap,mBinding,[]) |> List.existsSquared (isTupleTy cenv.g) then
+                FSComp.SR.tupleRequiredInAbstractMethod()
+            else ""
+
+        // Compute the argument counts of the member arguments
+        let _,synValInfo = GetNameAndArityOfObjExprBinding cenv env bind
+        let arity = 
+            match SynInfo.AritiesOfArgs synValInfo with            
+            | _::x::_ -> x
+            | _ -> 0
 
         match absSlotsByName with 
         | [] ->
@@ -6232,8 +6244,10 @@ and FreshenObjExprAbstractSlot cenv (_env: TcEnv) (implty:TType) virtNameAndArit
                 errorR(Error(FSComp.SR.tcMemberFoundIsNotAbstractOrVirtual(tcref.DisplayName, bindName, ErrorResolutionHints.FormatPredictions predictions),mBinding))                
             else
                 errorR(Error(FSComp.SR.tcNoAbstractOrVirtualMemberFound(bindName, ErrorResolutionHints.FormatPredictions predictions),mBinding))
-        | [(_,absSlot:MethInfo)]     -> errorR(Error(FSComp.SR.tcArgumentArityMismatch(bindName, (List.sum absSlot.NumArgs)),mBinding))
-        | (_,absSlot:MethInfo) :: _  -> errorR(Error(FSComp.SR.tcArgumentArityMismatchOneOverload(bindName, (List.sum absSlot.NumArgs)),mBinding))
+        | [(_,absSlot:MethInfo)]     ->
+            errorR(Error(FSComp.SR.tcArgumentArityMismatch(bindName, List.sum absSlot.NumArgs, arity, getSignature absSlot, getDetails absSlot),mBinding))
+        | (_,absSlot:MethInfo) :: _  ->
+            errorR(Error(FSComp.SR.tcArgumentArityMismatchOneOverload(bindName, List.sum absSlot.NumArgs, arity, getSignature absSlot, getDetails absSlot),mBinding))
 
         None
 

tests/fsharpqa/Source/Warnings/TupleInAbstractMethod.fs

@@ -0,0 +1,13 @@
+// #Warnings
+//<Expects status="Error" span="(10,16)" id="FS0768">The member 'Function' does not accept the correct number of arguments.</Expects>
+//<Expects>A tuple type is required for one or more arguments</Expects>
+
+type IInterface =
+    abstract Function : (int32 * int32) -> unit
+
+let x = 
+  { new IInterface with
+        member this.Function (i, j) = ()
+  }
+
+exit 0

tests/fsharpqa/Source/Warnings/env.lst

@@ -2,6 +2,7 @@
 	SOURCE=WarnIfMissingElseBranch.fs # WarnIfMissingElseBranch.fs
 	SOURCE=ReturnInsteadOfReturnBang.fs # ReturnInsteadOfReturnBang.fs
 	SOURCE=YieldInsteadOfYieldBang.fs # YieldInsteadOfYieldBang.fs
+	SOURCE=TupleInAbstractMethod.fs # TupleInAbstractMethod.fs
 	SOURCE=InvalidRecord.fs # InvalidRecord.fs
 	SOURCE=CommaInRecCtor.fs # CommaInRecCtor.fs
 	SOURCE=ValidCommaInRecCtor.fs # ValidCommaInRecCtor.fs