Propose to use :> instead of :?> - fixes dotnet#1127

src/fsharp/CompileOps.fs

@@ -119,6 +119,7 @@ let GetRangeOfError(err:PhasedError) =
       | ErrorFromAddingTypeEquation(_,_,_,_,_,m) 
       | ErrorFromApplyingDefault(_,_,_,_,_,m) 
       | ErrorsFromAddingSubsumptionConstraint(_,_,_,_,_,m) 
+      | ErrorsFromAddingSubsumptionConstraintWithDetails(_,_,_,_,_,_,m) 
       | FunctionExpected(_,_,m)
       | BakedInMemberConstraintName(_,m)
       | StandardOperatorRedefinitionWarning(_,m)
@@ -416,6 +417,9 @@ let SplitRelatedErrors(err:PhasedError) =
       | ErrorsFromAddingSubsumptionConstraint(g,denv,t1,t2,e,m) ->  
           let e,related = SplitRelatedException e
           ErrorsFromAddingSubsumptionConstraint(g,denv,t1,t2,e.Exception,m)|>ToPhased, related
+      | ErrorsFromAddingSubsumptionConstraintWithDetails(g,denv,t1,t2,e,details,m) ->  
+          let e,related = SplitRelatedException e
+          ErrorsFromAddingSubsumptionConstraintWithDetails(g,denv,t1,t2,e.Exception,details,m)|>ToPhased, related
       | ErrorFromAddingConstraint(x,e,m) ->  
           let e,related = SplitRelatedException e
           ErrorFromAddingConstraint(x,e.Exception,m)|>ToPhased, related
@@ -663,6 +667,14 @@ let OutputPhasedErrorR (os:System.Text.StringBuilder) (err:PhasedError) =
                   os.Append(ErrorsFromAddingSubsumptionConstraintE().Format t2 t1 tpcs) |> ignore
           )
           OutputExceptionR os e
+      | ErrorsFromAddingSubsumptionConstraintWithDetails(g,denv,t1,t2,e,details,_) ->  
+          if not (typeEquiv g t1 t2) then (
+              let t1,t2,tpcs = NicePrint.minimalStringsOfTwoTypes denv t1 t2
+              if t1 <> (t2 + tpcs) then 
+                  os.Append(ErrorsFromAddingSubsumptionConstraintE().Format t2 t1 tpcs) |> ignore                  
+          )
+          OutputExceptionR os e
+          os.Append(System.Environment.NewLine + details()) |> ignore
       | UpperCaseIdentifierInPattern(_) -> 
           os.Append(UpperCaseIdentifierInPatternE().Format) |> ignore
       | NotUpperCaseConstructor(_) -> 

src/fsharp/ConstraintSolver.fs

@@ -126,6 +126,7 @@ exception ConstraintSolverRelatedInformation of string option * range * exn
 exception ErrorFromApplyingDefault of TcGlobals * DisplayEnv * Tast.Typar * TType * exn * range
 exception ErrorFromAddingTypeEquation of TcGlobals * DisplayEnv * TType * TType * exn * range
 exception ErrorsFromAddingSubsumptionConstraint of TcGlobals * DisplayEnv * TType * TType * exn * range
+exception ErrorsFromAddingSubsumptionConstraintWithDetails of TcGlobals * DisplayEnv * TType * TType * exn * (unit -> string) * range
 exception ErrorFromAddingConstraint of  DisplayEnv * exn * range
 exception PossibleOverload of DisplayEnv * string * exn * range
 exception UnresolvedOverloading of DisplayEnv * exn list * string * range
@@ -1912,6 +1913,13 @@ and private SolveTypSubsumesTypWithReport (csenv:ConstraintSolverEnv) ndeep m tr
     TryD (fun () -> SolveTypSubsumesTypKeepAbbrevs csenv ndeep m trace ty1 ty2)
          (fun res -> ErrorD (ErrorsFromAddingSubsumptionConstraint(csenv.g,csenv.DisplayEnv,ty1,ty2,res,m)))
 
+and private SolveTypSubsumesTypWithSpecialReport (csenv:ConstraintSolverEnv) ndeep m trace ty1 ty2 = 
+    TryD (fun () -> SolveTypSubsumesTypKeepAbbrevs csenv ndeep m trace ty1 ty2)
+         (fun res ->
+            match SolveTypSubsumesTypKeepAbbrevs csenv ndeep m trace ty2 ty1 with // test if we can cast other way around
+            | OkResult _ -> ErrorD (ErrorsFromAddingSubsumptionConstraintWithDetails(csenv.g,csenv.DisplayEnv,ty1,ty2,res,FSComp.SR.considerDownCast,m))
+            | _ -> ErrorD (ErrorsFromAddingSubsumptionConstraint(csenv.g,csenv.DisplayEnv,ty1,ty2,res,m)))
+
 and private SolveTypEqualsTypWithReport (csenv:ConstraintSolverEnv) ndeep  m trace ty1 ty2 = 
     TryD (fun () -> SolveTypEqualsTypKeepAbbrevs csenv ndeep m trace ty1 ty2)
          (fun res -> ErrorD (ErrorFromAddingTypeEquation(csenv.g,csenv.DisplayEnv,ty1,ty2,res,m)))
@@ -2431,6 +2439,10 @@ let AddCxTypeMustSubsumeType denv css m trace ty1 ty2 =
     SolveTypSubsumesTypWithReport (MakeConstraintSolverEnv css m denv) 0 m trace ty1 ty2
     |> RaiseOperationResult
 
+let AddCxTypeMustSubsumeTypeSpecialReport denv css m trace ty1 ty2 = 
+    SolveTypSubsumesTypWithSpecialReport (MakeConstraintSolverEnv css m denv) 0 m trace ty1 ty2
+    |> RaiseOperationResult
+
 let AddCxMethodConstraint denv css m trace traitInfo  =
     TryD (fun () -> SolveMemberConstraint (MakeConstraintSolverEnv css m denv) false 0 m trace traitInfo ++ (fun _ -> CompleteD))
          (fun res -> ErrorD (ErrorFromAddingConstraint(denv,res,m)))

src/fsharp/ConstraintSolver.fsi

@@ -57,6 +57,7 @@ exception ConstraintSolverRelatedInformation            of string option * range
 exception ErrorFromApplyingDefault                      of TcGlobals * DisplayEnv * Typar * TType * exn * range
 exception ErrorFromAddingTypeEquation                   of TcGlobals * DisplayEnv * TType * TType * exn * range
 exception ErrorsFromAddingSubsumptionConstraint         of TcGlobals * DisplayEnv * TType * TType * exn * range
+exception ErrorsFromAddingSubsumptionConstraintWithDetails of TcGlobals * DisplayEnv * TType * TType * exn * (unit -> string) * range
 exception ErrorFromAddingConstraint                     of DisplayEnv * exn * range
 exception UnresolvedConversionOperator                  of DisplayEnv * TType * TType * range
 exception PossibleOverload                              of DisplayEnv * string * exn * range
@@ -97,6 +98,7 @@ val AddCxTypeEqualsType                       : DisplayEnv -> ConstraintSolverSt
 val AddCxTypeEqualsTypeUndoIfFailed           : DisplayEnv -> ConstraintSolverState -> range -> TType -> TType -> bool
 val AddCxTypeEqualsTypeMatchingOnlyUndoIfFailed : DisplayEnv -> ConstraintSolverState -> range -> TType -> TType -> bool
 val AddCxTypeMustSubsumeType                  : DisplayEnv -> ConstraintSolverState -> range -> OptionalTrace -> TType -> TType -> unit
+val AddCxTypeMustSubsumeTypeSpecialReport     : DisplayEnv -> ConstraintSolverState -> range -> OptionalTrace -> TType -> TType -> unit
 val AddCxTypeMustSubsumeTypeUndoIfFailed      : DisplayEnv -> ConstraintSolverState -> range -> TType -> TType -> bool
 val AddCxTypeMustSubsumeTypeMatchingOnlyUndoIfFailed : DisplayEnv -> ConstraintSolverState -> range -> TType -> TType -> bool
 val AddCxMethodConstraint                     : DisplayEnv -> ConstraintSolverState -> range -> OptionalTrace -> TraitConstraintInfo -> unit

src/fsharp/FSComp.txt

@@ -14,6 +14,7 @@ undefinedNameRecordLabel,"The record label '%s' is not defined"
 undefinedNameTypeParameter,"The type parameter '%s' is not defined"
 undefinedNamePatternDiscriminator,"The pattern discriminator '%s' is not defined"
 buildUnexpectedTypeArgs,"The non-generic type '%s' does not expect any type arguments, but here is given %d type argument(s)"
+considerDownCast,"Consider using the :> operator."
 203,buildInvalidWarningNumber,"Invalid warning number '%s'"
 204,buildInvalidVersionString,"Invalid version string '%s'"
 205,buildInvalidVersionFile,"Invalid version file '%s'"

src/fsharp/TypeChecker.fs

@@ -2834,7 +2834,7 @@ let TcRuntimeTypeTest isCast cenv denv m tgty srcTy =
     if isSealedTy cenv.g tgty ||
        isTyparTy cenv.g tgty ||
        not (isInterfaceTy cenv.g srcTy) then 
-        AddCxTypeMustSubsumeType denv cenv.css m NoTrace srcTy tgty
+        AddCxTypeMustSubsumeTypeSpecialReport denv cenv.css m NoTrace srcTy tgty
 
     if isErasedType cenv.g tgty then
         if isCast then