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

src/fsharp/CompileOps.fs

@@ -118,7 +118,7 @@ let GetRangeOfError(err:PhasedError) =
       | IndentationProblem(_,m)
       | ErrorFromAddingTypeEquation(_,_,_,_,_,_,m) 
       | ErrorFromApplyingDefault(_,_,_,_,_,m) 
-      | ErrorsFromAddingSubsumptionConstraint(_,_,_,_,_,m) 
+      | ErrorsFromAddingSubsumptionConstraint(_,_,_,_,_,_,m)
       | FunctionExpected(_,_,m)
       | BakedInMemberConstraintName(_,m)
       | StandardOperatorRedefinitionWarning(_,m)
@@ -369,6 +369,7 @@ let GetErrorNumber(err:PhasedError) =
 #if EXTENSIONTYPING
       | :? TypeProviderError as e -> e.Number
 #endif
+      | ErrorsFromAddingSubsumptionConstraint (_,_,_,_,_,ContextInfo.DowncastUsedInsteadOfUpcast _,_) -> fst (FSComp.SR.considerUpcast("",""))
       | _ -> 193
    GetFromException err.Exception
 
@@ -413,9 +414,9 @@ let SplitRelatedErrors(err:PhasedError) =
       | ErrorFromApplyingDefault(g,denv,tp,defaultType,e,m) ->  
           let e,related = SplitRelatedException e
           ErrorFromApplyingDefault(g,denv,tp,defaultType,e.Exception,m)|>ToPhased, related
-      | ErrorsFromAddingSubsumptionConstraint(g,denv,t1,t2,e,m) ->  
+      | ErrorsFromAddingSubsumptionConstraint(g,denv,t1,t2,e,contextInfo,m) ->  
           let e,related = SplitRelatedException e
-          ErrorsFromAddingSubsumptionConstraint(g,denv,t1,t2,e.Exception,m)|>ToPhased, related
+          ErrorsFromAddingSubsumptionConstraint(g,denv,t1,t2,e.Exception,contextInfo,m)|>ToPhased, related
       | ErrorFromAddingConstraint(x,e,m) ->  
           let e,related = SplitRelatedException e
           ErrorFromAddingConstraint(x,e.Exception,m)|>ToPhased, related
@@ -662,13 +663,23 @@ let OutputPhasedErrorR (os:System.Text.StringBuilder) (err:PhasedError) =
           os.Append(ErrorFromApplyingDefault1E().Format defaultType) |> ignore
           OutputExceptionR os e
           os.Append(ErrorFromApplyingDefault2E().Format) |> ignore
-      | ErrorsFromAddingSubsumptionConstraint(g,denv,t1,t2,e,_) ->  
-          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
+      | ErrorsFromAddingSubsumptionConstraint(g,denv,t1,t2,e,contextInfo,_) ->
+          match contextInfo with
+          | ContextInfo.DowncastUsedInsteadOfUpcast isOperator -> 
+              let t1,t2,_ = NicePrint.minimalStringsOfTwoTypes denv t1 t2
+              if isOperator then
+                  os.Append(FSComp.SR.considerUpcastOperator(t1,t2) |> snd) |> ignore
+              else
+                  os.Append(FSComp.SR.considerUpcast(t1,t2) |> snd) |> ignore
+          | _ ->
+              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
+                  else
+                      OutputExceptionR os e
+              else
+                  OutputExceptionR os e
       | UpperCaseIdentifierInPattern(_) -> 
           os.Append(UpperCaseIdentifierInPatternE().Format) |> ignore
       | NotUpperCaseConstructor(_) -> 

src/fsharp/ConstraintSolver.fs

@@ -128,6 +128,10 @@ type ContextInfo =
 | RecordFields
 /// The type equation comes from the verification of a tuple in record fields.
 | TupleInRecordFields
+/// The type equation comes from a runtime type test.
+| RuntimeTypeTest of bool
+/// The type equation comes from an downcast where a upcast could be used.
+| DowncastUsedInsteadOfUpcast of bool
 
 exception ConstraintSolverTupleDiffLengths of DisplayEnv * TType list * TType list * range  * range 
 exception ConstraintSolverInfiniteTypes of DisplayEnv * TType * TType * range * range
@@ -139,7 +143,7 @@ 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 * ContextInfo * range
-exception ErrorsFromAddingSubsumptionConstraint of TcGlobals * DisplayEnv * TType * TType * exn * range
+exception ErrorsFromAddingSubsumptionConstraint of TcGlobals * DisplayEnv * TType * TType * exn * ContextInfo * range
 exception ErrorFromAddingConstraint of  DisplayEnv * exn * range
 exception PossibleOverload of DisplayEnv * string * exn * range
 exception UnresolvedOverloading of DisplayEnv * exn list * string * range
@@ -1922,9 +1926,16 @@ and private DefinitelyEquiv (csenv:ConstraintSolverEnv) isConstraint calledArg (
 
 // Assert a subtype constraint, and wrap an ErrorsFromAddingSubsumptionConstraint error around any failure 
 // to allow us to report the outer types involved in the constraint 
-and private SolveTypSubsumesTypWithReport (csenv:ConstraintSolverEnv) ndeep m trace ty1 ty2 = 
+and private SolveTypSubsumesTypWithReport contextInfo (csenv:ConstraintSolverEnv) ndeep m trace ty1 ty2 =
     TryD (fun () -> SolveTypSubsumesTypKeepAbbrevs csenv ndeep m trace ty1 ty2)
-         (fun res -> ErrorD (ErrorsFromAddingSubsumptionConstraint(csenv.g,csenv.DisplayEnv,ty1,ty2,res,m)))
+         (fun res ->
+            match contextInfo with
+            | ContextInfo.RuntimeTypeTest isOperator ->
+                // test if we can cast other way around
+                match CollectThenUndo (fun newTrace -> SolveTypSubsumesTypKeepAbbrevs csenv ndeep m (OptionalTrace.WithTrace newTrace) ty2 ty1) with 
+                | OkResult _ -> ErrorD (ErrorsFromAddingSubsumptionConstraint(csenv.g,csenv.DisplayEnv,ty1,ty2,res,ContextInfo.DowncastUsedInsteadOfUpcast isOperator,m))
+                | _ -> ErrorD (ErrorsFromAddingSubsumptionConstraint(csenv.g,csenv.DisplayEnv,ty1,ty2,res,ContextInfo.NoContext,m))
+            | _ -> ErrorD (ErrorsFromAddingSubsumptionConstraint(csenv.g,csenv.DisplayEnv,ty1,ty2,res,contextInfo,m)))
 
 and private SolveTypEqualsTypWithReport contextInfo (csenv:ConstraintSolverEnv) ndeep  m trace ty1 ty2 = 
     TryD (fun () -> SolveTypEqualsTypKeepAbbrevs csenv ndeep m trace ty1 ty2)
@@ -1941,7 +1952,7 @@ and ArgsMustSubsumeOrConvert
     let g = csenv.g
     let m = callerArg.Range
     let calledArgTy = AdjustCalledArgType csenv.InfoReader isConstraint calledArg callerArg
-    SolveTypSubsumesTypWithReport csenv ndeep m trace calledArgTy callerArg.Type ++ (fun () -> 
+    SolveTypSubsumesTypWithReport ContextInfo.NoContext csenv ndeep m trace calledArgTy callerArg.Type ++ (fun () -> 
 
     if calledArg.IsParamArray && isArray1DTy g calledArgTy && not (isArray1DTy g callerArg.Type)
     then 
@@ -1957,10 +1968,10 @@ and MustUnifyInsideUndo csenv ndeep trace ty1 ty2 =
 
 and ArgsMustSubsumeOrConvertInsideUndo (csenv:ConstraintSolverEnv) ndeep trace isConstraint calledArg (CallerArg(callerArgTy,m,_,_) as callerArg) = 
     let calledArgTy = AdjustCalledArgType csenv.InfoReader isConstraint calledArg callerArg
-    SolveTypSubsumesTypWithReport csenv ndeep  m (WithTrace trace) calledArgTy callerArgTy 
+    SolveTypSubsumesTypWithReport ContextInfo.NoContext csenv ndeep  m (WithTrace trace) calledArgTy callerArgTy 
 
 and TypesMustSubsumeOrConvertInsideUndo (csenv:ConstraintSolverEnv) ndeep trace m calledArgTy callerArgTy = 
-    SolveTypSubsumesTypWithReport csenv ndeep m trace calledArgTy callerArgTy 
+    SolveTypSubsumesTypWithReport ContextInfo.NoContext csenv ndeep m trace calledArgTy callerArgTy 
 
 and ArgsEquivInsideUndo (csenv:ConstraintSolverEnv) _trace isConstraint calledArg (CallerArg(callerArgTy,m,_,_) as callerArg) = 
     let calledArgTy = AdjustCalledArgType csenv.InfoReader isConstraint calledArg callerArg
@@ -2439,10 +2450,8 @@ let AddCxTypeMustSubsumeTypeMatchingOnlyUndoIfFailed denv css m ty1 ty2 =
     let csenv = { csenv with MatchingOnly = true }
     UndoIfFailed (fun trace -> SolveTypSubsumesTypKeepAbbrevs csenv 0 m (WithTrace(trace)) ty1 ty2)
 
-
-
-let AddCxTypeMustSubsumeType denv css m trace ty1 ty2 = 
-    SolveTypSubsumesTypWithReport (MakeConstraintSolverEnv css m denv) 0 m trace ty1 ty2
+let AddCxTypeMustSubsumeType contextInfo denv css m trace ty1 ty2 = 
+    SolveTypSubsumesTypWithReport contextInfo (MakeConstraintSolverEnv css m denv) 0 m trace ty1 ty2
     |> RaiseOperationResult
 
 let AddCxMethodConstraint denv css m trace traitInfo  =

src/fsharp/ConstraintSolver.fsi

@@ -60,6 +60,10 @@ type ContextInfo =
 | RecordFields
 /// The type equation comes from the verification of a tuple in record fields.
 | TupleInRecordFields
+/// The type equation comes from a runtime type test.
+| RuntimeTypeTest of bool
+/// The type equation comes from an downcast where a upcast could be used.
+| DowncastUsedInsteadOfUpcast of bool
 
 exception ConstraintSolverTupleDiffLengths              of DisplayEnv * TType list * TType list * range * range
 exception ConstraintSolverInfiniteTypes                 of DisplayEnv * TType * TType * range * range
@@ -70,7 +74,7 @@ exception ConstraintSolverError                         of string * range * rang
 exception ConstraintSolverRelatedInformation            of string option * range * exn
 exception ErrorFromApplyingDefault                      of TcGlobals * DisplayEnv * Typar * TType * exn * range
 exception ErrorFromAddingTypeEquation                   of TcGlobals * DisplayEnv * TType * TType * exn * ContextInfo * range
-exception ErrorsFromAddingSubsumptionConstraint         of TcGlobals * DisplayEnv * TType * TType * exn * range
+exception ErrorsFromAddingSubsumptionConstraint         of TcGlobals * DisplayEnv * TType * TType * exn * ContextInfo * range
 exception ErrorFromAddingConstraint                     of DisplayEnv * exn * range
 exception UnresolvedConversionOperator                  of DisplayEnv * TType * TType * range
 exception PossibleOverload                              of DisplayEnv * string * exn * range
@@ -110,7 +114,7 @@ val AddConstraint                             : ConstraintSolverEnv -> int -> Ra
 val AddCxTypeEqualsType                       : ContextInfo -> DisplayEnv -> ConstraintSolverState -> range -> TType -> TType -> unit
 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 AddCxTypeMustSubsumeType                  : ContextInfo -> 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

@@ -1294,3 +1294,5 @@ estApplyStaticArgumentsForMethodNotImplemented,"A type provider implemented GetS
 3195,optsResponseFileNameInvalid,"Response file name '%s' is empty, contains invalid characters, has a drive specification without an absolute path, or is too long"
 3196,fsharpCoreNotFoundToBeCopied,"Cannot find FSharp.Core.dll in compiler's directory"
 3197,etMissingStaticArgumentsToMethod,"This provided method requires static parameters"
+3198,considerUpcast,"The conversion from %s to %s is a compile-time safe upcast, not a downcast. Consider using 'upcast' instead of 'downcast'."
+3198,considerUpcastOperator,"The conversion from %s to %s is a compile-time safe upcast, not a downcast. Consider using the :> (upcast) operator instead of the :?> (downcast) operator."