Speed up Constraint Solver's overload resolution (II) (#1650)

* Commit constraint solution before the type equality test.

* Commit constraint solution before the type equality test.
* SolveMemberConstraint will signal through an internal exception that it failed due to an unresolved overload.

* Adjust resulting error code and message from test.

* Use additional argument instead of permitWeakResolution

src/fsharp/ConstraintSolver.fs

@@ -357,6 +357,7 @@ let ShowAccessDomain ad =
 // Solve
 
 exception NonRigidTypar of DisplayEnv * string option * range * TType * TType * range
+exception LocallyAbortOperationThatFailsToResolveOverload
 exception LocallyAbortOperationThatLosesAbbrevs 
 let localAbortD = ErrorD LocallyAbortOperationThatLosesAbbrevs
 
@@ -730,7 +731,7 @@ and solveTypMeetsTyparConstraints (csenv:ConstraintSolverEnv) ndeep m2 trace ty
       | TyparConstraint.SimpleChoice(tys,m2)          -> SolveTypChoice                     csenv ndeep m2 trace ty tys
       | TyparConstraint.CoercesTo(ty2,m2)         -> SolveTypSubsumesTypKeepAbbrevs     csenv ndeep m2 trace None ty2 ty
       | TyparConstraint.MayResolveMember(traitInfo,m2) -> 
-          SolveMemberConstraint csenv false ndeep m2 trace traitInfo ++ (fun _ -> CompleteD) 
+          SolveMemberConstraint csenv false false ndeep m2 trace traitInfo ++ (fun _ -> CompleteD) 
     )))
 
 
@@ -740,14 +741,15 @@ and SolveTypEqualsTyp (csenv:ConstraintSolverEnv) ndeep m2 (trace: OptionalTrace
     let ndeep = ndeep + 1
     let aenv = csenv.EquivEnv
     let g = csenv.g
-    if ty1 === ty2 then CompleteD else
 
     match cxsln with
     | Some (traitInfo, traitSln) when traitInfo.Solution.IsNone -> 
         // If this is an overload resolution at this point it's safe to assume the candidate member being evaluated solves this member constraint.
         TransactMemberConstraintSolution traitInfo trace traitSln
     | _ -> ()
 
+    if ty1 === ty2 then CompleteD else
+
     let canShortcut = not trace.HasTrace
     let sty1 = stripTyEqnsA csenv.g canShortcut ty1
     let sty2 = stripTyEqnsA csenv.g canShortcut ty2
@@ -925,7 +927,7 @@ and SolveDimensionlessNumericType (csenv:ConstraintSolverEnv) ndeep m2 trace ty
 /// We pretend int and other types support a number of operators.  In the actual IL for mscorlib they 
 /// don't, however the type-directed static optimization rules in the library code that makes use of this 
 /// will deal with the problem. 
-and SolveMemberConstraint (csenv:ConstraintSolverEnv) permitWeakResolution ndeep m2 trace (TTrait(tys,nm,memFlags,argtys,rty,sln)) :  OperationResult<bool> =
+and SolveMemberConstraint (csenv:ConstraintSolverEnv) ignoreUnresolvedOverload permitWeakResolution ndeep m2 trace (TTrait(tys,nm,memFlags,argtys,rty,sln)) :  OperationResult<bool> =
     // Do not re-solve if already solved
     if sln.Value.IsSome then ResultD true else
     let g = csenv.g
@@ -1287,7 +1289,10 @@ and SolveMemberConstraint (csenv:ConstraintSolverEnv) permitWeakResolution ndeep
                   // If there's nothing left to learn then raise the errors 
                   (if (permitWeakResolution && List.isEmpty support) || List.isEmpty frees then errors  
                   // Otherwise re-record the trait waiting for canonicalization 
-                   else AddMemberConstraint csenv ndeep m2 trace traitInfo support frees) ++ (fun () -> ResultD TTraitUnsolved)
+                   else AddMemberConstraint csenv ndeep m2 trace traitInfo support frees) ++ (fun () -> 
+                       match errors with
+                       | ErrorResult (_,UnresolvedOverloading _) when not ignoreUnresolvedOverload && (not (nm = "op_Explicit" || nm = "op_Implicit")) -> ErrorD LocallyAbortOperationThatFailsToResolveOverload
+                       | _ -> ResultD TTraitUnsolved)
     ) 
     ++ 
     (fun res -> RecordMemberConstraintSolution csenv.SolverState m trace traitInfo res))
@@ -1429,7 +1434,7 @@ and SolveRelevantMemberConstraintsForTypar (csenv:ConstraintSolverEnv) ndeep per
 
     cxs |> AtLeastOneD (fun (traitInfo,m2) -> 
         let csenv = { csenv with m = m2 }
-        SolveMemberConstraint csenv permitWeakResolution (ndeep+1) m2 trace traitInfo)
+        SolveMemberConstraint csenv true permitWeakResolution (ndeep+1) m2 trace traitInfo)
 
 and CanonicalizeRelevantMemberConstraints (csenv:ConstraintSolverEnv) ndeep trace tps =
     SolveRelevantMemberConstraints csenv ndeep true trace tps 
@@ -1945,7 +1950,9 @@ and private SolveTypSubsumesTypWithReport (csenv:ConstraintSolverEnv) ndeep m tr
 
 and private SolveTypEqualsTypWithReport contextInfo (csenv:ConstraintSolverEnv) ndeep  m trace cxsln ty1 ty2 = 
     TryD (fun () -> SolveTypEqualsTypKeepAbbrevsWithCxsln csenv ndeep m trace cxsln ty1 ty2)
-         (fun res -> ErrorD (ErrorFromAddingTypeEquation(csenv.g,csenv.DisplayEnv,ty1,ty2,res,contextInfo,m)))
+         (function
+            | LocallyAbortOperationThatFailsToResolveOverload -> CompleteD
+            | res -> ErrorD (ErrorFromAddingTypeEquation(csenv.g,csenv.DisplayEnv,ty1,ty2,res,contextInfo,m)))
 
 and ArgsMustSubsumeOrConvert 
         (csenv:ConstraintSolverEnv)
@@ -2495,7 +2502,7 @@ let AddCxTypeMustSubsumeType contextInfo denv css m trace ty1 ty2 =
     |> RaiseOperationResult
 
 let AddCxMethodConstraint denv css m trace traitInfo  =
-    TryD (fun () -> SolveMemberConstraint (MakeConstraintSolverEnv ContextInfo.NoContext css m denv) false 0 m trace traitInfo ++ (fun _ -> CompleteD))
+    TryD (fun () -> SolveMemberConstraint (MakeConstraintSolverEnv ContextInfo.NoContext css m denv) true false 0 m trace traitInfo ++ (fun _ -> CompleteD))
          (fun res -> ErrorD (ErrorFromAddingConstraint(denv,res,m)))
     |> RaiseOperationResult
 
@@ -2550,7 +2557,7 @@ let CodegenWitnessThatTypSupportsTraitConstraint tcVal g amap m (traitInfo:Trait
                 ExtraCxs=HashMultiMap(10, HashIdentity.Structural)
                 InfoReader=new InfoReader(g,amap) }
     let csenv = MakeConstraintSolverEnv ContextInfo.NoContext css m (DisplayEnv.Empty g)
-    SolveMemberConstraint csenv true 0 m NoTrace traitInfo ++ (fun _res -> 
+    SolveMemberConstraint csenv true true 0 m NoTrace traitInfo ++ (fun _res -> 
         let sln = 
               match traitInfo.Solution with 
               | None -> Choice4Of4()

tests/fsharpqa/Source/Conformance/DeclarationElements/MemberDefinitions/OverloadingMembers/SlowOverloadResolution.fs

@@ -1,6 +1,6 @@
 // #Conformance #DeclarationElements #MemberDefinitions #Overloading 
 // https://github.com/Microsoft/visualfsharp/issues/351 - slow overlaod resolution
-//<Expects id="FS0001" status="error">No overloads match</Expects>
+//<Expects id="FS0003" status="error">This value is not a function and cannot be applied</Expects>
 type Switcher = Switcher
 
 let inline checker< ^s, ^r when (^s or ^r) : (static member pass : ^r -> unit)> (s : ^s) (r : ^r) = ()
@@ -22,4 +22,4 @@ let main argv =
     let res : unit = format () "text" 5 "more text" ()
     printfn "%A" res
     System.Console.ReadKey()
-    0 // return an integer exit code
+    0 // return an integer exit code