Remove double lookups from ConstraintSolver (#4732)

src/fsharp/ConstraintSolver.fs

@@ -778,7 +778,7 @@ and SolveTypEqualsTyp (csenv:ConstraintSolverEnv) ndeep m2 (trace: OptionalTrace
 
     match sty1, sty2 with 
     // type vars inside forall-types may be alpha-equivalent 
-    | TType_var tp1, TType_var tp2 when typarEq tp1 tp2 || (aenv.EquivTypars.ContainsKey tp1 && typeEquiv g aenv.EquivTypars.[tp1] ty2) -> CompleteD
+    | TType_var tp1, TType_var tp2 when typarEq tp1 tp2 || (match aenv.EquivTypars.TryFind tp1 with | Some v when typeEquiv g v ty2 -> true | _ -> false) -> CompleteD
 
     | TType_var tp1, TType_var tp2 when PreferUnifyTypar tp1 tp2 -> SolveTyparEqualsTyp csenv ndeep m2 trace sty1 ty2
     | TType_var tp1, TType_var tp2 when not csenv.MatchingOnly && PreferUnifyTypar tp2 tp1 -> SolveTyparEqualsTyp csenv ndeep m2 trace sty2 ty1
@@ -850,12 +850,17 @@ and SolveTypSubsumesTyp (csenv:ConstraintSolverEnv) ndeep m2 (trace: OptionalTra
     let aenv = csenv.EquivEnv
     let denv = csenv.DisplayEnv
     match sty1, sty2 with 
-    | TType_var tp1, _ when aenv.EquivTypars.ContainsKey tp1 -> 
-        SolveTypSubsumesTyp csenv ndeep m2 trace cxsln aenv.EquivTypars.[tp1] ty2 
-        
-    | TType_var r1, TType_var r2 when typarEq r1 r2 -> CompleteD
+    | TType_var tp1, _ ->
+        match aenv.EquivTypars.TryFind tp1 with
+        | Some v -> SolveTypSubsumesTyp csenv ndeep m2 trace cxsln v ty2
+        | _ ->
+        match sty2 with
+        | TType_var r2 when typarEq tp1 r2 -> CompleteD
+        | TType_var r when not csenv.MatchingOnly -> SolveTyparSubtypeOfType csenv ndeep m2 trace r ty1
+        | _ ->  SolveTypEqualsTypKeepAbbrevsWithCxsln csenv ndeep m2 trace cxsln ty1 ty2
+
     | _, TType_var r when not csenv.MatchingOnly -> SolveTyparSubtypeOfType csenv ndeep m2 trace r ty1
-    | TType_var _ , _ ->  SolveTypEqualsTypKeepAbbrevsWithCxsln csenv ndeep m2 trace cxsln ty1 ty2
+
     | TType_tuple (tupInfo1, l1)      , TType_tuple (tupInfo2, l2)      -> 
         if evalTupInfoIsStruct tupInfo1 <> evalTupInfoIsStruct tupInfo2 then ErrorD (ConstraintSolverError(FSComp.SR.tcTupleStructMismatch(), csenv.m, m2)) else
         SolveTypEqualsTypEqns csenv ndeep m2 trace cxsln l1 l2 (* nb. can unify since no variance *)

src/fsharp/TastOps.fs

@@ -43,6 +43,10 @@ type TyparMap<'T> =
         let (TPMap m) = tm
         m.ContainsKey(v.Stamp)
 
+    member tm.TryFind (v: Typar) =
+        let (TPMap m) = tm
+        m.TryFind(v.Stamp)
+
     member tm.Add (v: Typar, x) = 
         let (TPMap m) = tm
         TPMap (m.Add(v.Stamp, x))

src/fsharp/TastOps.fsi

@@ -306,6 +306,7 @@ type ValMultiMap<'T> =
 type TyparMap<'T>  =
     member Item : Typar -> 'T with get
     member ContainsKey : Typar -> bool
+    member TryFind : Typar -> 'T option
     member Add : Typar * 'T -> TyparMap<'T> 
     static member Empty : TyparMap<'T>