Remove always-on WitnessPassing language feature flag

src/Compiler/FSComp.txt

@@ -1557,7 +1557,6 @@ featureFromEndSlicing,"from-end slicing"
 featureNullnessChecking,"nullness checking"
 featureRuntimeAsync,"runtime async"
 featureStringInterpolation,"string interpolation"
-featureWitnessPassing,"witness passing for trait constraints in F# quotations"
 featureReallyLongList,"list literals of any size"
 featureErrorOnDeprecatedRequireQualifiedAccess,"give error on deprecated access of construct with RequireQualifiedAccess attribute"
 featureMatchNotAllowedForUnionCaseWithNoData,"Pattern match discard is not allowed for union case that takes no data."

src/Compiler/Facilities/LanguageFeatures.fs

@@ -18,7 +18,6 @@ module internal FSharp.Compiler.Features
 type LanguageFeature =
     | FromEndSlicing
     | RuntimeAsync
-    | WitnessPassing
     | StringInterpolation
     | ExpandedMeasurables
     | NullnessChecking
@@ -119,7 +118,6 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
         dict
             [
                 // F# 5.0
-                LanguageFeature.WitnessPassing, languageVersion50
                 LanguageFeature.StringInterpolation, languageVersion50
 
                 // F# 6.0
@@ -291,7 +289,6 @@ type LanguageVersion(versionText, ?disabledFeaturesArray: LanguageFeature array)
         | LanguageFeature.FromEndSlicing -> FSComp.SR.featureFromEndSlicing ()
         | LanguageFeature.NullnessChecking -> FSComp.SR.featureNullnessChecking ()
         | LanguageFeature.RuntimeAsync -> FSComp.SR.featureRuntimeAsync ()
-        | LanguageFeature.WitnessPassing -> FSComp.SR.featureWitnessPassing ()
         | LanguageFeature.StringInterpolation -> FSComp.SR.featureStringInterpolation ()
         | LanguageFeature.ExpandedMeasurables -> FSComp.SR.featureExpandedMeasurables ()
         | LanguageFeature.UnionIsPropertiesVisible -> FSComp.SR.featureUnionIsPropertiesVisible ()

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -8,7 +8,6 @@ module internal FSharp.Compiler.Features
 type LanguageFeature =
     | FromEndSlicing
     | RuntimeAsync
-    | WitnessPassing
     | StringInterpolation
     | ExpandedMeasurables
     | NullnessChecking

src/Compiler/Symbols/Exprs.fs

@@ -508,7 +508,7 @@ module FSharpExprConvert =
 
     and GetWitnessArgs cenv (env: ExprTranslationEnv) (vref: ValRef) m tps tyargs : FSharpExpr list =
         let g = cenv.g
-        if g.langVersion.SupportsFeature(Features.LanguageFeature.WitnessPassing) && not env.suppressWitnesses then
+        if not env.suppressWitnesses then
             /// There are two *conditional* properties a typar can have: equality and comparison.
             /// A generic type having that constraint may be conditional on whether a specific type parameter to that generic has that
             /// constraint.

src/Compiler/Symbols/Symbols.fs

@@ -2903,7 +2903,6 @@ type FSharpMemberOrFunctionOrValue(cenv, d:FSharpMemberOrValData, item) =
             | E _ | P _ | M _ | C _ -> []
         match witnessInfos with
         | [] -> None
-        | _ when not (cenv.g.langVersion.SupportsFeature(Features.LanguageFeature.WitnessPassing)) -> None
         | _ ->
         let witnessParams =
             ((Set.empty, 0), witnessInfos) ||> List.mapFold (fun (used,i) witnessInfo ->

src/Compiler/TypedTree/TcGlobals.fs

@@ -1915,7 +1915,7 @@ type TcGlobals(
   /// supports witness arguments.
   member g.generateWitnesses =
       compilingFSharpCore ||
-      ((ValRefForIntrinsic g.call_with_witnesses_info).TryDeref.IsSome && langVersion.SupportsFeature LanguageFeature.WitnessPassing)
+      (ValRefForIntrinsic g.call_with_witnesses_info).TryDeref.IsSome
 
   /// Indicates if we can use System.Array.Empty when emitting IL for empty array literals
   member val isArrayEmptyAvailable = v_Array_tcref.ILTyconRawMetadata.Methods.FindByName "Empty" |> List.isEmpty |> not

src/Compiler/TypedTree/TypedTreeOps.Transforms.fs

@@ -1022,16 +1022,15 @@ module internal Rewriting =
         remapTyconToNonLocal ctxt tmenv x
 
     (* Which constraints actually get compiled to .NET constraints? *)
-    let isCompiledOrWitnessPassingConstraint (g: TcGlobals) cx =
+    let isCompiledOrWitnessPassingConstraint cx =
         match cx with
         | TyparConstraint.SupportsNull _ // this implies the 'class' constraint
         | TyparConstraint.IsReferenceType _ // this is the 'class' constraint
         | TyparConstraint.IsNonNullableStruct _
-        | TyparConstraint.IsReferenceType _
         | TyparConstraint.RequiresDefaultConstructor _
         | TyparConstraint.IsUnmanaged _ //  implies "struct" and also causes a modreq
-        | TyparConstraint.CoercesTo _ -> true
-        | TyparConstraint.MayResolveMember _ when g.langVersion.SupportsFeature LanguageFeature.WitnessPassing -> true
+        | TyparConstraint.CoercesTo _
+        | TyparConstraint.MayResolveMember _ -> true
         | _ -> false
 
     // Is a value a first-class polymorphic value with .NET constraints, or witness-passing constraints?
@@ -1042,7 +1041,7 @@ module internal Rewriting =
         && v.Type
            |> destForallTy g
            |> fst
-           |> List.exists (fun tp -> HasConstraint (isCompiledOrWitnessPassingConstraint g) tp)
+           |> List.exists (HasConstraint isCompiledOrWitnessPassingConstraint)
 
     // Does a type support a given interface?
     type Entity with