Bugfix :: Fix 'Type inference problem too complicated' for SRTP with "T:null and T:struct" dummy constraint

docs/release-notes/.FSharp.Compiler.Service/9.0.300.md

@@ -13,6 +13,7 @@
 * Fix NRE when accessing nullable fields of types within their equals/hash/compare methods ([PR #18296](https://github.com/dotnet/fsharp/pull/18296))
 * Fix nullness warning for overrides of generic code with nullable type instance ([Issue #17988](https://github.com/dotnet/fsharp/issues/17988), [PR #18337](https://github.com/dotnet/fsharp/pull/18337))
 * Unsafe downcast from `obj` to generic `T` no longer requires `not null` constraint on `T`([Issue #18275](https://github.com/dotnet/fsharp/issues/18275), [PR #18343](https://github.com/dotnet/fsharp/pull/18343))
+* Fix "type inference problem too complicated" for SRTP with T:null and T:struct dummy constraint([Issue #18288](https://github.com/dotnet/fsharp/issues/18288), [PR #18345](https://github.com/dotnet/fsharp/pull/18345))
 * Fix for missing parse diagnostics in TransparentCompiler.ParseAndCheckProject ([PR #18366](https://github.com/dotnet/fsharp/pull/18366))
 
 ### Added

src/Compiler/Checking/AugmentWithHashCompare.fs

@@ -1647,13 +1647,7 @@ let rec TypeDefinitelyHasEquality g ty =
     match appTy with
     | ValueSome(tcref, _) when HasFSharpAttribute g g.attrib_NoEqualityAttribute tcref.Attribs -> false
     | _ ->
-        if
-            isTyparTy g ty
-            && (destTyparTy g ty).Constraints
-               |> List.exists (function
-                   | TyparConstraint.SupportsEquality _ -> true
-                   | _ -> false)
-        then
+        if ty |> IsTyparTyWithConstraint g _.IsSupportsEquality then
             true
         else
             match ty with

src/Compiler/Checking/CheckDeclarations.fs

@@ -2138,17 +2138,13 @@ module TyconConstraintInference =
 
                 // Is the field type a type parameter?
                 match tryDestTyparTy g ty with
-                | ValueSome tp ->
-                    // Look for an explicit 'comparison' constraint
-                    if tp.Constraints |> List.exists (function TyparConstraint.SupportsComparison _ -> true | _ -> false) then 
-                        true
-
+                | ValueSome tp when tp |> HasConstraint _.IsSupportsComparison -> true
+                | ValueSome tp ->                    
                     // Within structural types, type parameters can be optimistically assumed to have comparison
                     // We record the ones for which we have made this assumption.
-                    elif tycon.TyparsNoRange |> List.exists (fun tp2 -> typarRefEq tp tp2) then 
+                    if tycon.TyparsNoRange |> List.exists (fun tp2 -> typarRefEq tp tp2) then 
                         assumedTyparsAcc <- assumedTyparsAcc.Add(tp.Stamp)
-                        true
-
+                        true                    
                     else
                         false
                 | _ ->
@@ -2267,14 +2263,11 @@ module TyconConstraintInference =
             // and type parameters.
             let rec checkIfFieldTypeSupportsEquality (tycon: Tycon) (ty: TType) =
                 match tryDestTyparTy g ty with
+                | ValueSome tp when tp |> HasConstraint _.IsSupportsEquality -> true
                 | ValueSome tp ->
-                    // Look for an explicit 'equality' constraint
-                    if tp.Constraints |> List.exists (function TyparConstraint.SupportsEquality _ -> true | _ -> false) then 
-                        true
-
                     // Within structural types, type parameters can be optimistically assumed to have equality
                     // We record the ones for which we have made this assumption.
-                    elif tycon.Typars(tycon.Range) |> List.exists (fun tp2 -> typarRefEq tp tp2) then                     
+                    if tycon.Typars(tycon.Range) |> List.exists (fun tp2 -> typarRefEq tp tp2) then                     
                         assumedTyparsAcc <- assumedTyparsAcc.Add(tp.Stamp)
                         true
                     else

src/Compiler/Checking/ConstraintSolver.fs

@@ -958,13 +958,6 @@ let rec SolveTyparEqualsTypePart1 (csenv: ConstraintSolverEnv) m2 (trace: Option
         // Record the solution before we solve the constraints, since 
         // We may need to make use of the equation when solving the constraints. 
         // Record a entry in the undo trace if one is provided 
-
-        //let ty1AllowsNull = r.Constraints |> List.exists (function | TyparConstraint.SupportsNull _ -> true | _ -> false )
-        //let tyAllowsNull() = TypeNullIsExtraValueNew csenv.g m2 ty
-        //if ty1AllowsNull && not (tyAllowsNull()) then
-        //     trace.Exec (fun () -> r.typar_solution <- Some (ty |> replaceNullnessOfTy csenv.g.knownWithNull)) (fun () -> r.typar_solution <- None)
-        //else
-        //    trace.Exec (fun () -> r.typar_solution <- Some ty) (fun () -> r.typar_solution <- None)
         trace.Exec (fun () -> r.typar_solution <- Some ty) (fun () -> r.typar_solution <- None)
     }  
 
@@ -1295,8 +1288,9 @@ and SolveTypeEqualsType (csenv: ConstraintSolverEnv) ndeep m2 (trace: OptionalTr
                 SolveTyparEqualsType csenv ndeep m2 trace sty1 (replaceNullnessOfTy g.knownWithoutNull sty2)
             | ValueSome NullnessInfo.WithoutNull, ValueSome NullnessInfo.WithoutNull when 
                 csenv.IsSupportsNullFlex && 
-                isAppTy g sty2 && 
-                tp1.Constraints |> List.exists (function TyparConstraint.SupportsNull _ -> true | _ -> false) ->
+                isAppTy g sty2 &&
+                tp1 |> HasConstraint _.IsSupportsNull  &&
+                not(tp1 |> HasConstraint _.IsIsNonNullableStruct)->
                     let tpNew = NewCompGenTypar(TyparKind.Type, TyparRigidity.Flexible, TyparStaticReq.None, TyparDynamicReq.No, false)               
                     trackErrors {                    
                         do! SolveTypeEqualsType csenv ndeep m2 trace cxsln (TType_var(tpNew, g.knownWithoutNull)) sty2
@@ -1614,10 +1608,10 @@ and SolveTyparSubtypeOfType (csenv: ConstraintSolverEnv) ndeep m2 trace tp ty1 =
         else
             AddConstraint csenv ndeep m2 trace tp (TyparConstraint.CoercesTo(ty1, csenv.m))
 
-and DepthCheck ndeep m = 
-    if ndeep > 300 then 
-        error(Error(FSComp.SR.csTypeInferenceMaxDepth(), m)) 
-    else 
+and DepthCheck ndeep m =
+    if ndeep > 300 then
+        error(Error(FSComp.SR.csTypeInferenceMaxDepth(), m))
+    else
         CompleteD
 
 // If this is a type that's parameterized on a unit-of-measure (expected to be numeric), unify its measure with 1
@@ -2426,14 +2420,21 @@ and EnforceConstraintConsistency (csenv: ConstraintSolverEnv) ndeep m2 trace ret
             return! SolveTypeEqualsTypeKeepAbbrevs csenv ndeep m2 trace retTy1 retTy2
 
         | TyparConstraint.SupportsComparison _, TyparConstraint.IsDelegate _
-        | TyparConstraint.IsDelegate _, TyparConstraint.SupportsComparison _
+        | TyparConstraint.IsDelegate _, TyparConstraint.SupportsComparison _ ->
+            return! ErrorD (Error(FSComp.SR.csComparisonDelegateConstraintInconsistent(), m))
+
         | TyparConstraint.IsNonNullableStruct _, TyparConstraint.IsReferenceType _
         | TyparConstraint.IsReferenceType _, TyparConstraint.IsNonNullableStruct _   ->
             return! ErrorD (Error(FSComp.SR.csStructConstraintInconsistent(), m))
 
         | TyparConstraint.SupportsNull _, TyparConstraint.NotSupportsNull _
         | TyparConstraint.NotSupportsNull _, TyparConstraint.SupportsNull _   ->
             return! ErrorD (Error(FSComp.SR.csNullNotNullConstraintInconsistent(), m))
+
+        | TyparConstraint.SupportsNull _, TyparConstraint.IsNonNullableStruct _
+        | TyparConstraint.IsNonNullableStruct _, TyparConstraint.SupportsNull _   ->
+            ()
+            //return! WarnD (Error(FSComp.SR.csNullStructConstraintInconsistent(), m))
 
         | TyparConstraint.IsUnmanaged _, TyparConstraint.IsReferenceType _
         | TyparConstraint.IsReferenceType _, TyparConstraint.IsUnmanaged _ ->
@@ -2640,7 +2641,7 @@ and SolveTypeUseSupportsNull (csenv: ConstraintSolverEnv) ndeep m2 trace ty =
                     | ValueSome NullnessInfo.WithoutNull ->                      
                         return! AddConstraint csenv ndeep m2 trace tp (TyparConstraint.SupportsNull m)
                     | _ ->
-                        if tp.Constraints |> List.exists (function | TyparConstraint.IsReferenceType _ -> true | _ -> false) |> not then
+                        if not (tp |> HasConstraint _.IsIsReferenceType) then
                             do! AddConstraint csenv ndeep m2 trace tp (TyparConstraint.IsReferenceType m)
                         return! SolveNullnessSupportsNull csenv ndeep m2 trace ty nullness
                 | _ ->
@@ -2737,7 +2738,7 @@ and SolveTypeCanCarryNullness (csenv: ConstraintSolverEnv)  ty nullness =
         let strippedTy = stripTyEqnsA g true ty
         match tryAddNullnessToTy nullness strippedTy with
         | Some _ -> 
-            if isTyparTy g strippedTy && not (isReferenceTyparTy g strippedTy) then
+            if isTyparTy g strippedTy && not (IsReferenceTyparTy g strippedTy) then
                 return! AddConstraint csenv 0 m NoTrace (destTyparTy g strippedTy) (TyparConstraint.IsReferenceType m)
         | None -> 
             let tyString = NicePrint.minimalStringOfType csenv.DisplayEnv strippedTy
@@ -2978,10 +2979,11 @@ and SolveTypeRequiresDefaultValue (csenv: ConstraintSolverEnv) ndeep m2 trace or
     let g = csenv.g
     let m = csenv.m
     let ty = stripTyEqnsAndMeasureEqns g origTy
+
     if isTyparTy g ty then
-        if isNonNullableStructTyparTy g ty then
+        if IsNonNullableStructTyparTy g ty then
             SolveTypeRequiresDefaultConstructor csenv ndeep m2 trace ty 
-        elif isReferenceTyparTy g ty then
+        elif IsReferenceTyparTy g ty then
             SolveTypeUseSupportsNull csenv ndeep m2 trace ty
         else
             ErrorD (ConstraintSolverError(FSComp.SR.csGenericConstructRequiresStructOrReferenceConstraint(), m, m2))

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -2194,7 +2194,7 @@ module GeneralizationHelpers =
 
         let relevantUniqueSubtypeConstraint (tp: Typar) =
             // Find a single subtype constraint
-            match tp.Constraints |> List.partition (function TyparConstraint.CoercesTo _ -> true | _ -> false) with
+            match tp.Constraints |> List.partition _.IsCoercesTo with
             | [TyparConstraint.CoercesTo(tgtTy, _)], others ->
                  // Throw away null constraints if they are implied
                  if others |> List.exists (function TyparConstraint.SupportsNull _ -> not (TypeNullIsExtraValue g m tgtTy) | _ -> true)

src/Compiler/CodeGen/IlxGen.fs

@@ -5676,32 +5676,14 @@ and GenGenericParam cenv eenv (tp: Typar) =
         |> List.map (GenTypeAux cenv tp.Range eenv.tyenv VoidNotOK PtrTypesNotOK)
 
     let refTypeConstraint =
-        tp.Constraints
-        |> List.exists (function
-            | TyparConstraint.IsReferenceType _
-            // 'null' automatically implies 'not struct'
-            | TyparConstraint.SupportsNull _ -> true
-            | _ -> false)
+        tp |> HasConstraint(fun tc -> tc.IsIsReferenceType || tc.IsSupportsNull) // `null` implies not struct
 
-    let notNullableValueTypeConstraint =
-        tp.Constraints
-        |> List.exists (function
-            | TyparConstraint.IsNonNullableStruct _ -> true
-            | _ -> false)
+    let notNullableValueTypeConstraint = tp |> HasConstraint _.IsIsNonNullableStruct
 
     let nullnessOfTypar =
         if g.langFeatureNullness && g.checkNullness then
-            let hasNotSupportsNull =
-                tp.Constraints
-                |> List.exists (function
-                    | TyparConstraint.NotSupportsNull _ -> true
-                    | _ -> false)
-
-            let hasSupportsNull () =
-                tp.Constraints
-                |> List.exists (function
-                    | TyparConstraint.SupportsNull _ -> true
-                    | _ -> false)
+            let hasNotSupportsNull = tp |> HasConstraint _.IsNotSupportsNull
+            let hasSupportsNull () = tp |> HasConstraint _.IsSupportsNull
 
             if hasNotSupportsNull || notNullableValueTypeConstraint then
                 NullnessInfo.WithoutNull
@@ -5714,17 +5696,11 @@ and GenGenericParam cenv eenv (tp: Typar) =
             None
 
     let defaultConstructorConstraint =
-        tp.Constraints
-        |> List.exists (function
-            | TyparConstraint.RequiresDefaultConstructor _ -> true
-            | _ -> false)
+        tp |> HasConstraint _.IsRequiresDefaultConstructor
 
     let emitUnmanagedInIlOutput =
         cenv.g.langVersion.SupportsFeature(LanguageFeature.UnmanagedConstraintCsharpInterop)
-        && tp.Constraints
-           |> List.exists (function
-               | TyparConstraint.IsUnmanaged _ -> true
-               | _ -> false)
+        && tp |> HasConstraint _.IsIsUnmanaged
 
     let tpName =
         // use the CompiledName if given

src/Compiler/FSComp.txt

@@ -324,6 +324,8 @@ csMethodFoundButIsStatic,"The type '%s' has a method '%s' (full name '%s'), but
 csMethodFoundButIsNotStatic,"The type '%s' has a method '%s' (full name '%s'), but the method is not static"
 472,csStructConstraintInconsistent,"The constraints 'struct' and 'not struct' are inconsistent"
 473,csUnmanagedConstraintInconsistent,"The constraints 'unmanaged' and 'not struct' are inconsistent"
+474,csComparisonDelegateConstraintInconsistent,"The constraints 'comparison' and 'delegate' are inconsistent"
+475,csNullStructConstraintInconsistent,"The constraints 'struct' and 'null' are inconsistent"
 csTypeDoesNotHaveNull,"The type '%s' does not have 'null' as a proper value"
 csNullableTypeDoesNotHaveNull,"The type '%s' does not have 'null' as a proper value. To create a null value for a Nullable type use 'System.Nullable()'."
 csTypeDoesNotSupportComparison1,"The type '%s' does not support the 'comparison' constraint because it has the 'NoComparison' attribute"

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -9150,41 +9150,34 @@ let IsUnionTypeWithNullAsTrueValue (g: TcGlobals) (tycon: Tycon) =
 let TyconCompilesInstanceMembersAsStatic g tycon = IsUnionTypeWithNullAsTrueValue g tycon
 let TcrefCompilesInstanceMembersAsStatic g (tcref: TyconRef) = TyconCompilesInstanceMembersAsStatic g tcref.Deref
 
+let inline HasConstraint ([<InlineIfLambda>] predicate) (tp:Typar)  = 
+    tp.Constraints |> List.exists predicate
+
+let inline tryGetTyparTyWithConstraint g ([<InlineIfLambda>] predicate) ty = 
+    match tryDestTyparTy g ty with 
+    | ValueSome tp as x when HasConstraint predicate tp -> x
+    | _ -> ValueNone
+
+let inline IsTyparTyWithConstraint g ([<InlineIfLambda>] predicate) ty = 
+    match tryDestTyparTy g ty with 
+    | ValueSome tp -> HasConstraint predicate tp
+    | ValueNone -> false
+
 // Note, isStructTy does not include type parameters with the ': struct' constraint
 // This predicate is used to detect those type parameters.
-let isNonNullableStructTyparTy g ty = 
-    match tryDestTyparTy g ty with 
-    | ValueSome tp -> 
-        tp.Constraints |> List.exists (function TyparConstraint.IsNonNullableStruct _ -> true | _ -> false)
-    | ValueNone ->
-        false
+let IsNonNullableStructTyparTy g ty = ty |> IsTyparTyWithConstraint g _.IsIsNonNullableStruct
 
 // Note, isRefTy does not include type parameters with the ': not struct' or ': null' constraints
 // This predicate is used to detect those type parameters.
-let isReferenceTyparTy g ty = 
-    match tryDestTyparTy g ty with 
-    | ValueSome tp -> 
-        tp.Constraints |> List.exists (function
-            | TyparConstraint.IsReferenceType _ -> true
-            | TyparConstraint.SupportsNull _ -> true
-            | _ -> false)
-    | ValueNone ->
-        false
+let IsReferenceTyparTy g ty = ty |> IsTyparTyWithConstraint g (fun tc -> tc.IsIsReferenceType || tc.IsSupportsNull)
 
-let GetTyparTyIfSupportsNull g ty = 
-    if isReferenceTyparTy g ty then
-        let tp = destTyparTy g ty
-        if tp.Constraints |> List.exists (function TyparConstraint.SupportsNull _ -> true | _ -> false) then
-            ValueSome tp
-        else ValueNone
-    else
-        ValueNone
+let GetTyparTyIfSupportsNull g ty = ty |> tryGetTyparTyWithConstraint g _.IsSupportsNull
 
 let TypeNullNever g ty = 
     let underlyingTy = stripTyEqnsAndMeasureEqns g ty
     isStructTy g underlyingTy ||
     isByrefTy g underlyingTy ||
-    isNonNullableStructTyparTy g ty
+    IsNonNullableStructTyparTy g ty
 
 /// The pre-nullness logic about whether a type admits the use of 'null' as a value.
 let TypeNullIsExtraValue g m ty = 
@@ -9244,7 +9237,7 @@ let changeWithNullReqTyToVariable g reqTy =
 let reqTyForArgumentNullnessInference g actualTy reqTy =
     // Only change reqd nullness if actualTy is an inference variable
     match tryDestTyparTy g actualTy with
-    | ValueSome t when t.IsCompilerGenerated && not(t.Constraints |> List.exists(function | TyparConstraint.SupportsNull _ -> true | _ -> false))->
+    | ValueSome t when t.IsCompilerGenerated && not(t |> HasConstraint _.IsSupportsNull) ->
         changeWithNullReqTyToVariable g reqTy       
     | _ -> reqTy
 
@@ -9366,8 +9359,9 @@ let rec TypeHasDefaultValueAux isNew g m ty =
             true))
     || 
       // Check for type variables with the ":struct" and "(new : unit -> 'T)" constraints
-      (isNonNullableStructTyparTy g ty &&
-        (destTyparTy g ty).Constraints |> List.exists (function TyparConstraint.RequiresDefaultConstructor _ -> true | _ -> false))
+      ( match ty |> tryGetTyparTyWithConstraint g _.IsIsNonNullableStruct with
+        | ValueSome tp -> tp |> HasConstraint _.IsRequiresDefaultConstructor
+        | ValueNone -> false)        
 
 let TypeHasDefaultValue (g: TcGlobals) m ty = TypeHasDefaultValueAux false g m ty  
 
@@ -9984,7 +9978,7 @@ let isCompiledOrWitnessPassingConstraint (g: TcGlobals) cx =
 // FSharpTypeFunc, but rather bake a "local type function" for each TyLambda abstraction.
 let IsGenericValWithGenericConstraints g (v: Val) = 
     isForallTy g v.Type && 
-    v.Type |> destForallTy g |> fst |> List.exists (fun tp -> List.exists (isCompiledOrWitnessPassingConstraint g) tp.Constraints)
+    v.Type |> destForallTy g |> fst |> List.exists (fun tp -> HasConstraint (isCompiledOrWitnessPassingConstraint g) tp)
 
 // Does a type support a given interface? 
 type Entity with 

src/Compiler/TypedTree/TypedTreeOps.fsi

@@ -1734,13 +1734,18 @@ val isStructOrEnumTyconTy: TcGlobals -> TType -> bool
 ///
 /// Note, isStructTy does not include type parameters with the ': struct' constraint
 /// This predicate is used to detect those type parameters.
-val isNonNullableStructTyparTy: TcGlobals -> TType -> bool
+val IsNonNullableStructTyparTy: TcGlobals -> TType -> bool
+
+val inline HasConstraint: [<InlineIfLambda>] predicate: (TyparConstraint -> bool) -> Typar -> bool
+
+val inline IsTyparTyWithConstraint:
+    TcGlobals -> [<InlineIfLambda>] predicate: (TyparConstraint -> bool) -> TType -> bool
 
 /// Determine if a type is a variable type with the ': not struct' constraint.
 ///
 /// Note, isRefTy does not include type parameters with the ': not struct' constraint
 /// This predicate is used to detect those type parameters.
-val isReferenceTyparTy: TcGlobals -> TType -> bool
+val IsReferenceTyparTy: TcGlobals -> TType -> bool
 
 /// Determine if a type is an unmanaged type
 val isUnmanagedTy: TcGlobals -> TType -> bool