Fix runtime crash when using interfaces with unimplemented static abstract members as constrained type arguments

src/Compiler/Checking/InfoReader.fs

@@ -775,6 +775,26 @@ type InfoReader(g: TcGlobals, amap: ImportMap) as this =
             ))
            g amap m AllowMultiIntfInstantiations.Yes ty
 
+    let GetUnimplementedStaticAbstractMemberOfTypeUncached (_flags, m, interfaceTy) =
+        if not (isInterfaceTy g interfaceTy) then
+            None
+        else
+            let checkMembersOfInterface (ty: TType) =
+                let meths = this.GetIntrinsicMethInfosOfType None AccessibleFromSomeFSharpCode AllowMultiIntfInstantiations.Yes IgnoreOverrides m ty
+                meths |> List.tryPick (fun (minfo: MethInfo) ->
+                    // Static abstract non-sealed (non-DIM) members
+                    if not minfo.IsInstance && minfo.IsAbstract && not minfo.IsFinal then
+                        Some minfo.DisplayNameCore
+                    else
+                        None
+                )
+
+            match checkMembersOfInterface interfaceTy with
+            | Some name -> Some name
+            | None ->
+                let baseInterfaces = AllInterfacesOfType g amap m AllowMultiIntfInstantiations.Yes interfaceTy
+                baseInterfaces |> List.tryPick checkMembersOfInterface
+
     let hashFlags0 = 
         { new IEqualityComparer<string option * AccessorDomain * AllowMultiIntfInstantiations> with 
                member _.GetHashCode((filter: string option, ad: AccessorDomain, _allowMultiIntfInst1)) = hash filter + AccessorDomain.CustomGetHashCode ad
@@ -815,6 +835,11 @@ type InfoReader(g: TcGlobals, amap: ImportMap) as this =
     let primaryTypeHierarchyCache = MakeInfoCache "primaryTypeHierarchyCache" GetPrimaryTypeHierarchyUncached HashIdentity.Structural
     let implicitConversionCache = MakeInfoCache "implicitConversionCache" FindImplicitConversionsUncached hashFlags3
     let isInterfaceWithStaticAbstractMethodCache = MakeInfoCache "isInterfaceWithStaticAbstractMethodCache" IsInterfaceTypeWithMatchingStaticAbstractMemberUncached hashFlags4
+    let unimplementedStaticAbstractMemberCache =
+        MakeInfoCache
+            "unimplementedStaticAbstractMemberCache"
+            GetUnimplementedStaticAbstractMemberOfTypeUncached
+            hashFlags0
 
     // Runtime feature support
 
@@ -992,6 +1017,14 @@ type InfoReader(g: TcGlobals, amap: ImportMap) as this =
     member _.IsInterfaceTypeWithMatchingStaticAbstractMember m nm ad ty = 
         isInterfaceWithStaticAbstractMethodCache.Apply((ad, nm), m, ty)
 
+    member _.TryFindUnimplementedStaticAbstractMemberOfType (m: range) (interfaceTy: TType) : string option =
+        if not (isInterfaceTy g interfaceTy) then
+            None
+        elif not (g.langVersion.SupportsFeature LanguageFeature.InterfacesWithAbstractStaticMembers) then
+            None
+        else
+            unimplementedStaticAbstractMemberCache.Apply(((None, AccessibleFromSomewhere, AllowMultiIntfInstantiations.Yes), m, interfaceTy))
+
 let checkLanguageFeatureRuntimeAndRecover (infoReader: InfoReader) langFeature m =
     if not (infoReader.IsLanguageFeatureRuntimeSupported langFeature) then
         let featureStr = LanguageVersion.GetFeatureString langFeature

src/Compiler/Checking/InfoReader.fsi

@@ -225,6 +225,11 @@ type InfoReader =
     member IsInterfaceTypeWithMatchingStaticAbstractMember:
         m: range -> nm: string -> ad: AccessorDomain -> ty: TType -> bool
 
+    /// Check if an interface type has an unimplemented static abstract member.
+    /// Returns Some(memberLogicalName) if found, None otherwise.
+    /// Results are cached per interface type definition.
+    member TryFindUnimplementedStaticAbstractMemberOfType: m: range -> interfaceTy: TType -> string option
+
 val checkLanguageFeatureRuntimeAndRecover:
     infoReader: InfoReader -> langFeature: Features.LanguageFeature -> m: range -> unit
 

src/Compiler/Checking/PostInferenceChecks.fs

@@ -635,6 +635,24 @@ let rec mkArgsForAppliedExpr isBaseCall argsl x =
     | Expr.Op (TOp.Coerce, _, [f], _) -> mkArgsForAppliedExpr isBaseCall argsl f
     | _  -> []
 
+/// Check if a type argument is an interface with unimplemented static abstract members
+/// when used with a type parameter that has interface constraints.
+/// See: https://github.com/dotnet/fsharp/issues/19184
+let CheckInterfaceTypeArgForUnimplementedStaticAbstractMembers (cenv: cenv) m (typar: Typar) (typeArg: TType) =
+    if cenv.reportErrors then
+        // Only check if the type parameter has interface constraints
+        let hasInterfaceConstraint =
+            typar.Constraints |> List.exists (function
+                | TyparConstraint.CoercesTo(constraintTy, _) -> isInterfaceTy cenv.g constraintTy
+                | _ -> false)
+
+        if hasInterfaceConstraint && isInterfaceTy cenv.g typeArg then
+            match cenv.infoReader.TryFindUnimplementedStaticAbstractMemberOfType m typeArg with
+            | Some memberName ->
+                let interfaceTypeName = NicePrint.minimalStringOfType cenv.denv typeArg
+                errorR(Error(FSComp.SR.chkInterfaceWithUnimplementedStaticAbstractMemberUsedAsTypeArgument(interfaceTypeName, memberName), m))
+            | None -> ()
+
 /// Check types occurring in the TAST.
 let CheckTypeAux permitByRefLike (cenv: cenv) env m ty onInnerByrefError =
     if cenv.reportErrors then
@@ -681,6 +699,15 @@ let CheckTypeAux permitByRefLike (cenv: cenv) env m ty onInnerByrefError =
                         if isByrefTyconRef cenv.g tcref2 then
                             errorR(Error(FSComp.SR.chkNoByrefsOfByrefs(NicePrint.minimalStringOfType cenv.denv ty), m))
                 CheckTypesDeep cenv (visitType, None, None, None, None) cenv.g env tinst
+
+            // Check for interfaces with unimplemented static abstract members used as type arguments
+            // This only applies when the type parameter has an interface constraint - using interfaces
+            // with unconstrained generics (like List<ITest> or Dictionary<K, ITest>) is fine.
+            // See: https://github.com/dotnet/fsharp/issues/19184
+            if tcref.CanDeref then
+                let typars = tcref.Typars m
+                if typars.Length = tinst.Length then
+                    (typars, tinst) ||> List.iter2 (CheckInterfaceTypeArgForUnimplementedStaticAbstractMembers cenv m)
 
         let visitTraitSolution info =
             match info with
@@ -1374,6 +1401,20 @@ and CheckApplication cenv env expr (f, tyargs, argsl, m) ctxt =
     let env = { env with isInAppExpr = true }
 
     CheckTypeInstNoByrefs cenv env m tyargs
+
+    // Check for interfaces with unimplemented static abstract members used as type arguments
+    // See: https://github.com/dotnet/fsharp/issues/19184
+    if not tyargs.IsEmpty then
+        match f with
+        | Expr.Val (vref, _, _) ->
+            match vref.TryDeref with
+            | ValueSome v ->
+                let typars = v.Typars
+                if typars.Length = tyargs.Length then
+                    (typars, tyargs) ||> List.iter2 (CheckInterfaceTypeArgForUnimplementedStaticAbstractMembers cenv m)
+            | _ -> ()
+        | _ -> ()
+
     CheckExprNoByrefs cenv env f
 
     let hasReceiver =

src/Compiler/FSComp.txt

@@ -1783,6 +1783,7 @@ featureReuseSameFieldsInStructUnions,"Share underlying fields in a [<Struct>] di
 3865,parsOnlySimplePatternsAreAllowedInConstructors,"Only simple patterns are allowed in primary constructors"
 3866,chkStaticAbstractInterfaceMembers,"A static abstract non-virtual interface member should only be called via type parameter (for example: 'T.%s)."
 3867,chkStaticAbstractMembersOnClasses,"Classes cannot contain static abstract members."
+3868,chkInterfaceWithUnimplementedStaticAbstractMemberUsedAsTypeArgument,"The interface '%s' cannot be used as a type argument because the static abstract member '%s' does not have a most specific implementation in the interface."
 3868,tcActivePatternArgsCountNotMatchNoArgsNoPat,"This active pattern does not expect any arguments, i.e., it should be used like '%s' instead of '%s x'."
 3868,tcActivePatternArgsCountNotMatchOnlyPat,"This active pattern expects exactly one pattern argument, e.g., '%s pat'."
 3868,tcActivePatternArgsCountNotMatchArgs,"This active pattern expects %d expression argument(s), e.g., '%s%s'."