Add regression test: #12796, DefaultValue null on array type record field

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -19,6 +19,7 @@
 * Fixed how the source ranges of warn directives are reported (as trivia) in the parser output (by not reporting leading spaces). ([Issue #19405](https://github.com/dotnet/fsharp/issues/19405), [PR #19408]((https://github.com/dotnet/fsharp/pull/19408)))
 * Fix UoM value type `ToString()` returning garbage values when `--checknulls+` is enabled, caused by double address-taking in codegen. ([Issue #19435](https://github.com/dotnet/fsharp/issues/19435), [PR #19440](https://github.com/dotnet/fsharp/pull/19440))
 * Fix completion inconsistently showing some obsolete members (fields and events) while hiding others (methods and properties). All obsolete members are now consistently hidden by default. ([Issue #13512](https://github.com/dotnet/fsharp/issues/13512), [PR #19506](https://github.com/dotnet/fsharp/pull/19506))
+* Fix internal error `FS0192: encodeCustomAttrElemType` when using an empty array of user-defined type as a custom attribute argument (e.g. `[<DefaultValue([||] : A[])>]`). Empty arrays of arbitrary types are now encoded using `System.Object` as the element type. ([Issue #12796](https://github.com/dotnet/fsharp/issues/12796), [PR #19472](https://github.com/dotnet/fsharp/pull/19472))
 
 ### Added
 

src/Compiler/CodeGen/IlxGen.fs

@@ -10182,7 +10182,7 @@ and EmitRestoreStack cgbuf (savedStack, savedStackLocals) =
 //GenAttr: custom attribute generation
 //-------------------------------------------------------------------------
 
-and GenAttribArg amap g eenv x (ilArgTy: ILType) =
+and GenAttribArg amap (g: TcGlobals) eenv x (ilArgTy: ILType) =
 
     match stripDebugPoints x, ilArgTy with
     // Detect 'null' used for an array argument
@@ -10222,7 +10222,27 @@ and GenAttribArg amap g eenv x (ilArgTy: ILType) =
     // Detect '[| ... |]' nodes
     | Expr.Op(TOp.Array, [ elemTy ], args, m), _ ->
         let ilElemTy = GenType amap m eenv.tyenv elemTy
-        ILAttribElem.Array(ilElemTy, List.map (fun arg -> GenAttribArg amap g eenv arg ilElemTy) args)
+
+        // Check if element type can be encoded in custom attribute metadata (ECMA 335).
+        // Valid element types: primitives, enums, string, System.Type, System.Object.
+        let isUnencodableBoxedElemType =
+            match ilElemTy with
+            | ILType.Boxed tspec when
+                tspec.Name <> "System.String"
+                && tspec.Name <> "System.Object"
+                && tspec.Name <> "System.Type"
+                ->
+                true
+            | _ -> false
+
+        if isUnencodableBoxedElemType then
+            if args.IsEmpty then
+                // Empty arrays: substitute System.Object as element type since no elements need encoding.
+                ILAttribElem.Array(g.ilg.typ_Object, [])
+            else
+                error (Error(FSComp.SR.ilCustomAttrInvalidArrayElemType (ilElemTy.TypeRef.Name), m))
+        else
+            ILAttribElem.Array(ilElemTy, List.map (fun arg -> GenAttribArg amap g eenv arg ilElemTy) args)
 
     // Detect 'typeof<ty>' calls
     | TypeOfExpr g ty, _ -> ILAttribElem.Type(Some(GenType amap x.Range eenv.tyenv ty))

src/Compiler/FSComp.txt

@@ -1814,3 +1814,4 @@ featurePreprocessorElif,"#elif preprocessor directive"
 3882,lexHashElifMustBeFirst,"#elif directive must appear as the first non-whitespace character on a line"
 3883,lexHashElifMustHaveIdent,"#elif directive should be immediately followed by an identifier"
 3884,tcFunctionValueUsedAsInterpolatedStringArg,"This expression is a function value. When used in an interpolated string it will be formatted using its 'ToString' method, which is likely not the intended behavior. Consider applying the function to its arguments."
+3885,ilCustomAttrInvalidArrayElemType,"The type '%s' is not a valid custom attribute argument type. Custom attribute arrays must have elements of primitive types, enums, string, System.Type, or System.Object."