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

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

@@ -17,6 +17,7 @@
 * Fix `YieldFromFinal`/`ReturnFromFinal` being incorrectly called in non-tail positions (`for`, `use`, `use!`, `try/with` handler). ([Issue #19402](https://github.com/dotnet/fsharp/issues/19402), [PR #19403](https://github.com/dotnet/fsharp/pull/19403))
 * 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 internal error `FS0192: encodeCustomAttrElemType` when using an array of user-defined type as a custom attribute argument. Now reports a proper diagnostic (FS3885). ([Issue #12796](https://github.com/dotnet/fsharp/issues/12796), [PR #19472](https://github.com/dotnet/fsharp/pull/19472))
 
 ### Added
 

src/Compiler/CodeGen/IlxGen.fs

@@ -10223,6 +10223,18 @@ 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
+
+        // Validate element type is encodable in custom attribute metadata (ECMA 335).
+        // Only primitive types, enums, string, System.Type, and System.Object are valid.
+        match ilElemTy with
+        | ILType.Boxed tspec when
+            tspec.Name <> "System.String"
+            && tspec.Name <> "System.Object"
+            && tspec.Name <> "System.Type"
+            ->
+            error (Error(FSComp.SR.ilCustomAttrInvalidArrayElemType (tspec.Name), m))
+        | _ -> ()
+
         ILAttribElem.Array(ilElemTy, List.map (fun arg -> GenAttribArg amap g eenv arg ilElemTy) args)
 
     // Detect 'typeof<ty>' calls

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."