Fix compiler allowing setting private property on attribute

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -11411,6 +11411,9 @@ and TcAttributeEx canFail (cenv: cenv) (env: TcEnv) attrTgt attrEx (synAttr: Syn
                       | Item.Property (info = [pinfo]) ->
                           if not pinfo.HasSetter then
                             errorR(Error(FSComp.SR.tcPropertyCannotBeSet0(), m))
+                          // Check if the setter is accessible
+                          if not (IsPropInfoAccessible g cenv.amap m ad pinfo) then
+                            errorR(Error(FSComp.SR.tcPropertyCannotBeSetPrivateSetter(id.idText), m))
                           id.idText, true, pinfo.GetPropertyType(cenv.amap, m)
                       | Item.ILField finfo ->
                           CheckILFieldInfoAccessible g cenv.amap m ad finfo

src/Compiler/FSComp.txt

@@ -1523,6 +1523,7 @@ notAFunctionButMaybeDeclaration,"This value is not a function and cannot be appl
 3242,tcTypeDoesNotInheritAttribute,"This type does not inherit Attribute, it will not work correctly with other .NET languages."
 3243,parsInvalidAnonRecdExpr,"Invalid anonymous record expression"
 3244,parsInvalidAnonRecdType,"Invalid anonymous record type"
+3248,tcPropertyCannotBeSetPrivateSetter,"Property '%s' on attribute cannot be set because the setter is private"
 3245,tcCopyAndUpdateNeedsRecordType,"The input to a copy-and-update expression that creates an anonymous record must be either an anonymous record or a record"
 3246,tcAugmentationsCannotHaveAttributes,"Attributes cannot be applied to type extensions."
 3247,couldNotLoadDependencyManagerExtension,"The dependency manager extension %s could not be loaded. Message: %s"

tests/FSharp.Compiler.ComponentTests/Attributes/AttributePrivateSetterTests.fs

@@ -0,0 +1,24 @@
+module FSharp.Compiler.ComponentTests.AttributePrivateSetter
+
+open Xunit
+open FSharp.Test.Compiler
+
+[<Fact>]
+let ``Compiler disallows setting a value of a private property on attribute``() =
+    // Arrange
+    let mainCode = """
+module AttributeTest
+
+open System.Diagnostics
+
+[<Conditional("DEBUG", ConditionString = "RELEASE")>]
+type TestClass() = class end
+"""
+    // Act & Assert
+    FSharp mainCode
+    |> compile
+    |> shouldFail
+    |> withDiagnostics [
+        (Error, 3248, Line 5, Col 13, Line 5, Col 41, "Property 'ConditionString' on attribute cannot be set because the setter is private")
+    ]
+    |> ignore

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -312,6 +312,7 @@
     <Compile Include="CompilerService\AsyncMemoize.fs" />
     <Compile Include="CompilerService\FSharpWorkspace.fs" />
     <Compile Include="CompilerService\DependencyGraph.fs" />
+    <Compile Include="Attributes\AttributePrivateSetterTests.fs" />
     <Compile Include="Debugger\PortablePdbs.fs" />
     <Compile Include="Diagnostics\async.fs" />
     <Compile Include="Diagnostics\General.fs" />