Update union-case declaration AttributeTargets.Property (#16807)

Author: edgarfgp

src/Compiler/Checking/CheckDeclarations.fs

@@ -511,14 +511,6 @@ module TcRecdUnionAndEnumDeclarations =
 
     let TcUnionCaseDecl (cenv: cenv) env parent thisTy thisTyInst tpenv hasRQAAttribute (SynUnionCase(Attributes synAttrs, SynIdent(id, _), args, xmldoc, vis, m, _)) =
         let g = cenv.g
-        let attrs =
-            // The attributes of a union case decl get attached to the generated "static factory" method
-            // Enforce that the union-cases can only be targeted by attributes with AttributeTargets.Method
-            if g.langVersion.SupportsFeature(LanguageFeature.EnforceAttributeTargetsUnionCaseDeclarations) then
-                TcAttributes cenv env AttributeTargets.Method synAttrs
-            else
-                TcAttributes cenv env AttributeTargets.UnionCaseDecl synAttrs
-            
         let vis, _ = ComputeAccessAndCompPath g env None m vis None parent
         let vis = CombineReprAccess parent vis
 
@@ -571,6 +563,23 @@ module TcRecdUnionAndEnumDeclarations =
 
         let checkXmlDocs = cenv.diagnosticOptions.CheckXmlDocs
         let xmlDoc = xmldoc.ToXmlDoc(checkXmlDocs, Some names)
+        let attrs =
+            (*
+                The attributes of a union case decl get attached to the generated "static factory" method.
+                Enforce union-cases AttributeTargets:
+                - AttributeTargets.Method
+                    type SomeUnion =
+                    | Case1 of int // Compiles down to a static method
+                - AttributeTargets.Property
+                    type SomeUnion =
+                    | Case1 // Compiles down to a static property
+            *)
+            if g.langVersion.SupportsFeature(LanguageFeature.EnforceAttributeTargetsUnionCaseDeclarations) then
+                let target = if rfields.IsEmpty then AttributeTargets.Property else AttributeTargets.Method
+                TcAttributes cenv env target synAttrs
+            else
+                TcAttributes cenv env AttributeTargets.UnionCaseDecl synAttrs
+        
         Construct.NewUnionCase id rfields recordTy attrs xmlDoc vis
 
     let TcUnionCaseDecls (cenv: cenv) env (parent: ParentRef) (thisTy: TType) (thisTyInst: TypeInst) hasRQAAttribute tpenv unionCases =

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/AttributeUsage/AttributeTargetsIsProperty.fs

@@ -0,0 +1,10 @@
+
+open System
+
+[<AttributeUsage(AttributeTargets.Property)>]
+type PropertyLevelAttribute() =
+    inherit Attribute()
+
+type U =
+    | [<PropertyLevel>] A
+    | [<PropertyLevel>] B

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/AttributeUsage/AttributeUsage.fs

@@ -71,6 +71,21 @@ module CustomAttributes_AttributeUsage =
         |> withLangVersionPreview
         |> verifyCompileAndRun
         |> shouldSucceed
+        
+    // SOURCE=AttributeTargetsIsProperty.fs	# AttributeTargetsIsProperty.fs
+    [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"AttributeTargetsIsProperty.fs"|])>]
+    let ``AttributeTargetsIsProperty_fs`` compilation =
+        compilation
+        |> verifyCompile
+        |> shouldSucceed
+        
+    // SOURCE=AttributeTargetsIsProperty.fs	# AttributeTargetsIsProperty.fs
+    [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"AttributeTargetsIsProperty.fs"|])>]
+    let ``AttributeTargetsIsProperty_fs preview`` compilation =
+        compilation
+        |> withLangVersionPreview
+        |> verifyCompile
+        |> shouldSucceed
 
     // SOURCE=ConditionalAttribute.fs					# ConditionalAttribute.fs
     [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"ConditionalAttribute.fs"|])>]