From e2ca36c5212469ee1d4f16e6c8391643e069b3a4 Mon Sep 17 00:00:00 2001
From: ncave <777696+ncave@users.noreply.github.com>
Date: Mon, 10 Jun 2024 16:22:00 -0700
Subject: [PATCH] Fixed #3658

---
 src/Fable.Transforms/FSharp2Fable.fs | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/src/Fable.Transforms/FSharp2Fable.fs b/src/Fable.Transforms/FSharp2Fable.fs
index 22a134c048..5e3dfff915 100644
--- a/src/Fable.Transforms/FSharp2Fable.fs
+++ b/src/Fable.Transforms/FSharp2Fable.fs
@@ -1474,20 +1474,33 @@ let private transformExpr (com: IFableCompiler) (ctx: Context) appliedGenArgs fs
                 |> addErrorAndReturnNull com ctx.InlinePath (makeRangeFrom fsExpr)
     }
 
-let private isIgnoredNonAttachedMember (meth: FSharpMemberOrFunctionOrValue) =
-    Option.isSome meth.LiteralValue
-    || meth.Attributes
+let private isIgnoredNonAttachedMember (memb: FSharpMemberOrFunctionOrValue) =
+    Option.isSome memb.LiteralValue
+    || memb.Attributes
        |> Seq.exists (fun att ->
            match att.AttributeType.TryFullName with
            | Some(Atts.global_ | Naming.StartsWith Atts.import _ | Naming.StartsWith Atts.emit _) -> true
            | _ -> false
        )
     || (
-        match meth.DeclaringEntity with
+        match memb.DeclaringEntity with
         | Some ent -> isGlobalOrImportedFSharpEntity ent
         | None -> false
     )
 
+let private isErasedUnionCaseTester (memb: FSharpMemberOrFunctionOrValue) =
+    // if memb.IsUnionCaseTester then // TODO: this currently fails, use when fixed
+    if memb.IsPropertyGetterMethod && memb.LogicalName.StartsWith("get_Is") then
+        // currently returns true for any tester if any union case is erased
+        // TODO: return true only when the tester's own union case is erased
+        match memb.DeclaringEntity with
+        | Some ent when ent.IsFSharpUnion ->
+            ent.UnionCases
+            |> Seq.exists (fun unionCase -> hasAttrib Atts.erase unionCase.Attributes)
+        | _ -> false
+    else
+        false
+
 let private isCompilerGenerated (memb: FSharpMemberOrFunctionOrValue) (args: FSharpMemberOrFunctionOrValue list list) =
     memb.IsCompilerGenerated
     && memb.IsInstanceMember
@@ -1905,6 +1918,9 @@ let private transformMemberDecl
         []
     elif memb.IsImplicitConstructor then
         transformPrimaryConstructor com ctx memb args body
+    // Ignore union case testers for unions with erased union cases
+    elif isErasedUnionCaseTester memb then
+        []
     // Ignore members generated by the F# compiler (for comparison and equality)
     elif isCompilerGenerated memb args then
         []
@@ -2047,8 +2063,8 @@ let rec private transformDeclarations (com: FableCompiler) ctx fsDecls =
                         }
                 ]
             | sub -> transformDeclarations com ctx sub
-        | FSharpImplementationFileDeclaration.MemberOrFunctionOrValue(meth, args, body) ->
-            transformMemberDecl com ctx meth args body
+        | FSharpImplementationFileDeclaration.MemberOrFunctionOrValue(memb, args, body) ->
+            transformMemberDecl com ctx memb args body
         | FSharpImplementationFileDeclaration.InitAction fe ->
             let ctx = { ctx with UsedNamesInDeclarationScope = HashSet() }
             let e = transformExpr com ctx [] fe |> run