Fix FS2014 duplicate .cctor error by renaming and delegating static constructors

docs/release-notes/.FSharp.Compiler.Service/10.0.100.md

@@ -4,6 +4,7 @@
 
 ### Fixed
 
+* Fix duplicate .cctor issue for discriminated unions with generic statics ([Issue #18767](https://github.com/dotnet/fsharp/issues/18767))
 * Fix SignatureHash to include constant values in hash computation ([Issue #18758](https://github.com/dotnet/fsharp/issues/18758))
 * Fix parsing errors using anonymous records and units of measures ([PR #18543](https://github.com/dotnet/fsharp/pull/18543))
 * Fix parsing errors using anonymous records and code quotations ([PR #18603](https://github.com/dotnet/fsharp/pull/18603))

src/Compiler/AbstractIL/il.fs

@@ -4112,18 +4112,47 @@ let prependInstrsToMethod newCode md =
 let cdef_cctorCode2CodeOrCreate tag imports f (cd: ILTypeDef) =
     let mdefs = cd.Methods
 
-    let cctor =
+    let cctor, renamedCctors =
         match mdefs.FindByName ".cctor" with
-        | [ mdef ] -> mdef
+        | [ mdef ] -> mdef, []
         | [] ->
             let body = mkMethodBody (false, [], 1, nonBranchingInstrsToCode [], tag, imports)
-            mkILClassCtor body
-        | _ -> failwith "bad method table: more than one .cctor found"
+            mkILClassCtor body, []
+        | multipleCctors ->
+            // Handle multiple .cctor methods by renaming them and creating a new .cctor that calls them
+            // This resolves the "duplicate entry '.cctor' in method table" error (FS2014)
+            let renamedCctors = 
+                multipleCctors
+                |> List.mapi (fun i mdef ->
+                    let newName = 
+                        match i with
+                        | 0 -> "cctor_IncrClass"
+                        | 1 -> "cctor_UnionErasure" 
+                        | _ -> sprintf "cctor_%d" i
+                    mdef.With(name = newName))
+
+            // Create call instructions for each renamed .cctor
+            // Use a simple self-referencing type
+            let currentTypeRef = mkILTyRef (ILScopeRef.Local, cd.Name)
+            let currentType = mkILNonGenericBoxedTy currentTypeRef
+            let callInstrs = 
+                renamedCctors
+                |> List.map (fun mdef -> 
+                    let mspec = mkILNonGenericStaticMethSpecInTy (currentType, mdef.Name, [], ILType.Void)
+                    mkNormalCall mspec)
+
+            // Create new .cctor that calls all renamed methods
+            let newCctorInstrs = callInstrs @ [I_ret]
+            let newCctorBody = mkMethodBody (false, [], 8, nonBranchingInstrsToCode newCctorInstrs, tag, imports)
+            let newCctor = mkILClassCtor newCctorBody
+
+            newCctor, renamedCctors
 
     let methods =
         ILMethodDefs(fun () ->
             [|
                 yield f cctor
+                yield! renamedCctors
                 for md in mdefs do
                     if md.Name <> ".cctor" then
                         yield md

tests/FSharp.Compiler.ComponentTests/EmittedIL/Misc/DuplicateCctorFix.fs

@@ -0,0 +1,15 @@
+// #NoMono #NoMT #CodeGen #EmittedIL   
+// Regression test for duplicate .cctor issue: https://github.com/dotnet/fsharp/issues/18767
+// This test verifies that discriminated unions with generic statics and nullary cases 
+// generate only one .cctor method instead of failing with "duplicate entry '.cctor' in method table"
+
+type TestUnion<'T when 'T: comparison> =
+    | A of 'T
+    | B of string  
+    | C // nullary case that triggers union erasure .cctor for constant field initialization
+
+    // Static member that triggers incremental class .cctor generation
+    static member val StaticProperty = "test" with get, set
+
+    // Another static member to ensure .cctor has meaningful initialization
+    static member CompareStuff x y = compare x y

tests/FSharp.Compiler.ComponentTests/Misc.fs

@@ -30,3 +30,33 @@ IL_0005:  ret"""
                       """
 IL_0000:  call       !!0[] [runtime]System.Array::Empty<!!0>()
 IL_0005:  ret""" ]
+
+    [<Fact>]
+    let ``Discriminated union with generic statics generates single cctor calling renamed methods``() =
+        FSharp """
+module DuplicateCctorFix
+
+type TestUnion<'T when 'T: comparison> =
+    | A of 'T
+    | B of string  
+    | C // nullary case that triggers union erasure .cctor for constant field initialization
+
+    // Static member that triggers incremental class .cctor generation
+    static member val StaticProperty = "test" with get, set
+
+    // Another static member to ensure .cctor has meaningful initialization
+    static member CompareStuff x y = compare x y
+         """
+         |> compile
+         |> shouldSucceed
+         |> verifyIL [""".method private specialname rtspecialname static 
+          void  .cctor() cil managed
+  {
+    // Code size
+    IL_0000:  call       void DuplicateCctorFix/TestUnion`1::cctor_IncrClass()
+    IL_0005:  call       void DuplicateCctorFix/TestUnion`1::cctor_UnionErasure()
+    IL_000a:  ret
+  } // end of method TestUnion`1::.cctor
+
+  .method private static void cctor_IncrClass() cil managed
+  .method private static void cctor_UnionErasure() cil managed"""]