Complete fix for duplicate .cctor issue and add release notes

Co-authored-by: T-Gro <46543583+T-Gro@users.noreply.github.com>

Author: Copilot

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

@@ -4120,6 +4120,7 @@ let cdef_cctorCode2CodeOrCreate tag imports f (cd: ILTypeDef) =
             mkILClassCtor body
         | multipleCctors ->
             // Handle multiple .cctor methods by merging their instruction bodies
+            // This resolves the "duplicate entry '.cctor' in method table" error
             // Extract the instruction sequences from all .cctor methods (excluding the final 'ret')
             let allInstrs = 
                 multipleCctors

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 +30,24 @@ IL_0005:  ret"""
                       """
 IL_0000:  call       !!0[] [runtime]System.Array::Empty<!!0>()
 IL_0005:  ret""" ]
+
+    [<Fact>]
+    let ``Discriminated union with generic statics generates single merged cctor``() =
+        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"""]