Fix nativeptr type erasure to preserve element type in overload resolution

docs/release-notes/.Language/preview.md

@@ -16,5 +16,6 @@
 
 * Warn on uppercase identifiers in patterns. ([PR #15816](https://github.com/dotnet/fsharp/pull/15816))
 * Error on invalid declarations in type definitions.([Issue #10066](https://github.com/dotnet/fsharp/issues/10066), [PR #18813](https://github.com/dotnet/fsharp/pull/18813))
+* Fix type erasure logic for `nativeptr<'T>` overloads to properly preserve element type differences during duplicate member checking. ([Issue #<ISSUE_NUMBER>](https://github.com/dotnet/fsharp/issues/<ISSUE_NUMBER>), [PR #<PR_NUMBER>](https://github.com/dotnet/fsharp/pull/<PR_NUMBER>))
 
 ### Changed

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -803,7 +803,8 @@ let rec stripTyEqnsAndErase eraseFuncAndTuple (g: TcGlobals) ty =
             let reducedTy2 = addNullnessToTy nullness reducedTy
             stripTyEqnsAndErase eraseFuncAndTuple g reducedTy2
         elif tyconRefEq g tcref g.nativeptr_tcr && eraseFuncAndTuple then 
-            stripTyEqnsAndErase eraseFuncAndTuple g g.nativeint_ty
+            // Regression fix (issue #<ISSUE_NUMBER>): nativeptr<'T> erases to ilsigptr<'T>, not nativeint
+            stripTyEqnsAndErase eraseFuncAndTuple g (TType_app(g.ilsigptr_tcr, args, nullness))
         else
             ty
 

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OverloadingMembers/NativePtrOverloads01.fs

@@ -0,0 +1,9 @@
+module NativePtrOverloads01
+open Microsoft.FSharp.NativeInterop
+#nowarn "9"
+
+type P =
+    static member Do(p: nativeptr<int>) = 1
+    static member Do(p: nativeptr<int64>) = 2
+
+let _invoke (pi: nativeptr<int>) (pl: nativeptr<int64>) = P.Do pi + P.Do pl

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OverloadingMembers/NativePtrOverloads02.fs

@@ -0,0 +1,7 @@
+module NativePtrOverloads02
+open Microsoft.FSharp.NativeInterop
+#nowarn "9"
+
+type Q =
+    static member M(p: nativeptr<int>) = 0
+    static member M(p: nativeptr<int>) = 1  // expect duplicate member error

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OverloadingMembers/NativePtrOverloads03.fs

@@ -0,0 +1,8 @@
+module NativePtrOverloads03
+open Microsoft.FSharp.NativeInterop
+#nowarn "9"
+// Regression test for issue #<ISSUE_NUMBER>
+
+type R =
+    static member F(p: nativeptr<uint16>) = 0us
+    static member F(p: nativeptr<int64>) = 0L

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OverloadingMembers/NativePtrOverloads04.fs

@@ -0,0 +1,9 @@
+module NativePtrOverloads04
+open Microsoft.FSharp.NativeInterop
+#nowarn "9"
+[<Measure>] type kg
+[<Measure>] type m
+
+type S =
+    static member H(p: nativeptr<int<kg>>) = 1
+    static member H(p: nativeptr<int<m>>) = 2  // expect duplicate member error

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MemberDefinitions/OverloadingMembers/OverloadingMembers.fs

@@ -235,3 +235,49 @@ module MemberDefinitions_OverloadingMembers =
         compilation
         |> verifyCompileAndRun
         |> shouldSucceed
+
+    // Native pointer overload tests for regression fix
+
+    // NativePtrOverloads01.fs - distinct native pointer element types should compile
+    [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"NativePtrOverloads01.fs"|])>]
+    let ``NativePtrOverloads01_fs`` compilation =
+        compilation
+        |> asLibrary
+        |> withOptions ["--nowarn:9"]
+        |> compile
+        |> shouldSucceed
+
+    // NativePtrOverloads02.fs - duplicate exact signatures should fail
+    [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"NativePtrOverloads02.fs"|])>]
+    let ``NativePtrOverloads02_fs`` compilation =
+        compilation
+        |> asLibrary
+        |> withOptions ["--nowarn:9"]
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 438, Line 6, Col 17, Line 6, Col 18, "Duplicate method. The method 'M' has the same name and signature as another method in type 'Q'.")
+            (Error 438, Line 7, Col 17, Line 7, Col 18, "Duplicate method. The method 'M' has the same name and signature as another method in type 'Q'.")
+        ]
+
+    // NativePtrOverloads03.fs - regression test, previously failing overloads should now compile
+    [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"NativePtrOverloads03.fs"|])>]
+    let ``NativePtrOverloads03_fs`` compilation =
+        compilation
+        |> asLibrary
+        |> withOptions ["--nowarn:9"]
+        |> compile
+        |> shouldSucceed
+
+    // NativePtrOverloads04.fs - erased differences via measures should still fail
+    [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"NativePtrOverloads04.fs"|])>]
+    let ``NativePtrOverloads04_fs`` compilation =
+        compilation
+        |> asLibrary
+        |> withOptions ["--nowarn:9"]
+        |> compile
+        |> shouldFail
+        |> withDiagnostics [
+            (Error 438, Line 7, Col 17, Line 7, Col 18, "Duplicate method. The method 'H' has the same name and signature as another method in type 'S'.")
+            (Error 438, Line 8, Col 17, Line 8, Col 18, "Duplicate method. The method 'H' has the same name and signature as another method in type 'S'.")
+        ]