diff --git a/tests/ui/not_unpin/negative_impls_stable.rs b/tests/ui/not_unpin/negative_impls_stable.rs
new file mode 100644
index 00000000..1a5b4258
--- /dev/null
+++ b/tests/ui/not_unpin/negative_impls_stable.rs
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+// https://github.com/taiki-e/pin-project/issues/340#issuecomment-2428002670
+
+#[pin_project::pin_project(!Unpin)]
+struct Foo<Pinned, Unpinned> {
+    #[pin]
+    pinned: Pinned,
+    unpinned: Unpinned,
+}
+
+struct MyPhantomPinned(::core::marker::PhantomPinned);
+impl Unpin for MyPhantomPinned where for<'cursed> str: Sized {}
+impl Unpin for Foo<MyPhantomPinned, ()> {}
+
+fn is_unpin<T: Unpin>() {}
+
+fn main() {
+    is_unpin::<Foo<MyPhantomPinned, ()>>()
+}
diff --git a/tests/ui/not_unpin/negative_impls_stable.stderr b/tests/ui/not_unpin/negative_impls_stable.stderr
new file mode 100644
index 00000000..79e75fe1
--- /dev/null
+++ b/tests/ui/not_unpin/negative_impls_stable.stderr
@@ -0,0 +1,8 @@
+error[E0119]: conflicting implementations of trait `Unpin` for type `Foo<MyPhantomPinned, ()>`
+  --> tests/ui/not_unpin/negative_impls_stable.rs:5:28
+   |
+5  | #[pin_project::pin_project(!Unpin)]
+   |                            ^^^^^^ conflicting implementation for `Foo<MyPhantomPinned, ()>`
+...
+14 | impl Unpin for Foo<MyPhantomPinned, ()> {}
+   | --------------------------------------- first implementation here
diff --git a/tests/ui/unsafe_unpin/negative_impls_stable.rs b/tests/ui/unsafe_unpin/negative_impls_stable.rs
new file mode 100644
index 00000000..88faacc0
--- /dev/null
+++ b/tests/ui/unsafe_unpin/negative_impls_stable.rs
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+// https://github.com/taiki-e/pin-project/issues/340#issuecomment-2428002670
+
+#[pin_project::pin_project(UnsafeUnpin)]
+struct Foo<Pinned, Unpinned> {
+    #[pin]
+    pinned: Pinned,
+    unpinned: Unpinned,
+}
+
+unsafe impl<Pinned: Unpin, Unpinned> pin_project::UnsafeUnpin for Foo<Pinned, Unpinned> {}
+
+struct MyPhantomPinned(::core::marker::PhantomPinned);
+impl Unpin for MyPhantomPinned where for<'cursed> str: Sized {}
+impl Unpin for Foo<MyPhantomPinned, ()> {}
+
+fn is_unpin<T: Unpin>() {}
+
+fn main() {
+    is_unpin::<Foo<MyPhantomPinned, ()>>()
+}
diff --git a/tests/ui/unsafe_unpin/negative_impls_stable.stderr b/tests/ui/unsafe_unpin/negative_impls_stable.stderr
new file mode 100644
index 00000000..ac81ad64
--- /dev/null
+++ b/tests/ui/unsafe_unpin/negative_impls_stable.stderr
@@ -0,0 +1,8 @@
+error[E0119]: conflicting implementations of trait `Unpin` for type `Foo<MyPhantomPinned, ()>`
+  --> tests/ui/unsafe_unpin/negative_impls_stable.rs:5:28
+   |
+5  | #[pin_project::pin_project(UnsafeUnpin)]
+   |                            ^^^^^^^^^^^ conflicting implementation for `Foo<MyPhantomPinned, ()>`
+...
+16 | impl Unpin for Foo<MyPhantomPinned, ()> {}
+   | --------------------------------------- first implementation here