diff --git a/tests/ui/resolve/auxiliary/proc_macro_generate_packed.rs b/tests/ui/resolve/auxiliary/proc_macro_generate_packed.rs
new file mode 100644
index 0000000000000..c0b24706dcba6
--- /dev/null
+++ b/tests/ui/resolve/auxiliary/proc_macro_generate_packed.rs
@@ -0,0 +1,27 @@
+//@ force-host
+//@ no-prefer-dynamic
+//@ compile-flags: --crate-type proc-macro
+
+extern crate proc_macro;
+
+use proc_macro::*;
+
+#[proc_macro_attribute]
+pub fn proc_macro_attribute_that_generates_repr_packed(
+    _attr: TokenStream,
+    item: TokenStream,
+) -> TokenStream {
+    let repr = vec![TokenTree::Ident(Ident::new("packed", Span::call_site()))].into_iter();
+    let attr = vec![
+        TokenTree::Ident(Ident::new("repr", Span::call_site())),
+        TokenTree::Group(Group::new(Delimiter::Parenthesis, repr.collect())),
+    ]
+    .into_iter();
+    vec![
+        TokenTree::Punct(Punct::new('#', Spacing::Alone)),
+        TokenTree::Group(Group::new(Delimiter::Bracket, attr.collect())),
+    ]
+    .into_iter()
+    .chain(item)
+    .collect()
+}
diff --git a/tests/ui/resolve/multiple_definitions_attribute_merging.rs b/tests/ui/resolve/multiple_definitions_attribute_merging.rs
new file mode 100644
index 0000000000000..523717861e1ba
--- /dev/null
+++ b/tests/ui/resolve/multiple_definitions_attribute_merging.rs
@@ -0,0 +1,19 @@
+//! This test ICEs because the `repr(packed)` attributes
+//! end up on the `Dealigned` struct's attribute list, but the
+//! derive didn't see that.
+
+//@known-bug: #120873
+//@ failure-status: 101
+//@ normalize-stderr-test "note: .*\n\n" -> ""
+//@ normalize-stderr-test "thread 'rustc' panicked.*\n" -> ""
+//@ normalize-stderr-test "(error: internal compiler error: [^:]+):\d+:\d+: " -> "$1:LL:CC: "
+//@ rustc-env:RUST_BACKTRACE=0
+
+#[repr(packed)]
+struct Dealigned<T>(u8, T);
+
+#[derive(PartialEq)]
+#[repr(C)]
+struct Dealigned<T>(u8, T);
+
+fn main() {}
diff --git a/tests/ui/resolve/multiple_definitions_attribute_merging.stderr b/tests/ui/resolve/multiple_definitions_attribute_merging.stderr
new file mode 100644
index 0000000000000..b2d20af883abb
--- /dev/null
+++ b/tests/ui/resolve/multiple_definitions_attribute_merging.stderr
@@ -0,0 +1,26 @@
+error[E0428]: the name `Dealigned` is defined multiple times
+  --> $DIR/multiple_definitions_attribute_merging.rs:17:1
+   |
+LL | struct Dealigned<T>(u8, T);
+   | --------------------------- previous definition of the type `Dealigned` here
+...
+LL | struct Dealigned<T>(u8, T);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Dealigned` redefined here
+   |
+   = error: internal compiler error: compiler/rustc_mir_transform/src/check_packed_ref.rs:LL:CC: builtin derive created an unaligned reference
+  --> $DIR/multiple_definitions_attribute_merging.rs:17:25
+   |
+LL | #[derive(PartialEq)]
+   |          --------- in this derive macro expansion
+LL | #[repr(C)]
+LL | struct Dealigned<T>(u8, T);
+   |                         ^
+   |
+   = Box<dyn Any>
+query stack during panic:
+#0 [mir_const] preparing `<impl at $DIR/multiple_definitions_attribute_merging.rs:15:10: 15:19>::eq` for borrow checking
+#1 [mir_promoted] promoting constants in MIR for `<impl at $DIR/multiple_definitions_attribute_merging.rs:15:10: 15:19>::eq`
+end of query stack
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0428`.
diff --git a/tests/ui/resolve/proc_macro_generated_packed.rs b/tests/ui/resolve/proc_macro_generated_packed.rs
new file mode 100644
index 0000000000000..34a7e4db6030d
--- /dev/null
+++ b/tests/ui/resolve/proc_macro_generated_packed.rs
@@ -0,0 +1,20 @@
+//! This test ICEs because the `repr(packed)` attribute
+//! was generated by a proc macro, so `#[derive]` didn't see it.
+
+//@aux-build: proc_macro_generate_packed.rs
+//@known-bug: #120873
+//@ failure-status: 101
+//@ normalize-stderr-test "note: .*\n\n" -> ""
+//@ normalize-stderr-test "thread 'rustc' panicked.*\n" -> ""
+//@ normalize-stderr-test "(error: internal compiler error: [^:]+):\d+:\d+: " -> "$1:LL:CC: "
+//@ rustc-env:RUST_BACKTRACE=0
+
+extern crate proc_macro_generate_packed;
+use proc_macro_generate_packed::proc_macro_attribute_that_generates_repr_packed;
+
+#[derive(PartialEq)]
+#[repr(C)]
+#[proc_macro_attribute_that_generates_repr_packed]
+struct Dealigned<T>(u8, T);
+
+fn main() {}
diff --git a/tests/ui/resolve/proc_macro_generated_packed.stderr b/tests/ui/resolve/proc_macro_generated_packed.stderr
new file mode 100644
index 0000000000000..507e5867c904b
--- /dev/null
+++ b/tests/ui/resolve/proc_macro_generated_packed.stderr
@@ -0,0 +1,16 @@
+error: internal compiler error: compiler/rustc_mir_transform/src/check_packed_ref.rs:LL:CC: builtin derive created an unaligned reference
+  --> $DIR/proc_macro_generated_packed.rs:18:25
+   |
+LL | #[derive(PartialEq)]
+   |          --------- in this derive macro expansion
+...
+LL | struct Dealigned<T>(u8, T);
+   |                         ^
+   |
+   = Box<dyn Any>
+query stack during panic:
+#0 [mir_const] preparing `<impl at $DIR/proc_macro_generated_packed.rs:15:10: 15:19>::eq` for borrow checking
+#1 [mir_promoted] promoting constants in MIR for `<impl at $DIR/proc_macro_generated_packed.rs:15:10: 15:19>::eq`
+end of query stack
+error: aborting due to 1 previous error
+