diff --git a/tests/ui/associated-types/projection-dyn-associated-type.rs b/tests/ui/associated-types/projection-dyn-associated-type.rs
new file mode 100644
index 0000000000000..3b981e7987e0e
--- /dev/null
+++ b/tests/ui/associated-types/projection-dyn-associated-type.rs
@@ -0,0 +1,28 @@
+// Regression test for the projection bug in <https://github.com/rust-lang/rust/issues/123953>
+//
+//@ compile-flags: -Zincremental-verify-ich=yes
+//@ incremental
+
+pub trait A {}
+pub trait B: A {}
+
+pub trait Mirror {
+    type Assoc: ?Sized;
+}
+
+impl<T: ?Sized> Mirror for A {
+    //~^ ERROR the type parameter `T` is not constrained by the impl trait, self type, or predicates [E0207]
+    //~| WARN trait objects without an explicit `dyn` are deprecated [bare_trait_objects]
+    //~| WARN this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+    type Assoc = T;
+}
+
+pub fn foo<'a>(
+    x: &'a <dyn A + 'static as Mirror>::Assoc
+) -> &'a <dyn B + 'static as Mirror>::Assoc {
+    //~^ ERROR the trait bound `(dyn B + 'static): Mirror` is not satisfied [E0277]
+    //~| ERROR the trait bound `(dyn B + 'static): Mirror` is not satisfied [E0277]
+    static
+} //~ ERROR expected identifier, found `}`
+
+pub fn main() {}
diff --git a/tests/ui/associated-types/projection-dyn-associated-type.stderr b/tests/ui/associated-types/projection-dyn-associated-type.stderr
new file mode 100644
index 0000000000000..1ac2beb0414e2
--- /dev/null
+++ b/tests/ui/associated-types/projection-dyn-associated-type.stderr
@@ -0,0 +1,52 @@
+error: expected identifier, found `}`
+  --> $DIR/projection-dyn-associated-type.rs:26:1
+   |
+LL | }
+   | ^ expected identifier
+
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/projection-dyn-associated-type.rs:13:28
+   |
+LL | impl<T: ?Sized> Mirror for A {
+   |                            ^
+   |
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2021/warnings-promoted-to-error.html>
+   = note: `#[warn(bare_trait_objects)]` (part of `#[warn(rust_2021_compatibility)]`) on by default
+help: if this is a dyn-compatible trait, use `dyn`
+   |
+LL | impl<T: ?Sized> Mirror for dyn A {
+   |                            +++
+help: alternatively use a blanket implementation to implement `Mirror` for all types that also implement `A`
+   |
+LL - impl<T: ?Sized> Mirror for A {
+LL + impl<T: ?Sized, U: A> Mirror for U {
+   |
+
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/projection-dyn-associated-type.rs:13:6
+   |
+LL | impl<T: ?Sized> Mirror for A {
+   |      ^ unconstrained type parameter
+
+error[E0277]: the trait bound `(dyn B + 'static): Mirror` is not satisfied
+  --> $DIR/projection-dyn-associated-type.rs:22:6
+   |
+LL | ) -> &'a <dyn B + 'static as Mirror>::Assoc {
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Mirror` is not implemented for `(dyn B + 'static)`
+   |
+   = help: the trait `Mirror` is implemented for `dyn A`
+
+error[E0277]: the trait bound `(dyn B + 'static): Mirror` is not satisfied
+  --> $DIR/projection-dyn-associated-type.rs:22:6
+   |
+LL | ) -> &'a <dyn B + 'static as Mirror>::Assoc {
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Mirror` is not implemented for `(dyn B + 'static)`
+   |
+   = help: the trait `Mirror` is implemented for `dyn A`
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 4 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0207, E0277.
+For more information about an error, try `rustc --explain E0207`.