3 files changed, 46 insertions, 3 deletions
diff --git a/tests/ui/traits/object/constrain-via-unnecessary-bound.rs b/tests/ui/traits/object/constrain-via-unnecessary-bound.rs
new file mode 100644
index 00000000000..4640d6b3ed5
--- /dev/null
+++ b/tests/ui/traits/object/constrain-via-unnecessary-bound.rs
@@ -0,0 +1,24 @@
+//@ check-pass
+
+// Regression test for <https://github.com/rust-lang/rust/issues/140645>.
+// Test that we lower impossible-to-satisfy associated type bounds, which
+// may for example constrain impl parameters.
+
+pub trait Other {}
+
+pub trait Trait {
+    type Assoc
+    where
+        Self: Sized;
+}
+
+impl Other for dyn Trait {}
+// `dyn Trait<Assoc = ()>` is a different "nominal type" than `dyn Trait`.
+impl Other for dyn Trait<Assoc = ()> {}
+//~^ WARN unnecessary associated type bound for dyn-incompatible associated type
+
+// I hope it's clear that `dyn Trait` (w/o `Assoc`) wouldn't match this impl.
+impl<T> dyn Trait<Assoc = T> {}
+//~^ WARN unnecessary associated type bound for dyn-incompatible associated type
+
+fn main() {}
diff --git a/tests/ui/traits/object/constrain-via-unnecessary-bound.stderr b/tests/ui/traits/object/constrain-via-unnecessary-bound.stderr
new file mode 100644
index 00000000000..4383ca869ea
--- /dev/null
+++ b/tests/ui/traits/object/constrain-via-unnecessary-bound.stderr
@@ -0,0 +1,19 @@
+warning: unnecessary associated type bound for dyn-incompatible associated type
+  --> $DIR/constrain-via-unnecessary-bound.rs:17:26
+   |
+LL | impl Other for dyn Trait<Assoc = ()> {}
+   |                          ^^^^^^^^^^ help: remove this bound
+   |
+   = note: this associated type has a `where Self: Sized` bound, and while the associated type can be specified, it cannot be used because trait objects are never `Sized`
+   = note: `#[warn(unused_associated_type_bounds)]` on by default
+
+warning: unnecessary associated type bound for dyn-incompatible associated type
+  --> $DIR/constrain-via-unnecessary-bound.rs:21:19
+   |
+LL | impl<T> dyn Trait<Assoc = T> {}
+   |                   ^^^^^^^^^ help: remove this bound
+   |
+   = note: this associated type has a `where Self: Sized` bound, and while the associated type can be specified, it cannot be used because trait objects are never `Sized`
+
+warning: 2 warnings emitted
+
diff --git a/tests/ui/traits/object/pretty.stderr b/tests/ui/traits/object/pretty.stderr
index 37fe142951d..2f9fdf151f0 100644
--- a/tests/ui/traits/object/pretty.stderr
+++ b/tests/ui/traits/object/pretty.stderr
@@ -154,12 +154,12 @@ error[E0308]: mismatched types
   --> $DIR/pretty.rs:41:56
    |
 LL | fn dyn_has_gat(x: &dyn HasGat<u8, Assoc<bool> = ()>) { x }
-   |                                                     -  ^ expected `()`, found `&dyn HasGat<u8>`
+   |                                                     -  ^ expected `()`, found `&dyn HasGat<u8, Assoc<bool> = ()>`
    |                                                     |
-   |                                                     help: try adding a return type: `-> &dyn HasGat<u8>`
+   |                                                     help: try adding a return type: `-> &dyn HasGat<u8, Assoc<bool> = ()>`
    |
    = note: expected unit type `()`
-              found reference `&dyn HasGat<u8>`
+              found reference `&dyn HasGat<u8, Assoc<bool> = ()>`
 
 error: aborting due to 14 previous errors; 1 warning emitted