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diff --git a/tests/ui/generics/duplicate-generic-parameter-error-86756.stderr b/tests/ui/generics/duplicate-generic-parameter-error-86756.stderr
new file mode 100644
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+++ b/tests/ui/generics/duplicate-generic-parameter-error-86756.stderr
@@ -0,0 +1,48 @@
+error[E0403]: the name `T` is already used for a generic parameter in this item's generic parameters
+  --> $DIR/duplicate-generic-parameter-error-86756.rs:3:14
+   |
+LL | trait Foo<T, T = T> {}
+   |           -  ^ already used
+   |           |
+   |           first use of `T`
+
+error[E0412]: cannot find type `dyn` in this scope
+  --> $DIR/duplicate-generic-parameter-error-86756.rs:7:10
+   |
+LL |     eq::<dyn, Foo>
+   |          ^^^ not found in this scope
+
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/duplicate-generic-parameter-error-86756.rs:7:15
+   |
+LL |     eq::<dyn, Foo>
+   |               ^^^
+   |
+   = 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 |     eq::<dyn, dyn Foo>
+   |               +++
+
+error[E0107]: missing generics for trait `Foo`
+  --> $DIR/duplicate-generic-parameter-error-86756.rs:7:15
+   |
+LL |     eq::<dyn, Foo>
+   |               ^^^ expected at least 1 generic argument
+   |
+note: trait defined here, with at least 1 generic parameter: `T`
+  --> $DIR/duplicate-generic-parameter-error-86756.rs:3:7
+   |
+LL | trait Foo<T, T = T> {}
+   |       ^^^ -
+help: add missing generic argument
+   |
+LL |     eq::<dyn, Foo<T>>
+   |                  +++
+
+error: aborting due to 3 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0107, E0403, E0412.
+For more information about an error, try `rustc --explain E0107`.