clean up issue-21950 (dyn trait cast without assoc type at the cast)

2 files changed, 31 insertions, 0 deletions

diff --git a/tests/ui/traits/cast-as-dyn-trait-wo-assoc-type-issue-21950.rs b/tests/ui/traits/cast-as-dyn-trait-wo-assoc-type-issue-21950.rs
new file mode 100644
index 00000000000..3c381505450
--- /dev/null
+++ b/tests/ui/traits/cast-as-dyn-trait-wo-assoc-type-issue-21950.rs
@@ -0,0 +1,19 @@
+//! Tests that compiler yields error E0191 when value with existing trait implementation
+//! is cast as same `dyn` trait without specifying associated type at the cast.
+//!
+//! # Context
+//! Original issue: https://github.com/rust-lang/rust/issues/21950
+
+trait Add<Rhs=Self> {
+    type Output;
+}
+
+impl Add for i32 {
+    type Output = i32;
+}
+
+fn main() {
+    let x = &10 as &dyn Add<i32, Output = i32>; //OK
+    let x = &10 as &dyn Add;
+    //~^ ERROR E0191
+}
diff --git a/tests/ui/traits/cast-as-dyn-trait-wo-assoc-type-issue-21950.stderr b/tests/ui/traits/cast-as-dyn-trait-wo-assoc-type-issue-21950.stderr
new file mode 100644
index 00000000000..5f4974e6f23
--- /dev/null
+++ b/tests/ui/traits/cast-as-dyn-trait-wo-assoc-type-issue-21950.stderr
@@ -0,0 +1,12 @@
+error[E0191]: the value of the associated type `Output` in `Add` must be specified
+  --> $DIR/cast-as-dyn-trait-wo-assoc-type-issue-21950.rs:17:25
+   |
+LL |     type Output;
+   |     ----------- `Output` defined here
+...
+LL |     let x = &10 as &dyn Add;
+   |                         ^^^ help: specify the associated type: `Add<Output = Type>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0191`.