Don't require specifying unrelated assoc types when trait alias is in dyn type

2 files changed, 18 insertions, 5 deletions

diff --git a/tests/ui/associated-type-bounds/overlaping-bound-suggestion.stderr b/tests/ui/associated-type-bounds/overlaping-bound-suggestion.stderr
index 2a308f83731..03d72f2ae2c 100644
--- a/tests/ui/associated-type-bounds/overlaping-bound-suggestion.stderr
+++ b/tests/ui/associated-type-bounds/overlaping-bound-suggestion.stderr
@@ -1,11 +1,8 @@
-error[E0191]: the value of the associated types `Item`, `Item`, `IntoIter` and `IntoIter` in `IntoIterator` must be specified
+error[E0191]: the value of the associated types `Item` and `IntoIter` in `IntoIterator` must be specified
   --> $DIR/overlaping-bound-suggestion.rs:7:13
    |
 LL |     inner: <IntoIterator<Item: IntoIterator<Item: >>::IntoIterator as Item>::Core,
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |             |                  |
-   |             |                  associated types `Item`, `IntoIter` must be specified
-   |             associated types `Item`, `IntoIter` must be specified
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: specify the associated types: `IntoIterator<Item: IntoIterator<Item: >, Item = Type, IntoIter = Type>`
 
 error[E0223]: ambiguous associated type
   --> $DIR/overlaping-bound-suggestion.rs:7:13
diff --git a/tests/ui/traits/alias/only-require-assocs-from-supertraits.rs b/tests/ui/traits/alias/only-require-assocs-from-supertraits.rs
new file mode 100644
index 00000000000..35149fdfba0
--- /dev/null
+++ b/tests/ui/traits/alias/only-require-assocs-from-supertraits.rs
@@ -0,0 +1,16 @@
+//@ check-pass
+
+#![feature(trait_alias)]
+
+trait Foo<T> {}
+trait Bar { type Assoc; }
+
+trait Alias<T: Bar> = Foo<T>;
+
+// Check that an alias only requires us to specify the associated types
+// of the principal's supertraits. For example, we shouldn't require
+// specifying the type `Assoc` on trait `Bar` just because we have some
+// `T: Bar` where clause on the alias... because that makes no sense.
+fn use_alias<T: Bar>(x: &dyn Alias<T>) {}
+
+fn main() {}