diff --git a/src/test/ui/existential_types/generic_duplicate_param_use8.rs b/src/test/ui/existential_types/generic_duplicate_param_use8.rs
new file mode 100644
index 000000000000..83501ad8c41a
--- /dev/null
+++ b/src/test/ui/existential_types/generic_duplicate_param_use8.rs
@@ -0,0 +1,16 @@
+#![feature(existential_type)]
+
+use std::fmt::Debug;
+
+fn main() {}
+
+existential type Two<T, U>: Debug;
+
+fn two<T: Debug, U: Debug>(t: T, _: U) -> Two<T, U> {
+    (t, 4u32)
+}
+
+fn three<T: Debug, U: Debug>(_: T, u: U) -> Two<T, U> {
+//~^ concrete type differs from previous
+    (u, 4u32)
+}
diff --git a/src/test/ui/existential_types/generic_duplicate_param_use8.stderr b/src/test/ui/existential_types/generic_duplicate_param_use8.stderr
new file mode 100644
index 000000000000..80c7441c857d
--- /dev/null
+++ b/src/test/ui/existential_types/generic_duplicate_param_use8.stderr
@@ -0,0 +1,19 @@
+error: concrete type differs from previous defining existential type use
+  --> $DIR/generic_duplicate_param_use8.rs:13:1
+   |
+LL | / fn three<T: Debug, U: Debug>(_: T, u: U) -> Two<T, U> {
+LL | | //~^ concrete type differs from previous
+LL | |     (u, 4u32)
+LL | | }
+   | |_^ expected `(T, u32)`, got `(U, u32)`
+   |
+note: previous use here
+  --> $DIR/generic_duplicate_param_use8.rs:9:1
+   |
+LL | / fn two<T: Debug, U: Debug>(t: T, _: U) -> Two<T, U> {
+LL | |     (t, 4u32)
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/existential_types/not_a_defining_use.rs b/src/test/ui/existential_types/not_a_defining_use.rs
new file mode 100644
index 000000000000..413cc6788c28
--- /dev/null
+++ b/src/test/ui/existential_types/not_a_defining_use.rs
@@ -0,0 +1,39 @@
+#![feature(existential_type)]
+
+use std::fmt::Debug;
+
+fn main() {}
+
+existential type Two<T, U>: Debug;
+
+fn two<T: Debug>(t: T) -> Two<T, u32> {
+    (t, 4i8)
+}
+
+fn three<T: Debug, U>(t: T) -> Two<T, U> {
+    (t, 5i8)
+}
+
+trait Bar {
+    type Blub: Debug;
+    const FOO: Self::Blub;
+}
+
+impl Bar for u32 {
+    type Blub = i32;
+    const FOO: i32 = 42;
+}
+
+// this should work! But it requires `two` and `three` not to be defining uses,
+// just restricting uses
+fn four<T: Debug, U: Bar>(t: T) -> Two<T, U> { //~ concrete type differs from previous
+    (t, <U as Bar>::FOO)
+}
+
+fn is_sync<T: Sync>() {}
+
+fn asdfl() {
+    //FIXME(oli-obk): these currently cause cycle errors
+    //is_sync::<Two<i32, u32>>();
+    //is_sync::<Two<i32, *const i32>>();
+}
diff --git a/src/test/ui/existential_types/not_a_defining_use.stderr b/src/test/ui/existential_types/not_a_defining_use.stderr
new file mode 100644
index 000000000000..a6ed5dbe0a9d
--- /dev/null
+++ b/src/test/ui/existential_types/not_a_defining_use.stderr
@@ -0,0 +1,18 @@
+error: concrete type differs from previous defining existential type use
+  --> $DIR/not_a_defining_use.rs:29:1
+   |
+LL | / fn four<T: Debug, U: Bar>(t: T) -> Two<T, U> { //~ concrete type differs from previous
+LL | |     (t, <U as Bar>::FOO)
+LL | | }
+   | |_^ expected `(T, i8)`, got `(T, <U as Bar>::Blub)`
+   |
+note: previous use here
+  --> $DIR/not_a_defining_use.rs:9:1
+   |
+LL | / fn two<T: Debug>(t: T) -> Two<T, u32> {
+LL | |     (t, 4i8)
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+