1 files changed, 19 insertions, 0 deletions
diff --git a/src/test/ui/type-alias-impl-trait/no_inferrable_concrete_type.rs b/src/test/ui/type-alias-impl-trait/no_inferrable_concrete_type.rs
new file mode 100644
index 000000000..46621362e
--- /dev/null
+++ b/src/test/ui/type-alias-impl-trait/no_inferrable_concrete_type.rs
@@ -0,0 +1,19 @@
+// Issue 52985: user code provides no use case that allows a type alias `impl Trait`
+// We now emit a 'unconstrained opaque type' error
+
+#![feature(type_alias_impl_trait)]
+
+mod foo {
+    pub type Foo = impl Copy;
+    //~^ ERROR unconstrained opaque type
+
+    // make compiler happy about using 'Foo'
+    pub fn bar(x: Foo) -> Foo {
+        x
+    }
+}
+
+fn main() {
+    let _: foo::Foo = std::mem::transmute(0u8);
+    //~^ ERROR cannot transmute between types of different sizes, or dependently-sized types
+}