Test that incomplete inference for TAITs fail

src/test/ui/type-alias-impl-trait/incomplete-inference.rs

@@ -0,0 +1,15 @@
+#![feature(type_alias_impl_trait)]
+
+type Foo = impl Sized;
+
+fn bar() -> Foo {
+    None
+    //~^ ERROR: type annotations needed [E0282]
+}
+
+fn baz() -> Foo {
+    //~^ ERROR: concrete type differs from previous defining opaque type use
+    Some(())
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/incomplete-inference.stderr

@@ -0,0 +1,21 @@
+error[E0282]: type annotations needed
+  --> $DIR/incomplete-inference.rs:6:5
+   |
+LL |     None
+   |     ^^^^ cannot infer type for type parameter `T` declared on the enum `Option`
+
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/incomplete-inference.rs:10:1
+   |
+LL | fn baz() -> Foo {
+   | ^^^^^^^^^^^^^^^ expected `[type error]`, got `Option<()>`
+   |
+note: previous use here
+  --> $DIR/incomplete-inference.rs:5:1
+   |
+LL | fn bar() -> Foo {
+   | ^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0282`.