Be sure that we do not allow too much

src/test/ui/coherence/coherence-overlap-trait-alias.rs

@@ -0,0 +1,20 @@
+#![feature(rustc_attrs)]
+#![feature(trait_alias)]
+
+trait A {}
+trait B {}
+trait AB = A + B;
+
+impl A for u32 {}
+impl B for u32 {}
+
+trait C {}
+#[rustc_strict_coherence]
+impl<T: AB> C for T {}
+#[rustc_strict_coherence]
+impl C for u32 {}
+//~^ ERROR
+// FIXME it's giving an ungreat error but unsure if we care given that it's using an internal rustc
+// attribute and an artificial code path for testing purposes
+
+fn main() {}

src/test/ui/coherence/coherence-overlap-trait-alias.stderr

@@ -0,0 +1,16 @@
+error[E0283]: type annotations needed
+  --> $DIR/coherence-overlap-trait-alias.rs:15:6
+   |
+LL | impl C for u32 {}
+   |      ^ cannot infer type for type `u32`
+   |
+   = note: cannot satisfy `u32: C`
+note: required by a bound in `C`
+  --> $DIR/coherence-overlap-trait-alias.rs:11:1
+   |
+LL | trait C {}
+   | ^^^^^^^ required by this bound in `C`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0283`.