pre-expansion gate trait_alias.

src/test/ui/feature-gates/feature-gate-trait-alias.rs

@@ -1,4 +1,13 @@
 trait Foo = Default;
 //~^ ERROR trait aliases are experimental
 
+macro_rules! accept_item {
+    ($i:item) => {}
+}
+
+accept_item! {
+    trait Foo = Ord + Eq;
+    //~^ ERROR trait aliases are experimental
+}
+
 fn main() {}

src/test/ui/feature-gates/feature-gate-trait-alias.stderr

@@ -7,6 +7,15 @@ LL | trait Foo = Default;
    = note: for more information, see https://github.com/rust-lang/rust/issues/41517
    = help: add `#![feature(trait_alias)]` to the crate attributes to enable
 
-error: aborting due to previous error
+error[E0658]: trait aliases are experimental
+  --> $DIR/feature-gate-trait-alias.rs:9:5
+   |
+LL |     trait Foo = Ord + Eq;
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/41517
+   = help: add `#![feature(trait_alias)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0658`.