Revert "pre-expansion gate trait_alias."

This reverts commit 2d182b8.

src/libsyntax/feature_gate/check.rs

@@ -446,6 +446,15 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                    "auto traits are experimental and possibly buggy");
             }
 
+            ast::ItemKind::TraitAlias(..) => {
+                gate_feature_post!(
+                    &self,
+                    trait_alias,
+                    i.span,
+                    "trait aliases are experimental"
+                );
+            }
+
             ast::ItemKind::MacroDef(ast::MacroDef { legacy: false, .. }) => {
                 let msg = "`macro` is experimental";
                 gate_feature_post!(&self, decl_macro, i.span, msg);

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

@@ -1,13 +1,4 @@
 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

@@ -7,15 +7,6 @@ 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[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
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0658`.