accept union inside enum if not followed by identifier

src/librustc_parse/parser/item.rs

@@ -1727,9 +1727,10 @@ impl<'a> Parser<'a> {
     /// Checks if current token is one of tokens which cannot be nested like `kw::Enum`. In case
     /// it is, we try to parse the item and report error about nested types.
     fn recover_nested_adt_item(&mut self, keyword: Symbol) -> PResult<'a, bool> {
-        if self.token.is_keyword(kw::Enum) ||
+        if (self.token.is_keyword(kw::Enum) ||
             self.token.is_keyword(kw::Struct) ||
-            self.token.is_keyword(kw::Union)
+            self.token.is_keyword(kw::Union))
+           && self.look_ahead(1, |t| t.is_ident())
         {
             let kw_token = self.token.clone();
             let kw_str = pprust::token_to_string(&kw_token);

src/test/ui/enum/union-in-enum.rs

@@ -0,0 +1,13 @@
+// This test checks that the union keyword
+// is accepted as the name of an enum variant
+// when not followed by an identifier
+// This special case exists because `union` is a contextual keyword.
+
+#![allow(warnings)]
+
+// check-pass
+
+enum A { union }
+enum B { union {} }
+enum C { union() }
+fn main(){}