Disallow {...} arguments for annotations

This mimics scalac's behavior (but not the official syntax; in fact the
official syntax seems to be ambiguous.

docs/SyntaxSummary.txt

@@ -249,7 +249,7 @@ grammar.
   AccessModifier    ::=  (`private' | `protected') [AccessQualifier]
   AccessQualifier   ::=  `[' (id | `this') `]'
 
-  Annotation        ::=  `@' SimpleType {ArgumentExprs}                         Apply(tpe, args)
+  Annotation        ::=  `@' SimpleType {ParArgumentExprs}                      Apply(tpe, args)
 
   TemplateBody      ::=  [nl] `{' [SelfType] TemplateStat {semi TemplateStat} `} (self, stats)
   TemplateStat      ::=  Import

src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1429,10 +1429,10 @@ object Parsers {
       else tree1
     }
 
-    /** Annotation        ::=  `@' SimpleType {ArgumentExprs}
+    /** Annotation        ::=  `@' SimpleType {ParArgumentExprs}
      */
     def annot() =
-      adjustStart(accept(AT)) { ensureApplied(argumentExprss(wrapNew(simpleType()))) }
+      adjustStart(accept(AT)) { ensureApplied(parArgumentExprss(wrapNew(simpleType()))) }
 
     def annotations(skipNewLines: Boolean = false): List[Tree] = {
       if (skipNewLines) newLineOptWhenFollowedBy(AT)

tests/pos/annot.scala

@@ -1,7 +1,10 @@
 import java.beans.Transient
+import annotation.unchecked.uncheckedVariance
 
 class Test {
 
+// testing combinations of annotation syntax
+
   @SuppressWarnings(Array("hi")) def foo() = ??? // evalutation of annotation on type cannot be deferred as requires implicit resolution(only generic Array$.apply applies here)
 
   @SuppressWarnings(Array("hi", "foo")) def foo2() = ??? //can be deferred as there is a non-generic method
@@ -11,5 +14,15 @@ class Test {
   @Transient(false) def bar = ???
 
   @Transient() def baz = ???
+
+// testing annotations in types
+
+  class A
+  trait B
+
+  val x: A @uncheckedVariance with B @uncheckedVariance = ???
+
+  //class C extends A @uncheckedVariance () with B @uncheckedVariance { val x = 10 }
+
 }