Allow to use (...) around GivenTypes

Author: odersky

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

@@ -2190,7 +2190,19 @@ object Parsers {
         }
         val isContextual = initialMods.is(Given)
         newLineOptWhenFollowedBy(LPAREN)
-        if (in.token == LPAREN) {
+        def isParamClause: Boolean =
+          !isContextual || {
+            val lookahead = in.lookaheadScanner
+            lookahead.nextToken()
+            paramIntroTokens.contains(lookahead.token) && {
+              lookahead.token != IDENTIFIER ||
+              lookahead.name == nme.inline || {
+                lookahead.nextToken()
+                lookahead.token == COLON
+              }
+            }
+          }
+        if (in.token == LPAREN && isParamClause) {
           if (ofInstance && !isContextual)
             syntaxError(em"parameters of instance definitions must come after `given'")
           val params = paramClause(

compiler/src/dotty/tools/dotc/parsing/Tokens.scala

@@ -240,6 +240,8 @@ object Tokens extends TokensCommon {
 
   final val modifierFollowers = modifierTokens | defIntroTokens
 
+  final val paramIntroTokens: TokenSet = modifierTokens | identifierTokens | BitSet(AT, VAL, VAR, IMPLICIT)
+
   /** Is token only legal as start of statement (eof also included)? */
   final val mustStartStatTokens: TokenSet = defIntroTokens | modifierTokens | BitSet(IMPORT, EXPORT, PACKAGE)