Merge pull request #15559 from som-snytt/tweak/using

Tweak optional using spec

Author: odersky

docs/_docs/internals/syntax.md

@@ -287,7 +287,8 @@ TypeSplice        ::= spliceId
 ExprsInParens     ::=  ExprInParens {‘,’ ExprInParens}
 ExprInParens      ::=  PostfixExpr ‘:’ Type                                     -- normal Expr allows only RefinedType here
                     |  Expr
-ParArgumentExprs  ::=  ‘(’ [‘using’] ExprsInParens ‘)’                          exprs
+ParArgumentExprs  ::=  ‘(’ [ExprsInParens] ‘)’                          exprs
+                    |  ‘(’ ‘using’ ExprsInParens ‘)’
                     |  ‘(’ [ExprsInParens ‘,’] PostfixExpr ‘*’ ‘)’              exprs :+ Typed(expr, Ident(wildcardStar))
 ArgumentExprs     ::=  ParArgumentExprs
                     |  BlockExpr

docs/_docs/reference/syntax.md

@@ -275,7 +275,8 @@ ExprSplice        ::= spliceId
 ExprsInParens     ::=  ExprInParens {‘,’ ExprInParens}
 ExprInParens      ::=  PostfixExpr ‘:’ Type
                     |  Expr
-ParArgumentExprs  ::=  ‘(’ [‘using’] ExprsInParens ‘)’
+ParArgumentExprs  ::=  ‘(’ [ExprsInParens] ‘)’
+                    |  ‘(’ ‘using’ ExprsInParens ‘)’
                     |  ‘(’ [ExprsInParens ‘,’] PostfixExpr ‘*’ ‘)’
 ArgumentExprs     ::=  ParArgumentExprs
                     |  BlockExpr