Syntax change: multiple patterns in definition

Allow `val a, b, c = ...` only if a, b, c are identifiers.
It looks weird if they are general patterns, and it seems this gives
unnecessary power.

Author: odersky

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

@@ -2355,13 +2355,21 @@ object Parsers {
         tmplDef(start, mods)
     }
 
-    /** PatDef ::= Pattern2 {`,' Pattern2} [`:' Type] `=' Expr
-     *  VarDef ::= PatDef | id {`,' id} `:' Type `=' `_'
-     *  ValDcl ::= id {`,' id} `:' Type
-     *  VarDcl ::= id {`,' id} `:' Type
+    /** PatDef  ::=  ids [‘:’ Type] ‘=’ Expr
+     *            |  Pattern2 [‘:’ Type] ‘=’ Expr
+     *  VarDef  ::=  PatDef | id {`,' id} `:' Type `=' `_'
+     *  ValDcl  ::=  id {`,' id} `:' Type
+     *  VarDcl  ::=  id {`,' id} `:' Type
      */
     def patDefOrDcl(start: Offset, mods: Modifiers): Tree = atSpan(start, nameStart) {
-      val lhs = commaSeparated(pattern2)
+      val first = pattern2()
+      val lhs = first match {
+        case id: Ident if in.token == COMMA =>
+          in.nextToken()
+          id :: commaSeparated(() => termIdent())
+        case _ =>
+          first :: Nil
+      }
       val tpt = typedOpt()
       val rhs =
         if (tpt.isEmpty || in.token == EQUALS) {

docs/docs/internals/syntax.md

@@ -359,7 +359,8 @@ Def               ::=  ‘val’ PatDef
                     |  ‘type’ {nl} TypeDcl
                     |  TmplDef
                     |  INT
-PatDef            ::=  Pattern2 {‘,’ Pattern2} [‘:’ Type] ‘=’ Expr              PatDef(_, pats, tpe?, expr)
+PatDef            ::=  ids [‘:’ Type] ‘=’ Expr
+                    |  Pattern2 [‘:’ Type] ‘=’ Expr                PatDef(_, pats, tpe?, expr)
 VarDef            ::=  PatDef
                     |  ids ‘:’ Type ‘=’ ‘_’
 DefDef            ::=  DefSig [(‘:’ | ‘<:’) Type] ‘=’ Expr                      DefDef(_, name, tparams, vparamss, tpe, expr)

tests/neg/multi-patterns.scala

@@ -0,0 +1,4 @@
+object Test {
+  val (a :: as), bs = List(1, 2, 3) // error
+  val B @ List(), C: List[Int] = List() // error
+}

tests/pos/i3412.scala

@@ -1,5 +1,3 @@
 class Test {
   val A @ List() = List()
-  val B @ List(), C: List[Int] = List()
-  val D @ List(), E @ List() = List()
 }