Fix #4935: remove unsound optimization

Author: liufengyun

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -921,9 +921,7 @@ object desugar {
       }
       val ids = for ((named, _) <- vars) yield Ident(named.name)
       val caseDef = CaseDef(pat, EmptyTree, makeTuple(ids))
-      val matchExpr =
-        if (forallResults(rhs, isMatchingTuple)) rhs
-        else Match(rhsUnchecked, caseDef :: Nil)
+      val matchExpr = Match(rhsUnchecked, caseDef :: Nil)
       vars match {
         case Nil =>
           matchExpr

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -269,16 +269,6 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
    */
   def bodyKind(body: List[Tree])(implicit ctx: Context): FlagSet =
     (NoInitsInterface /: body)((fs, stat) => fs & defKind(stat))
-
-  /** Checks whether predicate `p` is true for all result parts of this expression,
-   *  where we zoom into Ifs, Matches, and Blocks.
-   */
-  def forallResults(tree: Tree, p: Tree => Boolean): Boolean = tree match {
-    case If(_, thenp, elsep) => forallResults(thenp, p) && forallResults(elsep, p)
-    case Match(_, cases) => cases forall (c => forallResults(c.body, p))
-    case Block(_, expr) => forallResults(expr, p)
-    case _ => p(tree)
-  }
 }
 
 trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped] =>

tests/neg/i4935.scala

@@ -0,0 +1,3 @@
+object Foo {
+  val (A, B) = () // error // error
+}

tests/run/i4935.scala

@@ -0,0 +1,5 @@
+object Test {
+  val (Some(a), b) = (Some(3), 6)
+
+  def main(args: Array[String]) = assert(a == 3)
+}