RHS of var def is an expr

Leading underscore short-circuited `var f: Int => Int = _ + 1`.

Also remove an extraneous `toList`.

The test for default var value should narrowly
accept only the correct syntax.

Ports scala/bug#11437

Author: som-snytt

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

@@ -3146,14 +3146,20 @@ object Parsers {
         }
         else emptyType
       val rhs =
-        if (tpt.isEmpty || in.token == EQUALS)
+        if tpt.isEmpty || in.token == EQUALS then
           endMarkerScope(first) {
             accept(EQUALS)
-            if (in.token == USCORE && !tpt.isEmpty && mods.is(Mutable) &&
-                (lhs.toList forall (_.isInstanceOf[Ident])))
-              wildcardIdent()
+            val rhs0 = subExpr()
+            val defaultOK = !tpt.isEmpty && mods.is(Mutable) && lhs.forall(_.isInstanceOf[Ident])
+            def isDefaultSyntax(t: Tree) = t match {
+              case Ident(name) => placeholderParams.nonEmpty && name == placeholderParams.head.name
+              case _           => false
+            }
+            if defaultOK && isDefaultSyntax(rhs0) then
+              placeholderParams = placeholderParams.tail
+              atSpan(rhs0.span) { Ident(nme.WILDCARD) }
             else
-              subExpr()
+              rhs0
           }
         else EmptyTree
       lhs match {

tests/neg/t11437.scala

@@ -0,0 +1,6 @@
+
+class Regress {
+  var v: Int = _
+  def f = 42
+  var w: Int = (_)    // error: not default value syntax
+}

tests/pos/t11437.scala

@@ -0,0 +1,11 @@
+
+trait T {
+  val adder0: Int => Int = _ + 3   // Works fine
+  var adder1: Int => Int = (_ + 3) // Works fine
+  var adder2: Int => Int = _ + 3   // was: Error
+}
+class Regress {
+  var v: Int = _
+  def f = 42
+  //var w: Int = (_)    //Unbound placeholder parameter; incorrect use of _
+}