Fix #2368: local variables must be initialized

Disallow var declarations initialized to a placholder, if
the var declaration is local.

`var x: Foo = _` is still allowed in fields.

Testing
=======
Added tests for both invariants above.

Author: Abel Nieto

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -490,7 +490,10 @@ object messages {
            |${"var a = _"}
            |
            |Note that this use of `_` is not placeholder syntax,
-           |but an uninitialized var definition"""
+           |but an uninitialized var definition.
+           |Only fields can be left uninitialized in this manner; local variables
+           |must be initialized.
+           |"""
   }
 
   case class IllegalStartSimpleExpr(illegalToken: String)(implicit ctx: Context)

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1175,7 +1175,15 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     completeAnnotations(vdef, sym)
     val tpt1 = checkSimpleKinded(typedType(tpt))
     val rhs1 = vdef.rhs match {
-      case rhs @ Ident(nme.WILDCARD) => rhs withType tpt1.tpe
+      case rhs @ Ident(nme.WILDCARD) =>
+        // Fields can be left uninitialized, but local variables can't.
+        val rhsTyp =
+          if (sym.owner.isTerm) {
+            errorType(UnboundPlaceholderParameter(), sym.pos)
+          } else {
+            tpt1.tpe
+          }
+        rhs withType rhsTyp
       case rhs => typedExpr(rhs, tpt1.tpe)
     }
     val vdef1 = assignType(cpy.ValDef(vdef)(name, tpt1, rhs1), sym)

tests/neg/t2368.scala

@@ -0,0 +1,5 @@
+class C {
+  def foo() = {
+    var x: String = _ // error: local variables can't be left uninitialized
+  }
+}

tests/pos/t2368.scala

@@ -0,0 +1,4 @@
+class C {
+  // A `var` field can be left uninitialized.
+  var x: String = _
+}