Merge pull request #3772 from dotty-staging/fix-#3012

Fix #3012: Avoid swallowing multiple ()'s

Author: odersky

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

@@ -62,8 +62,21 @@ object Typer {
     if (!tree.isEmpty && !tree.isInstanceOf[untpd.TypedSplice] && ctx.typerState.isGlobalCommittable)
       assert(tree.pos.exists, s"position not set for $tree # ${tree.uniqueId}")
 
+  /** A context property that indicates the owner of any expressions to be typed in the context
+   *  if that owner is different from the context's owner. Typically, a context with a class
+   *  as owner would have a local dummy as ExprOwner value.
+   */
   private val ExprOwner = new Property.Key[Symbol]
+
+  /** An attachment on a Select node with an `apply` field indicating that the `apply`
+   *  was inserted by the Typer.
+   */
   private val InsertedApply = new Property.Key[Unit]
+
+  /** An attachment on a tree `t` occurring as part of a `t()` where
+   *  the `()` was dropped by the Typer.
+   */
+  private val DroppedEmptyArgs = new Property.Key[Unit]
 }
 
 class Typer extends Namer with TypeAssigner with Applications with Implicits with Dynamic with Checking with Docstrings {
@@ -1862,6 +1875,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
    *
    *  0th strategy: If `tree` overrides a nullary method, mark the prototype
    *                so that the argument is dropped and return `tree` itself.
+   *                (but do this at most once per tree).
    *
    *  After that, two strategies are tried, and the first that is successful is picked.
    *  If neither of the strategies are successful, continues with`fallBack`.
@@ -1899,7 +1913,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
 
     pt match {
       case pt @ FunProto(Nil, _, _)
-      if tree.symbol.allOverriddenSymbols.exists(_.info.isNullaryMethod) =>
+      if tree.symbol.allOverriddenSymbols.exists(_.info.isNullaryMethod) &&
+         tree.getAttachment(DroppedEmptyArgs).isEmpty =>
+        tree.putAttachment(DroppedEmptyArgs, ())
         pt.markAsDropped()
         tree
       case _ =>

tests/neg/i3012/Fuzbar.java

@@ -0,0 +1,4 @@
+package fuz;
+public interface Fuzbar {
+  public String str();
+}

tests/neg/i3012/a.scala

@@ -0,0 +1,8 @@
+
+object a extends fuz.Fuzbar {
+  override def str = ""
+  str()()()()()()  // error: missing argument
+  str()() // error: missing argument
+  str() // ok
+  str // ok
+}