From ee265a645c17ee8ee3d5a5dd74a1b7371338b8df Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Sun, 7 Jan 2018 13:14:22 +0100
Subject: [PATCH 1/2] Fix #3012: Avoid swallowing multiple ()'s

Allow dropping at most one () if a method is called that exists
in both ()T and => T forms.
---
 compiler/src/dotty/tools/dotc/typer/Typer.scala | 6 +++++-
 tests/neg/i3012/Fuzbar.java                     | 4 ++++
 tests/neg/i3012/a.scala                         | 5 +++++
 3 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i3012/Fuzbar.java
 create mode 100644 tests/neg/i3012/a.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index a562e69acc00..8acacf1950eb 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -64,6 +64,7 @@ object Typer {
 
   private val ExprOwner = new Property.Key[Symbol]
   private val InsertedApply = new Property.Key[Unit]
+  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 +1863,7 @@ 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 +1901,9 @@ 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 _ =>
diff --git a/tests/neg/i3012/Fuzbar.java b/tests/neg/i3012/Fuzbar.java
new file mode 100644
index 000000000000..2818d4f6455e
--- /dev/null
+++ b/tests/neg/i3012/Fuzbar.java
@@ -0,0 +1,4 @@
+package fuz;
+public interface Fuzbar {
+  public String str();
+}
\ No newline at end of file
diff --git a/tests/neg/i3012/a.scala b/tests/neg/i3012/a.scala
new file mode 100644
index 000000000000..a02998d1c16b
--- /dev/null
+++ b/tests/neg/i3012/a.scala
@@ -0,0 +1,5 @@
+
+object a extends fuz.Fuzbar {
+  override def str = ""
+  str()()()()()()  // error: missing argument
+}

From c299da9ba8a2fe69b3a7bb41b3aae2740685da0d Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Mon, 8 Jan 2018 10:18:41 +0100
Subject: [PATCH 2/2] Address review comments

---
 compiler/src/dotty/tools/dotc/typer/Typer.scala | 12 ++++++++++++
 tests/neg/i3012/a.scala                         |  3 +++
 2 files changed, 15 insertions(+)

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 8acacf1950eb..c9458c547aec 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -62,8 +62,20 @@ 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]
 }
 
diff --git a/tests/neg/i3012/a.scala b/tests/neg/i3012/a.scala
index a02998d1c16b..29f65430f05c 100644
--- a/tests/neg/i3012/a.scala
+++ b/tests/neg/i3012/a.scala
@@ -2,4 +2,7 @@
 object a extends fuz.Fuzbar {
   override def str = ""
   str()()()()()()  // error: missing argument
+  str()() // error: missing argument
+  str() // ok
+  str // ok
 }