Added two new compiler options:

  -Ywarn-adapted-args   // also included in -Xlint
  -Yno-adapted-args

The former warns when a () is inserted or an argument list is implicitly
tupled. The latter errors under the same conditions. Using these options
I found several bugs in the distribution which would otherwise be nearly
impossible to spot. These bugs were innocuous (I think) but similar bugs
could easily be (and have been) otherwise.

Certain particularly threatening scenarios are at minimum warned about
regardless of options given. Closes SI-4851, no review.

5 files changed, 93 insertions, 0 deletions

diff --git a/test/files/neg/bug4851.check b/test/files/neg/bug4851.check
new file mode 100644
index 0000000000..8011350f23
--- /dev/null
+++ b/test/files/neg/bug4851.check
@@ -0,0 +1,43 @@
+S.scala:2: error: Adapting argument list by inserting (): leaky (Object-receiving) target makes this especially dangerous.
+        signature: J(x: Any): J
+  given arguments: <none>
+ after adaptation: new J((): Unit)
+  val x1 = new J
+           ^
+S.scala:3: error: Adapting argument list by inserting (): leaky (Object-receiving) target makes this especially dangerous.
+        signature: J(x: Any): J
+  given arguments: <none>
+ after adaptation: new J((): Unit)
+  val x2 = new J()
+           ^
+S.scala:4: error: Adapting argument list by creating a 5-tuple: this may not be what you want.
+        signature: J(x: Any): J
+  given arguments: 1, 2, 3, 4, 5
+ after adaptation: new J((1, 2, 3, 4, 5): (Int, Int, Int, Int, Int))
+  val x3 = new J(1, 2, 3, 4, 5)
+           ^
+S.scala:6: error: Adapting argument list by creating a 3-tuple: this may not be what you want.
+        signature: Some.apply[A](x: A): Some[A]
+  given arguments: 1, 2, 3
+ after adaptation: Some((1, 2, 3): (Int, Int, Int))
+  val y1 = Some(1, 2, 3)
+               ^
+S.scala:7: error: Adapting argument list by creating a 3-tuple: this may not be what you want.
+        signature: Some(x: A): Some[A]
+  given arguments: 1, 2, 3
+ after adaptation: new Some((1, 2, 3): (Int, Int, Int))
+  val y2 = new Some(1, 2, 3)
+           ^
+S.scala:9: error: Adapting argument list by inserting (): this is unlikely to be what you want.
+        signature: J2[T](x: T): J2[T]
+  given arguments: <none>
+ after adaptation: new J2((): Unit)
+  val z1 = new J2
+           ^
+S.scala:10: error: Adapting argument list by inserting (): this is unlikely to be what you want.
+        signature: J2[T](x: T): J2[T]
+  given arguments: <none>
+ after adaptation: new J2((): Unit)
+  val z2 = new J2()
+           ^
+7 errors found
diff --git a/test/files/neg/bug4851.flags b/test/files/neg/bug4851.flags
new file mode 100644
index 0000000000..0545cb8b84
--- /dev/null
+++ b/test/files/neg/bug4851.flags
@@ -0,0 +1 @@
+-Ywarn-adapted-args -Xfatal-warnings
diff --git a/test/files/neg/bug4851/J.java b/test/files/neg/bug4851/J.java
new file mode 100644
index 0000000000..9c35b8a16e
--- /dev/null
+++ b/test/files/neg/bug4851/J.java
@@ -0,0 +1,15 @@
+public class J {
+  Object x;
+
+  public J(Object x) {
+    this.x = x;
+  }
+
+  public J(int x1, int x2, int x3, int x4, int x5, int x6) {
+    this.x = null;
+  }
+
+  public String toString() {
+    return "J:" + x.getClass();
+  }
+}
\ No newline at end of file
diff --git a/test/files/neg/bug4851/J2.java b/test/files/neg/bug4851/J2.java
new file mode 100644
index 0000000000..82954d9489
--- /dev/null
+++ b/test/files/neg/bug4851/J2.java
@@ -0,0 +1,11 @@
+public class J2<T> {
+  T x;
+
+  public <T> J(T x) {
+    this.x = x;
+  }
+
+  public String toString() {
+    return "J2:" + x.getClass();
+  }
+}
\ No newline at end of file
diff --git a/test/files/neg/bug4851/S.scala b/test/files/neg/bug4851/S.scala
new file mode 100644
index 0000000000..deb42980ab
--- /dev/null
+++ b/test/files/neg/bug4851/S.scala
@@ -0,0 +1,23 @@
+object Test {
+  val x1 = new J
+  val x2 = new J()
+  val x3 = new J(1, 2, 3, 4, 5)
+
+  val y1 = Some(1, 2, 3)
+  val y2 = new Some(1, 2, 3)
+
+  val z1 = new J2
+  val z2 = new J2()
+  val z3 = new J2(())
+
+  def main(args: Array[String]): Unit = {
+    println(x1)
+    println(x2)
+    println(x3)
+    println(y1)
+
+    println(z1)
+    println(z2)
+    println(z3)
+  }
+}