Remove eta-expansion of zero-arg method values

Fixes scala/bug#7187

scala#5327 deprecates eta-expansion of zero-arg method values. This removes it in 2.13.

spec/06-expressions.md

@@ -1400,16 +1400,16 @@ type $T$ by evaluating the expression to which $m$ is bound.
 If the method takes only implicit parameters, implicit
 arguments are passed following the rules [here](07-implicits.html#implicit-parameters).
 
+###### Empty Application
+Otherwise, if $e$ has method type $()T$, it is implicitly applied to the empty
+argument list, yielding $e()$.
+
 ###### Eta Expansion
 Otherwise, if the method is not a constructor,
 and the expected type $\mathit{pt}$ is a function type
 $(\mathit{Ts}') \Rightarrow T'$, [eta-expansion](#eta-expansion)
 is performed on the expression $e$.
 
-###### Empty Application
-Otherwise, if $e$ has method type $()T$, it is implicitly applied to the empty
-argument list, yielding $e()$.
-
 ### Overloading Resolution
 
 If an identifier or selection $e$ references several members of a

src/compiler/scala/tools/nsc/typechecker/Adaptations.scala

@@ -70,16 +70,11 @@ trait Adaptations {
 
       if (settings.noAdaptedArgs)
         context.warning(t.pos, adaptWarningMessage("No automatic adaptation here: use explicit parentheses."))
-      else if (args.isEmpty) {
-        if (settings.future)
-          context.error(t.pos, adaptWarningMessage("Adaptation of argument list by inserting () has been removed.", showAdaptation = false))
-        else {
-          val msg = "Adaptation of argument list by inserting () is deprecated: " + (
-          if (isLeakyTarget) "leaky (Object-receiving) target makes this especially dangerous."
-          else "this is unlikely to be what you want.")
-          context.deprecationWarning(t.pos, t.symbol, adaptWarningMessage(msg), "2.11.0")
-        }
-      } else if (settings.warnAdaptedArgs)
+      else if (args.isEmpty)
+        context.error(t.pos,
+          adaptWarningMessage(s"Adaptation of argument list by inserting () has been removed.",
+          showAdaptation = false))
+      else if (settings.warnAdaptedArgs)
         context.warning(t.pos, adaptWarningMessage(
           s"Adapting argument list by creating a ${args.size}-tuple: this may not be what you want.")
         )

test/files/neg/checksensible.check

@@ -1,21 +1,3 @@
-checksensible.scala:45: warning: Adaptation of argument list by inserting () is deprecated: this is unlikely to be what you want.
-        signature: Any.==(x$1: Any): Boolean
-  given arguments: <none>
- after adaptation: Any.==((): Unit)
-  () == ()
-     ^
-checksensible.scala:48: warning: Adaptation of argument list by inserting () is deprecated: this is unlikely to be what you want.
-        signature: Object.!=(x$1: Any): Boolean
-  given arguments: <none>
- after adaptation: Object.!=((): Unit)
-  scala.runtime.BoxedUnit.UNIT != ()
-                               ^
-checksensible.scala:49: warning: Adaptation of argument list by inserting () is deprecated: this is unlikely to be what you want.
-        signature: Any.!=(x$1: Any): Boolean
-  given arguments: <none>
- after adaptation: Any.!=((): Unit)
-  (scala.runtime.BoxedUnit.UNIT: java.io.Serializable) != () // shouldn't warn
-                                                       ^
 checksensible.scala:13: warning: comparing a fresh object using `eq' will always yield false
   (new AnyRef) eq (new AnyRef)
                ^
@@ -62,59 +44,53 @@ checksensible.scala:44: warning: comparing values of types Unit and Boolean usin
   () == true
      ^
 checksensible.scala:45: warning: comparing values of types Unit and Unit using `==' will always yield true
-  () == ()
-     ^
-checksensible.scala:46: warning: comparing values of types Unit and Unit using `==' will always yield true
   () == println
      ^
-checksensible.scala:47: warning: comparing values of types Unit and scala.runtime.BoxedUnit using `==' will always yield true
+checksensible.scala:46: warning: comparing values of types Unit and scala.runtime.BoxedUnit using `==' will always yield true
   () == scala.runtime.BoxedUnit.UNIT // these should warn for always being true/false
      ^
-checksensible.scala:48: warning: comparing values of types scala.runtime.BoxedUnit and Unit using `!=' will always yield false
-  scala.runtime.BoxedUnit.UNIT != ()
-                               ^
-checksensible.scala:51: warning: comparing values of types Int and Unit using `!=' will always yield true
+checksensible.scala:48: warning: comparing values of types Int and Unit using `!=' will always yield true
   (1 != println)
      ^
-checksensible.scala:52: warning: comparing values of types Int and Symbol using `!=' will always yield true
+checksensible.scala:49: warning: comparing values of types Int and Symbol using `!=' will always yield true
   (1 != 'sym)
      ^
-checksensible.scala:58: warning: comparing a fresh object using `==' will always yield false
+checksensible.scala:55: warning: comparing a fresh object using `==' will always yield false
   ((x: Int) => x + 1) == null
                       ^
-checksensible.scala:59: warning: comparing a fresh object using `==' will always yield false
+checksensible.scala:56: warning: comparing a fresh object using `==' will always yield false
   Bep == ((_: Int) + 1)
       ^
-checksensible.scala:61: warning: comparing a fresh object using `==' will always yield false
+checksensible.scala:58: warning: comparing a fresh object using `==' will always yield false
   new Object == new Object
              ^
-checksensible.scala:62: warning: comparing a fresh object using `==' will always yield false
+checksensible.scala:59: warning: comparing a fresh object using `==' will always yield false
   new Object == "abc"
              ^
-checksensible.scala:63: warning: comparing a fresh object using `!=' will always yield true
+checksensible.scala:60: warning: comparing a fresh object using `!=' will always yield true
   new Exception() != new Exception()
                   ^
-checksensible.scala:66: warning: comparing values of types Int and Null using `==' will always yield false
+checksensible.scala:63: warning: comparing values of types Int and Null using `==' will always yield false
   if (foo.length == null) "plante" else "plante pas"
                  ^
-checksensible.scala:71: warning: comparing values of types Bip and Bop using `==' will always yield false
+checksensible.scala:68: warning: comparing values of types Bip and Bop using `==' will always yield false
   (x1 == x2)
       ^
-checksensible.scala:81: warning: comparing values of types EqEqRefTest.this.C3 and EqEqRefTest.this.Z1 using `==' will always yield false
+checksensible.scala:78: warning: comparing values of types EqEqRefTest.this.C3 and EqEqRefTest.this.Z1 using `==' will always yield false
   c3 == z1
      ^
-checksensible.scala:82: warning: comparing values of types EqEqRefTest.this.Z1 and EqEqRefTest.this.C3 using `==' will always yield false
+checksensible.scala:79: warning: comparing values of types EqEqRefTest.this.Z1 and EqEqRefTest.this.C3 using `==' will always yield false
   z1 == c3
      ^
-checksensible.scala:83: warning: comparing values of types EqEqRefTest.this.Z1 and EqEqRefTest.this.C3 using `!=' will always yield true
+checksensible.scala:80: warning: comparing values of types EqEqRefTest.this.Z1 and EqEqRefTest.this.C3 using `!=' will always yield true
   z1 != c3
      ^
-checksensible.scala:84: warning: comparing values of types EqEqRefTest.this.C3 and String using `!=' will always yield true
+checksensible.scala:81: warning: comparing values of types EqEqRefTest.this.C3 and String using `!=' will always yield true
   c3 != "abc"
      ^
-checksensible.scala:95: warning: comparing values of types Unit and Int using `!=' will always yield true
+checksensible.scala:92: warning: comparing values of types Unit and Int using `!=' will always yield true
     while ((c = in.read) != -1)
                          ^
 error: No warnings can be incurred under -Xfatal-warnings.
-36 warnings found
+31 warnings found
 one error found

test/files/neg/checksensible.scala

@@ -42,11 +42,8 @@ class EqEqValTest {
 
   1 != true
   () == true
-  () == ()
   () == println
   () == scala.runtime.BoxedUnit.UNIT // these should warn for always being true/false
-  scala.runtime.BoxedUnit.UNIT != ()
-  (scala.runtime.BoxedUnit.UNIT: java.io.Serializable) != () // shouldn't warn
 
   (1 != println)
   (1 != 'sym)

test/files/neg/t4851.check

@@ -1,13 +1,11 @@
-S.scala:2: warning: Adaptation of argument list by inserting () is deprecated: leaky (Object-receiving) target makes this especially dangerous.
+S.scala:2: error: Adaptation of argument list by inserting () has been removed.
         signature: J(x: Any): J
   given arguments: <none>
- after adaptation: new J((): Unit)
   val x1 = new J
            ^
-S.scala:3: warning: Adaptation of argument list by inserting () is deprecated: leaky (Object-receiving) target makes this especially dangerous.
+S.scala:3: error: Adaptation of argument list by inserting () has been removed.
         signature: J(x: Any): J
   given arguments: <none>
- after adaptation: new J((): Unit)
   val x2 = new J()
            ^
 S.scala:4: warning: Adapting argument list by creating a 5-tuple: this may not be what you want.
@@ -28,16 +26,14 @@ S.scala:7: warning: Adapting argument list by creating a 3-tuple: this may not b
  after adaptation: new Some((1, 2, 3): (Int, Int, Int))
   val y2 = new Some(1, 2, 3)
            ^
-S.scala:9: warning: Adaptation of argument list by inserting () is deprecated: this is unlikely to be what you want.
+S.scala:9: error: Adaptation of argument list by inserting () has been removed.
         signature: J2(x: T): J2[T]
   given arguments: <none>
- after adaptation: new J2((): Unit)
   val z1 = new J2
            ^
-S.scala:10: warning: Adaptation of argument list by inserting () is deprecated: this is unlikely to be what you want.
+S.scala:10: error: Adaptation of argument list by inserting () has been removed.
         signature: J2(x: T): J2[T]
   given arguments: <none>
- after adaptation: new J2((): Unit)
   val z2 = new J2()
            ^
 S.scala:14: warning: Adapting argument list by creating a 3-tuple: this may not be what you want.
@@ -46,6 +42,5 @@ S.scala:14: warning: Adapting argument list by creating a 3-tuple: this may not
  after adaptation: Test.anyId((1, 2, 3): (Int, Int, Int))
   val w1 = anyId(1, 2 ,3)
                 ^
-error: No warnings can be incurred under -Xfatal-warnings.
-8 warnings found
-one error found
+four warnings found
+four errors found