Fast-track deprecating nullary method eta-expansion

Eta-expansion of nullary methods is dropped in Scala 3:
https://dotty.epfl.ch/docs/reference/changed-features/eta-expansion.html

So it must be deprecated in 2.13 first, and error under -Xsource:3.

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

@@ -459,6 +459,14 @@ trait ContextErrors {
       }
 
       //typedEta
+      def UnderscoreNullaryEtaMessage(tpe: Type) =
+        s"Only function types can be followed by _ but the current expression has type $tpe"
+
+      def UnderscoreNullaryEtaError(tree: Tree, tpe: Type) = {
+        issueNormalTypeError(tree, UnderscoreNullaryEtaMessage(tpe))
+        setError(tree)
+      }
+
       def UnderscoreEtaError(tree: Tree) = {
         issueNormalTypeError(tree, "_ must follow method; cannot follow " + tree.tpe)
         setError(tree)

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

@@ -4916,7 +4916,15 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           val funSym = context.owner.newAnonymousFunctionValue(pos)
           new ChangeOwnerTraverser(context.owner, funSym) traverse methodValue
 
-          typed(Function(List(), methodValue) setSymbol funSym setPos pos, mode, pt)
+          val result = typed(Function(List(), methodValue) setSymbol funSym setPos pos, mode, pt)
+          val restpe = result.tpe.typeArgs.headOption.getOrElse(methodValue.tpe)
+
+          if (currentRun.isScala3) {
+            UnderscoreNullaryEtaError(methodValue, restpe)
+          } else {
+            context.deprecationWarning(pos, NoSymbol, UnderscoreNullaryEtaMessage(restpe), "2.13.2")
+            result
+          }
 
         case ErrorType =>
           methodValue

test/files/neg/t7187-3.check

@@ -3,6 +3,9 @@ t7187-3.scala:13: error: type mismatch;
  required: () => Any
   val t1: () => Any  = m1   // error
                        ^
+t7187-3.scala:27: error: Only function types can be followed by _ but the current expression has type Int
+  val t7 = m1 _ // error: Only function types can be followed by _ but the current expression has type Int
+           ^
 t7187-3.scala:14: warning: An unapplied 0-arity method was eta-expanded (due to the expected type () => Any), rather than applied to `()`.
 Write m2() to invoke method m2, or change the expected type.
   val t2: () => Any  = m2   // eta-expanded with lint warning
@@ -21,4 +24,4 @@ Write m2() to invoke method m2, or change the expected type.
   val t2Sam: SamZero = m2   // eta-expanded with lint warning
                        ^
 4 warnings
-1 error
+2 errors

test/files/neg/t7187-3.scala

@@ -24,11 +24,11 @@ class EtaExpand214 {
   val t5a: Int        = t5 // ok
   val t6a: Int => Any = t6 // ok
 
-  val t7 = m1 _
+  val t7 = m1 _ // error: Only function types can be followed by _ but the current expression has type Int
   val t8 = m2 _
   val t9 = m3 _
 
-  val t7a: () => Any  = t7 // ok
+  val t7a: () => Any  = t7 // error: t7 is an error
   val t8a: () => Any  = t8 // ok
   val t9a: Int => Any = t9 // ok
 }

test/files/neg/t7187-deprecation.check

@@ -3,6 +3,9 @@ t7187-deprecation.scala:17: error: type mismatch;
  required: () => Any
   val t1: () => Any  = m1   // error
                        ^
+t7187-deprecation.scala:31: error: Only function types can be followed by _ but the current expression has type Int
+  val t7 = m1 _ // error: Only function types can be followed by _ but the current expression has type Int
+           ^
 t7187-deprecation.scala:24: warning: Auto-application to `()` is deprecated. Supply the empty argument list `()` explicitly to invoke method m2,
 or remove the empty argument list from its definition (Java-defined methods are exempt).
 In Scala 3, an unapplied method like this will be eta-expanded into a function.
@@ -14,4 +17,4 @@ In Scala 3, an unapplied method like this will be eta-expanded into a function.
   a.boom // error
     ^
 2 warnings
-1 error
+2 errors

test/files/neg/t7187-deprecation.scala

@@ -28,11 +28,11 @@ class EtaExpand214 {
   val t5a: Int        = t5 // ok
   val t6a: Int => Any = t6 // ok
 
-  val t7 = m1 _
+  val t7 = m1 _ // error: Only function types can be followed by _ but the current expression has type Int
   val t8 = m2 _
   val t9 = m3 _
 
-  val t7a: () => Any  = t7 // ok
+  val t7a: () => Any  = t7 // error: t7 is an error
   val t8a: () => Any  = t8 // ok
   val t9a: Int => Any = t9 // ok
 

test/files/neg/t7187.check

@@ -26,6 +26,12 @@ t7187.scala:12: warning: An unapplied 0-arity method was eta-expanded (due to th
 Write foo() to invoke method foo, or change the expected type.
   val t1b: () => Any = foo   // eta-expansion, but lint warning
                        ^
+t7187.scala:21: warning: Only function types can be followed by _ but the current expression has type String
+  val t2c: () => Any = bar _ // warning: Only function types can be followed by _ but the current expression has type String
+                       ^
+t7187.scala:22: warning: Only function types can be followed by _ but the current expression has type String
+  val t2d: Any       = bar _ // warning: Only function types can be followed by _ but the current expression has type String
+                       ^
 t7187.scala:26: warning: An unapplied 0-arity method was eta-expanded (due to the expected type () => Any), rather than applied to `()`.
 Write baz() to invoke method baz, or change the expected type.
   val t3a: () => Any = baz   // eta-expansion, but lint warning
@@ -43,5 +49,5 @@ even though trait AcciSamOne is not annotated with `@FunctionalInterface`;
 to suppress warning, add the annotation or write out the equivalent function literal.
   val t5AcciSam: AcciSamOne = zup // ok, but warning
                               ^
-5 warnings
+7 warnings
 6 errors

test/files/neg/t7187.scala

@@ -1,4 +1,4 @@
-// scalac: -Xlint:eta-zero -Xlint:eta-sam
+// scalac: -deprecation -Xlint:eta-zero -Xlint:eta-sam
 //
 
 trait AcciSamOne { def apply(x: Int): Int }
@@ -18,8 +18,8 @@ class EtaExpandZeroArg {
   def bar = ""
   val t2a: () => Any = bar   // error: no eta-expansion of zero-arglist-methods (nullary methods)
   val t2b: () => Any = bar() // error: bar doesn't take arguments, so expanded to bar.apply(), which misses an argument
-  val t2c: () => Any = bar _ // ok
-  val t2d: Any       = bar _ // ok
+  val t2c: () => Any = bar _ // warning: Only function types can be followed by _ but the current expression has type String
+  val t2d: Any       = bar _ // warning: Only function types can be followed by _ but the current expression has type String
   val t2e: Any       = bar() _ // error: not enough arguments for method apply
 
   def baz() = ""

test/files/run/amp.scala

@@ -2,12 +2,12 @@ object Test extends App {
 
   def foo() = {
     def f: Int = 1
-    val x = f _
+    val x = () => f
     x
   }
 
   def bar(g: => Int) = {
-    g _
+    () => g
   }
 
   Console.println((bar{ Console.println("g called"); 42 })())

test/files/run/byname.scala

@@ -8,7 +8,7 @@ def test[A](name: String, expect: A, actual: => A): Unit = {
 def testNoBraces = 1
 test("no braces", 1, testNoBraces)
 
-val testNoBracesR = testNoBraces _
+val testNoBracesR = () => testNoBraces
 test("no braces r", 1, testNoBracesR())
 
 def testPlain(x: String, y: String): String = x + y

test/files/run/existentials3-new.scala

@@ -77,6 +77,6 @@ object Misc {
     def f1 = { trait A extends Seq[U forSome { type U <: Bippy }] ; abstract class B extends A ; trait C extends B ; (null: C) }
     def f2 = f1.head.bippy
   }
-  def g1 = o1.f1 _
-  def g2 = o1.f2 _
+  def g1 = () => o1.f1
+  def g2 = () => o1.f2
 }

test/files/run/existentials3-old.scala

@@ -70,6 +70,6 @@ object Misc {
     def f1 = { trait A extends Seq[U forSome { type U <: Bippy }] ; abstract class B extends A ; trait C extends B ; (null: C) }
     def f2 = f1.head.bippy
   }
-  def g1 = o1.f1 _
-  def g2 = o1.f2 _
+  def g1 = () => o1.f1
+  def g2 = () => o1.f2
 }

test/files/run/pr7593.scala

@@ -1,7 +1,7 @@
 object Test {
   def main(args: Array[String]): Unit = {
     def foo = synchronized { "bar" }
-    val eta = foo _
-    println(eta())
+    val bar = () => foo
+    println(bar())
   }
 }

test/files/run/sammy_cbn.scala

@@ -1,7 +1,7 @@
 trait F0[T] { def apply(): T }
 
 object Test extends App {
-  def delay[T](v: => T) = (v _): F0[T]
+  def delay[T](v: => T) = (() => v): F0[T]
 
   // should not fail with ClassCastException: $$Lambda$6279/897871870 cannot be cast to F0
   // (also, should not say boe!)

test/files/run/t1247.scala

@@ -1,7 +1,7 @@
 object Test extends App {
   val f = () => 5
   def test(g: => Int): Unit = {
-    val gFunc = g _
+    val gFunc = () => g
     val isSameClosureClass = gFunc.getClass == f.getClass
     val isSame = gFunc eq f
     println("Is same closure class: "+isSameClosureClass+" is same closure: "+isSame)