SI-7187 deprecate eta-expansion of zero-arg method values

For backwards compatiblity with 2.11, we already
don't adapt a zero-arg method value to a SAM.
In 2.13, we won't do any eta-expansion for zero-arg method values,
but we should deprecate first.

1 files changed, 30 insertions, 17 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index ba104fb7a6..91cff54bc7 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -872,16 +872,32 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           case Block(_, tree1) => tree1.symbol
           case _               => tree.symbol
         }
-        def shouldEtaExpandToSam: Boolean = {
-          // SI-9536 don't adapt parameterless method types to a to SAM's, fall through to empty application
-          // instead for backwards compatiblity with 2.11. See comments of that ticket and SI-7187
-          // for analogous trouble with non-SAM eta expansion. Suggestions there are: a) deprecate eta expansion to Function0,
-          // or b) switch the order of eta-expansion and empty application in this adaptation.
-          !mt.params.isEmpty && samOf(pt).exists
-        }
-        if (!meth.isConstructor && (isFunctionType(pt) || shouldEtaExpandToSam)) { // (4.2)
+
+        def cantAdapt =
+          if (context.implicitsEnabled) MissingArgsForMethodTpeError(tree, meth)
+          else setError(tree)
+
+        // constructors do not eta-expand
+        if (meth.isConstructor) cantAdapt
+        // (4.2) eta-expand method value when function or sam type is expected
+        else if (isFunctionType(pt) || (!mt.params.isEmpty && samOf(pt).exists)) {
+          // SI-9536 `!mt.params.isEmpty &&`: for backwards compatiblity with 2.11,
+          // we don't adapt a zero-arg method value to a SAM
+          // In 2.13, we won't do any eta-expansion for zero-arg method values, but we should deprecate first
+
           debuglog(s"eta-expanding $tree: ${tree.tpe} to $pt")
           checkParamsConvertible(tree, tree.tpe)
+
+          // SI-7187 eta-expansion of zero-arg method value is deprecated, switch order of (4.3) and (4.2) in 2.13
+          def isExplicitEtaExpansion = original match {
+            case Typed(_, Function(Nil, EmptyTree)) => true // tree shape for `f _`
+            case _ => false
+          }
+          if (mt.params.isEmpty && !isExplicitEtaExpansion) {
+            currentRun.reporting.deprecationWarning(tree.pos, NoSymbol,
+              s"Eta-expansion of zero-argument method values is deprecated. Did you intend to write ${Apply(tree, Nil)}?", "2.12.0")
+          }
+
           val tree0 = etaExpand(context.unit, tree, this)
 
           // #2624: need to infer type arguments for eta expansion of a polymorphic method
@@ -895,12 +911,9 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           else
             typed(tree0, mode, pt)
         }
-        else if (!meth.isConstructor && mt.params.isEmpty) // (4.3)
-          adapt(typed(Apply(tree, Nil) setPos tree.pos), mode, pt, original)
-        else if (context.implicitsEnabled)
-          MissingArgsForMethodTpeError(tree, meth)
-        else
-          setError(tree)
+        // (4.3) apply to empty argument list -- TODO 2.13: move this one case up to avoid eta-expanding at arity 0
+        else if (mt.params.isEmpty) adapt(typed(Apply(tree, Nil) setPos tree.pos), mode, pt, original)
+        else cantAdapt
       }
 
       def adaptType(): Tree = {
@@ -4398,11 +4411,11 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
         *   (2) If $e$ is a parameterless method or call-by-name parameter of type `=>$T$`, `$e$ _` represents
         *       the function of type `() => $T$`, which evaluates $e$ when it is applied to the empty parameterlist `()`.
         */
-      def typedEta(methodValue: Tree): Tree = methodValue.tpe match {
+      def typedEta(methodValue: Tree, original: Tree): Tree = methodValue.tpe match {
         case tp@(MethodType(_, _) | PolyType(_, MethodType(_, _))) => // (1)
           val formals = tp.params
           if (isFunctionType(pt) || samMatchesFunctionBasedOnArity(samOf(pt), formals)) methodValue
-          else adapt(methodValue, mode, checkArity(methodValue)(functionTypeWildcard(formals.length)))
+          else adapt(methodValue, mode, checkArity(methodValue)(functionTypeWildcard(formals.length)), original)
 
         case TypeRef(_, ByNameParamClass, _) |  NullaryMethodType(_) => // (2)
           val pos = methodValue.pos
@@ -5106,7 +5119,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           case Typed(expr, Function(Nil, EmptyTree)) =>
             typed1(suppressMacroExpansion(expr), mode, pt) match {
               case macroDef if treeInfo.isMacroApplication(macroDef) => MacroEtaError(macroDef)
-              case methodValue                                       => typedEta(checkDead(methodValue))
+              case methodValue                                       => typedEta(checkDead(methodValue), tree)
             }
           case Typed(expr, tpt) =>
             val tpt1  = typedType(tpt, mode)                           // type the ascribed type first