Keep Function when CBN arg thunk targets a SAM

The body of `def delay[T](v: => T) = (v _): F0[T]`
becomes `() => v` during `typedEta`, and then uncurry
considers whether to strip the function wrapper since
`v` is known to be a `Function0` thunk. Stripping is sound
when the expected type is `Function0` for this expression,
but that's no longer a given, since we could be expecting any
nullary SAM.

Also sweep up a bit around `typedEta`.

Encapsulate the, erm, creative  encoding of
`m _` as `Typed(m, Function(Nil, EmptyTree))`.

2 files changed, 4 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index d4715471f6..9c0174d89b 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
@@ -1713,9 +1713,7 @@ self =>
           }
           simpleExprRest(app, canApply = true)
         case USCORE =>
-          atPos(t.pos.start, in.skipToken()) {
-            Typed(stripParens(t), Function(Nil, EmptyTree))
-          }
+          atPos(t.pos.start, in.skipToken()) { makeMethodValue(stripParens(t)) }
         case _ =>
           t
       }
diff --git a/src/compiler/scala/tools/nsc/ast/parser/TreeBuilder.scala b/src/compiler/scala/tools/nsc/ast/parser/TreeBuilder.scala
index 473a40f42a..1e9a1762eb 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/TreeBuilder.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/TreeBuilder.scala
@@ -35,6 +35,9 @@ abstract class TreeBuilder {
   def repeatedApplication(tpe: Tree): Tree =
     AppliedTypeTree(rootScalaDot(tpnme.REPEATED_PARAM_CLASS_NAME), List(tpe))
 
+  // represents `expr _`, as specified in Method Values of spec/06-expressions.md
+  def makeMethodValue(expr: Tree): Tree = Typed(expr, Function(Nil, EmptyTree))
+
   def makeImportSelector(name: Name, nameOffset: Int): ImportSelector =
     ImportSelector(name, nameOffset, name, nameOffset)