Flag trailing `_' following non-function as an error

If `x` is not a function or method, then `x _` should be disallowed.
scalac accepts this and converts it to () => x instead. I'd like
to drop this because it's unnecessary and non-obvious. If
-language:Scala2 is on, the behavior is like Scala 2's but
a migration warning is issued.

1 files changed, 13 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 936c006fc..b689c8750 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -1051,8 +1051,19 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     }
   }
 
-  def typedAsFunction(tree: untpd.Tree, pt: Type)(implicit ctx: Context): Tree =
-    typed(tree, if (defn.isFunctionType(pt)) pt else AnyFunctionProto)
+  def typedAsFunction(tree: untpd.Tree, pt: Type)(implicit ctx: Context): Tree = {
+    val pt1 = if (defn.isFunctionType(pt)) pt else AnyFunctionProto
+    var res = typed(tree, pt1)
+    if (pt1.eq(AnyFunctionProto) && !defn.isFunctionType(res.tpe)) {
+      def msg = "not a function; cannot be followed by `_'"
+      if (ctx.scala2Mode) {
+        ctx.migrationWarning(msg, tree.pos)
+        res = typed(untpd.Function(Nil, untpd.TypedSplice(res)))
+      }
+      else ctx.error(msg, tree.pos)
+    }
+    res
+  }
 
   /** Retrieve symbol attached to given tree */
   protected def retrieveSym(tree: untpd.Tree)(implicit ctx: Context) = tree.removeAttachment(SymOfTree) match {