Fix #4157: Broken error message when trying to eta-expand

Avoid leaking error types in the printed error messages

Author: smarter

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -1835,10 +1835,10 @@ object messages {
     }
   }
 
-  case class OnlyFunctionsCanBeFollowedByUnderscore(pt: Type)(implicit ctx: Context)
+  case class OnlyFunctionsCanBeFollowedByUnderscore(tp: Type)(implicit ctx: Context)
     extends Message(OnlyFunctionsCanBeFollowedByUnderscoreID) {
     val kind = "Syntax"
-    val msg = hl"Not a function: $pt: cannot be followed by ${"_"}"
+    val msg = hl"Only function types can be followed by ${"_"} but the current expression has type $tp"
     val explanation =
       hl"""The syntax ${"x _"} is no longer supported if ${"x"} is not a function.
           |To convert to a function value, you need to explicitly write ${"() => x"}"""

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1645,9 +1645,10 @@ class Typer extends Namer
     val nestedCtx = ctx.fresh.setNewTyperState()
     val res = typed(qual, pt1)(nestedCtx)
     res match {
-      case res @ closure(_, _, _) =>
+      case closure(_, _, _) =>
       case _ =>
-        ctx.errorOrMigrationWarning(OnlyFunctionsCanBeFollowedByUnderscore(res.tpe), tree.pos)
+        val recovered = typed(qual)(ctx.fresh.setExploreTyperState())
+        ctx.errorOrMigrationWarning(OnlyFunctionsCanBeFollowedByUnderscore(recovered.tpe.widen), tree.pos)
         if (ctx.scala2Mode) {
           // Under -rewrite, patch `x _` to `(() => x)`
           patch(Position(tree.pos.start), "(() => ")

compiler/test-resources/repl/errmsgs

@@ -72,3 +72,11 @@ scala> val a: iDontExist = 1
 1 | val a: iDontExist = 1
   |        ^^^^^^^^^^
   |        not found: type iDontExist
+scala> def foo1(x: => Int) = x _
+1 | def foo1(x: => Int) = x _
+  |                      ^^^
+  |Only function types can be followed by _ but the current expression has type Int
+scala> def foo2(x: => Int): () => Int = x _
+1 | def foo2(x: => Int): () => Int = x _
+  |                                 ^^^
+  |Only function types can be followed by _ but the current expression has type Int