Better diagnosis for cyclic references caused by implicit search

Since we now allow to drop the explicit type of a local implicit val
it can happen that this causes a cyclic reference, namely when the
typechecking of the right-hand side involves an implicit search.
It's unpractical and fragile to avoid this. Instead we give now
a nice error message explaining the problem and how to fix it in
source code.

Author: odersky

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

@@ -46,7 +46,13 @@ object ErrorReporting {
             errorMsg(msg, cx.outer)
         }
       } else msg
-    errorMsg(ex.show, ctx)
+
+      if (cycleSym.is(Implicit, butNot = Method) && cycleSym.owner.isTerm)
+        em"""cyclic reference involving implicit $cycleSym
+            |This happens when the right hand-side of $cycleSym's definition involves an implicit search.
+            |To avoid the error, give $cycleSym an explicit type."""
+      else
+        errorMsg(ex.show, ctx)
   }
 
   def wrongNumberOfArgs(fntpe: Type, kind: String, expectedArgs: List[TypeParamInfo], actual: List[untpd.Tree], pos: Position)(implicit ctx: Context) =

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

@@ -1001,7 +1001,6 @@ class Namer { typer: Typer =>
         def missingType(modifier: String) = {
           ctx.error(s"${modifier}type of implicit definition needs to be given explicitly", mdef.pos)
           sym.resetFlag(Implicit)
-
         }
         if (sym is Implicit)
           mdef match {

tests/neg/implicitDefs.scala

@@ -8,4 +8,11 @@ object implicitDefs {
   implicit val x = 2 // error: type of implicit definition needs to be given explicitly
   implicit def y(x: Int) = 3 // error: result type of implicit definition needs to be given explicitly
   implicit def z(a: x.type): String = "" // error: implicit conversion may not have a parameter of singleton type
+
+  def foo(implicit x: String) = 1
+
+  def bar() = {
+    implicit val x = foo
+    x
+  }
 }