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.

2 files changed, 7 insertions, 2 deletions

diff --git a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
index a18c83ff8..a066fc04a 100644
--- a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
+++ b/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) =
diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index 4ca9f9e10..4bcdd5071 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/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 {