Fix stack overflow when testing for shadowing

Shadowing tests could go into an infinite recursion when
the found sahdwoing member itself needs an implicit that
is resolved and then shadowed again by the same member.

A test case is neg/arrayclone-new.scala. This caused
a SO before, now gives two errors.

Author: odersky

src/dotty/tools/dotc/typer/Implicits.scala

@@ -491,7 +491,7 @@ trait Implicits { self: Typer =>
             pt)
         val generated1 = adapt(generated, pt)
         lazy val shadowing =
-          typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)(nestedContext.setNewTyperState)
+          typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)(nestedContext.setNewTyperState.addMode(Mode.ImplicitShadowing))
         def refMatches(shadowing: Tree): Boolean =
           ref.symbol == closureBody(shadowing).symbol || {
             shadowing match {
@@ -501,7 +501,8 @@ trait Implicits { self: Typer =>
           }
         if (ctx.typerState.reporter.hasErrors)
           nonMatchingImplicit(ref)
-        else if (contextual && !shadowing.tpe.isError && !refMatches(shadowing)) {
+        else if (contextual && !ctx.mode.is(Mode.ImplicitShadowing) &&
+                 !shadowing.tpe.isError && !refMatches(shadowing)) {
           implicits.println(i"SHADOWING $ref in ${ref.termSymbol.owner} is shadowed by $shadowing in ${shadowing.symbol.owner}")
           shadowedImplicit(ref, methPart(shadowing).tpe)
         }

src/dotty/tools/dotc/typer/Mode.scala

@@ -68,5 +68,10 @@ object Mode {
    */
   val Printing = newMode(10, "Printing")
 
+  /** We are currently typechecking an ident to determine whether some implicit
+   *  is shadowed - don't do any other shadowing tests.
+   */
+  val ImplicitShadowing = newMode(11, "ImplicitShadowing")
+
   val PatternOrType = Pattern | Type
 }

test/dotc/tests.scala

@@ -137,6 +137,7 @@ class tests extends CompilerTest {
   @Test def neg_escapingRefs = compileFile(negDir, "escapingRefs", xerrors = 2)
   @Test def neg_instantiateAbstract = compileFile(negDir, "instantiateAbstract", xerrors = 8)
   @Test def neg_selfInheritance = compileFile(negDir, "selfInheritance", xerrors = 5)
+  @Test def neg_shadowedImplicits = compileFile(negDir, "arrayclone-new", xerrors = 2)
 
   @Test def run_all = runFiles(runDir)