Drop implicit shadowing checks

Since implicit selection is now inner to outer, shadowing checks
are less important than before.

Shadowing is necessary if we treat implicit search as a synthesis for untyped
terms. I.e. come up with an untyped term and then check whether that term is
typeable and refers to the original implicit. If there's a nested definition
with the same name, that fails.

This viewpoint is very easy to spec but a bit unnatural. In all other instances
of meta programming we deal with typed terms. If we synthesize a typed
term directly, then name resolution is already done and shadowing is immaterial.
Besides, shadowing tests,  if they fail, are almost always much more suprising
than enlightening.

Author: odersky

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

@@ -1105,7 +1105,8 @@ trait Implicits { self: Typer =>
           }
         }
       }
-      else if (contextual && !ctx.mode.is(Mode.ImplicitShadowing) &&
+      else if (false &&
+               contextual && !ctx.mode.is(Mode.ImplicitShadowing) &&
                 !shadowing.tpe.isError && !refSameAs(shadowing)) {
         implicits.println(i"SHADOWING $ref in ${ref.termSymbol.maybeOwner} is shadowed by $shadowing in ${shadowing.symbol.maybeOwner}")
         SearchFailure(generated1.withTypeUnchecked(

tests/neg/implicit-shadowing.scala renamed to tests/pos/implicit-shadowing.scala

@@ -4,8 +4,8 @@ object Test {
 
   def outer(implicit c: C) = {
 
-    def f(c: C) = implicitly[C]   // error: shadowing
-    def g(c: Int) = implicitly[C] // error: shadowing (even though type is different)
+    def f(c: C) = implicitly[C]   // now ok: shadowing no longer tested
+    def g(c: Int) = implicitly[C] // now ok: shadowing no longer tested
 
     f(new C)
   }

tests/run/i5224.check

@@ -1,2 +1,2 @@
 barInt
-bar
+barInt

tests/run/i5224.scala

@@ -16,5 +16,7 @@ object Test extends App {
     def barInt: Unit = ???
 
     implicitly[Bar[Int]]
+      // used to resolve to bar, but
+      // resolves to barInt now, since shadowing is no longer tested
   }
 }