Fix #877

Use freshName to name evidence parameters.

Author: odersky

src/dotty/tools/dotc/ast/Desugar.scala

@@ -132,7 +132,7 @@ object desugar {
       case tparam @ TypeDef(_, ContextBounds(tbounds, cxbounds)) =>
         for (cxbound <- cxbounds) {
           val paramFlags: FlagSet = if (isPrimaryConstructor) PrivateLocalParamAccessor else Param
-          val epname = (nme.EVIDENCE_PARAM_PREFIX.toString + epbuf.length).toTermName
+          val epname = ctx.freshName(nme.EVIDENCE_PARAM_PREFIX).toTermName
           epbuf += ValDef(epname, cxbound, EmptyTree).withFlags(paramFlags | Implicit)
         }
         cpy.TypeDef(tparam)(rhs = tbounds)

tests/pos/i877.scala

@@ -0,0 +1,11 @@
+class First[A]
+class Second[A]
+
+class Foo {
+  def foo[A: First] = {
+    def bar[B: Second] = {
+      val fst: First[A] = implicitly[First[A]]
+      val snd: Second[B] = implicitly[Second[B]]
+    }
+  }
+}