SI-6780 Better handling of cycles in in-scope implicit search

Implicit searches in the body of implicit members with inferred
types were leading to cycles. Before we used to resolve that
by saying there were no implicits in scope at all; now we just
skip the current context and still include the enclosing implicits.
Care is taken not to cache results under these circumstances.

This entails reworking `Context#implicitss` so that:

  - the implicit info cache only contains implicits from the current
    level. The List[List[_]] is now contructed on demand;
  - we can detect cycles by setting `implicitsCacheRunId` to -1 during
    the computation. The outer implicits when we encounter that.
  - we avoid caching when we hit a cycle or when the owner is uninitialized.

3 files changed, 45 insertions, 9 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
index 2be6d92ed0..b7a942fc0d 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
@@ -773,7 +773,7 @@ trait Contexts { self: Analyzer =>
     // Implicit collection
     //
 
-    private var implicitsCache: List[List[ImplicitInfo]] = null
+    private var implicitsCache: List[ImplicitInfo] = null
     private var implicitsRunId = NoRunId
 
     def resetCache() {
@@ -834,14 +834,24 @@ trait Contexts { self: Analyzer =>
     def implicitss: List[List[ImplicitInfo]] = {
       val imports = this.imports
       val nextOuter = this.nextOuter
-      if (implicitsRunId != currentRunId) {
-        implicitsRunId = currentRunId
+      def withOuter(is: List[ImplicitInfo]): List[List[ImplicitInfo]] =
+        is match {
+          case Nil => nextOuter.implicitss
+          case _ => is :: nextOuter.implicitss
+        }
+
+      val CycleMarker = NoRunId - 1
+      if (implicitsRunId == CycleMarker) {
+        debuglog(s"cycle while collecting implicits at owner ${owner}, probably due to an implicit without an explicit return type. Continuing with implicits from enclosing contexts.")
+        withOuter(Nil)
+      } else if (implicitsRunId != currentRunId) {
+        implicitsRunId = CycleMarker
         implicitsCache = List()
+        var canCache = true
         val newImplicits: List[ImplicitInfo] =
           if (owner != nextOuter.owner && owner.isClass && !owner.isPackageClass && !inSelfSuperCall) {
-            if (!owner.isInitialized) return nextOuter.implicitss
-            // debuglog("collect member implicits " + owner + ", implicit members = " + owner.thisType.implicitMembers)//DEBUG
-            savingEnclClass(this) {
+            if (!owner.isInitialized) { canCache = false; Nil }
+            else savingEnclClass(this) {
               // !!! In the body of `class C(implicit a: A) { }`, `implicitss` returns `List(List(a), List(a), List(<predef..)))`
               //     it handled correctly by implicit search, which considers the second `a` to be shadowed, but should be
               //     remedied nonetheless.
@@ -857,10 +867,15 @@ trait Contexts { self: Analyzer =>
             // the corresponding package object may contain implicit members.
             collectImplicits(owner.tpe.implicitMembers, owner.tpe)
           } else List()
-        implicitsCache = if (newImplicits.isEmpty) nextOuter.implicitss
-                         else newImplicits :: nextOuter.implicitss
+
+        if (canCache) {
+          implicitsRunId = currentRunId
+          implicitsCache = newImplicits
+        } else implicitsRunId = NoRunId
+
+        withOuter(newImplicits)
       }
-      implicitsCache
+      else withOuter(implicitsCache)
     }
 
     //
diff --git a/test/files/neg/t712.check b/test/files/neg/t712.check
index 6819dc0ce9..831e943063 100644
--- a/test/files/neg/t712.check
+++ b/test/files/neg/t712.check
@@ -1,4 +1,5 @@
 t712.scala:10: error: value self is not a member of B.this.ParentImpl
+ Note: implicit method coerce is not applicable here because it comes after the application point and it lacks an explicit result type
   implicit def coerce(p : ParentImpl) = p.self;
                                           ^
 one error found
diff --git a/test/files/pos/t6780.scala b/test/files/pos/t6780.scala
new file mode 100644
index 0000000000..4a358046c6
--- /dev/null
+++ b/test/files/pos/t6780.scala
@@ -0,0 +1,20 @@
+object O {
+  implicit def i: Int = 0
+}
+
+import O._
+
+trait Foo {
+  implicit val v1: Any
+  implicit def d1: Any
+           val v2: Any
+  implicit val v3: Any
+}
+
+trait Bar1 extends Foo {
+  implicit val v1      = {implicitly[Int]; ()} // failed due to cycle in Context#implicits being broken with Nil.
+           def d1      = {implicitly[Int]; ()} // okay
+  implicit val v2      = {implicitly[Int]; ()} // okay
+  implicit val v3: Any = {implicitly[Int]; ()} // okay
+
+}