Fix #9928: Don't treat import contexts as properly nested

This avoids surprises since context stacks sometimes have an import context in more
deeply nested position than the enclosing definition scope. It also corresponds to
the rule for name resolution, where we treat an inner import and an outer definition
of the same name as ambiguous.

Author: odersky

compiler/src/dotty/tools/dotc/core/Contexts.scala

@@ -247,7 +247,7 @@ object Contexts {
             else
               outer.implicits
           if (implicitRefs.isEmpty) outerImplicits
-          else new ContextualImplicits(implicitRefs, outerImplicits)(this)
+          else new ContextualImplicits(implicitRefs, outerImplicits, isImportContext)(this)
         }
       implicitsCache
     }
@@ -777,7 +777,7 @@ object Contexts {
 
   @sharable object NoContext extends Context(null) {
     source = NoSource
-    override val implicits: ContextualImplicits = new ContextualImplicits(Nil, null)(this)
+    override val implicits: ContextualImplicits = new ContextualImplicits(Nil, null, false)(this)
   }
 
   /** A context base defines state and associated methods that exist once per

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

@@ -295,7 +295,10 @@ object Implicits:
    *                   name, b, whereas the name of the symbol is the original name, a.
    *  @param outerCtx  the next outer context that makes visible further implicits
    */
-  class ContextualImplicits(val refs: List[ImplicitRef], val outerImplicits: ContextualImplicits)(initctx: Context) extends ImplicitRefs(initctx) {
+  class ContextualImplicits(
+      val refs: List[ImplicitRef],
+      val outerImplicits: ContextualImplicits,
+      isImport: Boolean)(initctx: Context) extends ImplicitRefs(initctx) {
     private val eligibleCache = EqHashMap[Type, List[Candidate]]()
 
     /** The level increases if current context has a different owner or scope than
@@ -304,7 +307,8 @@ object Implicits:
      */
     override val level: Int =
       if outerImplicits == null then 1
-      else if migrateTo3(using irefCtx)
+      else if isImport
+              || migrateTo3(using irefCtx)
               || (irefCtx.owner eq outerImplicits.irefCtx.owner)
                  && (irefCtx.scope eq outerImplicits.irefCtx.scope)
                  && !refs.head.implicitName.is(LazyImplicitName)
@@ -370,7 +374,7 @@ object Implicits:
         val outerExcluded = outerImplicits exclude root
         if (irefCtx.importInfo.site.termSymbol == root) outerExcluded
         else if (outerExcluded eq outerImplicits) this
-        else new ContextualImplicits(refs, outerExcluded)(irefCtx)
+        else new ContextualImplicits(refs, outerExcluded, isImport)(irefCtx)
       }
   }
 

tests/neg/i9928.scala

@@ -0,0 +1,26 @@
+trait Magic[F]:
+  extension (x: Int) def read: F
+
+object Magic:
+  given Magic[String]:
+    extension(x: Int) def read: String =
+      println("In string")
+      s"$x"
+
+opaque type Foo = String
+object Foo:
+  import Magic.{given _}
+  def apply(s: String): Foo = s
+
+  given Magic[Foo]:
+    extension (x: Int) def read: Foo =
+      println("In foo")
+      Foo(s"$x")
+
+  def test: Unit =
+    (3.read: Foo)  // error: ambiguous
+
+
+@main def Test = {
+  Foo.test
+}