Fix constructor completion problem detected in t0054

Constructors need to be completed in the context which immediately encloses
a class. Otherwise type references in the constructor see the wrong types,
as is demonstrated in t0054. The difficulty here is that the inner class B
nested in A also extends from A. Then it makes a difference whether the constructor
parameter types of B are resolved in the context of B or in the context of A.
Added explanation for context handling of constructors.

Author: odersky

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -148,9 +148,9 @@ object SymDenotations {
       // completions.println(s"completing ${this.debugString}")
       try completer.complete(this)
       catch {
-      case ex: CyclicReference =>
-        completions.println(s"error while completing ${this.debugString}")
-        throw ex
+        case ex: CyclicReference =>
+          completions.println(s"error while completing ${this.debugString}")
+          throw ex
       }
       // completions.println(s"completed ${this.debugString}")
     }

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

@@ -216,9 +216,18 @@ class Namer { typer: Typer =>
         checkNoConflict(name)
         val deferred = if (lacksDefinition(tree)) Deferred else EmptyFlags
         val method = if (tree.isInstanceOf[DefDef]) Method else EmptyFlags
+
+        // to complete a constructor, move one context further out -- this
+        // is the context enclosing the class. Note that the context in which a
+        // constructor is recorded and the context in which it is completed are
+        // different: The former must have the class as owner (because the
+        // constructor is owned by the class), the latter must not (because
+        // constructor parameters are interpreted as if they are outside the class).
+        val cctx = if (tree.name == nme.CONSTRUCTOR) ctx.outer else ctx
+
         record(ctx.newSymbol(
           ctx.owner, name, tree.mods.flags | deferred | method,
-          adjustIfModule(new Completer(tree), tree),
+          adjustIfModule(new Completer(tree)(cctx), tree),
           privateWithinClass(tree.mods), tree.pos))
       case tree: Import =>
         record(ctx.newSymbol(

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

@@ -699,9 +699,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     val TypeBoundsTree(lo, hi) = desugar.typeBoundsTree(tree)
     val lo1 = typed(lo)
     val hi1 = typed(hi)
-    if (false) // need to do in later phase, as this might cause a cyclic reference error. See pos/t0039.scala
-      if (!(lo1.tpe <:< hi1.tpe))
-        ctx.error(i"lower bound ${lo1.tpe} does not conform to upper bound ${hi1.tpe}", tree.pos)
+    // need to do in later phase, as this might cause a cyclic reference error. See pos/t0039.scala
+    //  if (!(lo1.tpe <:< hi1.tpe))
+    //    ctx.error(i"lower bound ${lo1.tpe} does not conform to upper bound ${hi1.tpe}", tree.pos)
     assignType(cpy.TypeBoundsTree(tree, lo1, hi1), lo1, hi1)
   }
 

tests/pos/t0054.scala

@@ -0,0 +1,4 @@
+class A {
+  case class B(x: C) extends A { val z: A.this.C = x }
+  class C {}
+}