adjustModuleCompleter: Avoid capturing Context

Previously we computed the scope in `findModuleBuddy` using
`this.effectiveScope`, this means that we captured `this` which has a
self-type of `Context`, replacing it by `ctx.effectiveScope`
would be wrong since we are interested in the scope at the time
`adjustModuleCompleter` was called, not the scope at the time the
completer is called. Therefore, we have to eagerly compute
`this.effectiveScope` so that we don't capture the Context but can use
the right scope in `findModuleBuddy`.

We also move `findModuleBuddy` to a companion object to avoid accidental
captures of `this`.

This capture lead to crashes in the IDE.

Author: smarter

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

@@ -24,6 +24,7 @@ import language.implicitConversions
 import reporting.diagnostic.messages._
 
 trait NamerContextOps { this: Context =>
+  import NamerContextOps._
 
   /** Enter symbol into current class, if current class is owner of current context,
    *  or into current scope, if not. Should always be called instead of scope.enter
@@ -119,22 +120,25 @@ trait NamerContextOps { this: Context =>
     else monotpe
   }
 
-  /** Find moduleClass/sourceModule in effective scope */
-  private def findModuleBuddy(name: Name)(implicit ctx: Context) = {
-    val scope = effectiveScope
-    val it = scope.lookupAll(name).filter(_ is Module)
-    assert(it.hasNext, s"no companion $name in $scope")
-    it.next
-  }
-
   /** Add moduleClass or sourceModule functionality to completer
    *  for a module or module class
    */
-  def adjustModuleCompleter(completer: LazyType, name: Name) =
+  def adjustModuleCompleter(completer: LazyType, name: Name) = {
+    val scope = this.effectiveScope
     if (name.isTermName)
-      completer withModuleClass (_ => findModuleBuddy(name.moduleClassName))
+      completer withModuleClass (implicit ctx => findModuleBuddy(name.moduleClassName, scope))
     else
-      completer withSourceModule (_ => findModuleBuddy(name.sourceModuleName))
+      completer withSourceModule (implicit ctx => findModuleBuddy(name.sourceModuleName, scope))
+  }
+}
+
+object NamerContextOps {
+  /** Find moduleClass/sourceModule in effective scope */
+  private def findModuleBuddy(name: Name, scope: Scope)(implicit ctx: Context) = {
+    val it = scope.lookupAll(name).filter(_ is Module)
+    assert(it.hasNext, s"no companion $name in $scope")
+    it.next
+  }
 }
 
 /** This class creates symbols from definitions and imports and gives them