Fix #1990: Handle case where inlining changes class of outer

The new situation in the test was that outer of the inlined method
was `A` but it's as seen from type is a subtype `B`.

We need two fixes:

 - Ignore outerSelects in TreeChecker. These are treated as having fixed symbols.
 - Adapt the outer-path logic to deal with code that's moved to another context.

Author: odersky

compiler/src/dotty/tools/dotc/transform/ExplicitOuter.scala

@@ -60,7 +60,7 @@ class ExplicitOuter extends MiniPhaseTransform with InfoTransformer { thisTransf
   /** Convert a selection of the form `qual.C_<OUTER>` to an outer path from `qual` to `C` */
   override def transformSelect(tree: Select)(implicit ctx: Context, info: TransformerInfo) =
     if (tree.name.isOuterSelect)
-      outer.path(tree.tpe.widen.classSymbol, tree.qualifier).ensureConforms(tree.tpe)
+      outer.path(tree.tpe.widen.classSymbol, tree.qualifier, outOfContext = true).ensureConforms(tree.tpe)
     else tree
 
   /** First, add outer accessors if a class does not have them yet and it references an outer this.
@@ -355,14 +355,21 @@ object ExplicitOuter {
     }
 
     /** The path of outer accessors that references `toCls.this` starting from
-     *  the context owner's this node.
+     *  node `start`, which defaults to the context owner's this node.
+     *  @param  outOfContext  When true, we take the `path` in code that has been inlined
+     *                        from somewhere else. In that case, we need to stop not
+     *                        just when `toCls` is reached exactly, but also in any superclass
+     *                        of `treeCls`. This compensates the `asSeenFrom` logic
+     *                        used to compute this-proxies in Inliner.
      */
-    def path(toCls: Symbol, start: Tree = This(ctx.owner.lexicallyEnclosingClass.asClass)): Tree = try {
+    def path(toCls: Symbol,
+             start: Tree = This(ctx.owner.lexicallyEnclosingClass.asClass),
+             outOfContext: Boolean = true): Tree = try {
       def loop(tree: Tree): Tree = {
         val treeCls = tree.tpe.widen.classSymbol
         val outerAccessorCtx = ctx.withPhaseNoLater(ctx.lambdaLiftPhase) // lambdalift mangles local class names, which means we cannot reliably find outer acessors anymore
         ctx.log(i"outer to $toCls of $tree: ${tree.tpe}, looking for ${outerAccName(treeCls.asClass)(outerAccessorCtx)} in $treeCls")
-        if (treeCls == toCls) tree
+        if (treeCls == toCls || outOfContext && toCls.derivesFrom(treeCls)) tree
         else {
           val acc = outerAccessor(treeCls.asClass)(outerAccessorCtx)
           assert(acc.exists,

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -12,6 +12,7 @@ import core.Types._
 import core.Flags._
 import core.Constants._
 import core.StdNames._
+import core.NameOps._
 import core.Decorators._
 import core.TypeErasure.isErasedType
 import core.Phases.Phase
@@ -336,7 +337,10 @@ class TreeChecker extends Phase with SymTransformer {
       assert(tree.isTerm || !ctx.isAfterTyper, tree.show + " at " + ctx.phase)
       val tpe = tree.typeOpt
       val sym = tree.symbol
-      if (!tpe.isInstanceOf[WithFixedSym] && sym.exists && !sym.is(Private)) {
+      if (!tpe.isInstanceOf[WithFixedSym] &&
+          sym.exists && !sym.is(Private) &&
+          !tree.name.isOuterSelect // outer selects have effectively fixed symbols
+          ) {
         val qualTpe = tree.qualifier.typeOpt
         val member =
           if (sym.is(Private)) qualTpe.member(tree.name)

tests/pos/i1990.scala

@@ -0,0 +1,12 @@
+class A {
+  class Foo {
+    inline def inlineMeth: Unit = {
+      new Bar
+    }
+  }
+  class Bar
+}
+
+class B extends A {
+  (new Foo).inlineMeth
+}