Exploring inner modules for binary incompatibilities. Fix #127

The former implementation of PackageInfo#accessibleClasses expects that for any
module `Foo` compiled by scalac, both `Foo.class` and `Foo$.class` classfiles
are produced. Because `Foo$.class` and `Foo.class` are essentially mirrors, and
users never access directly `Foo$.class`, MiMa only traverses the `Foo.class`. This
works well for top-level modules, but it breaks for inner modules, because no
mirror class is created for inner modules (only the module class is created).

The fix consist in treating inner modules differently, hence making sure inner
modules are part of the set returned by PackageInfo#accessibleClasses.

core/src/main/scala/com/typesafe/tools/mima/core/PackageInfo.scala

@@ -64,8 +64,9 @@ abstract class PackageInfo(val owner: PackageInfo) {
   def classes: mutable.Map[String, ClassInfo]
 
   lazy val accessibleClasses: Set[ClassInfo] = {
+    // FIXME: Make this a tailrecursive implementation
     /** Fixed point iteration for finding all accessible classes. */
-    def accessibleClassesUnder(prefix: Set[ClassInfo]): Set[ClassInfo] = {
+    def accessibleClassesUnder(prefix: Set[ClassInfo]): Set[ClassInfo] = { 
       val vclasses = (classes.valuesIterator filter (isAccessible(_, prefix))).toSet
       if (vclasses.isEmpty) vclasses
       else vclasses union accessibleClassesUnder(vclasses)
@@ -77,7 +78,28 @@ abstract class PackageInfo(val owner: PackageInfo) {
         else {
           val idx = clazz.decodedName.lastIndexOf("$")
           if (idx < 0) prefix.isEmpty // class name contains no $
-          else prefix exists (_.decodedName == clazz.decodedName.substring(0, idx)) // prefix before dollar is an accessible class detected previously
+          else {
+            // A top-level module is compiled into a class together with a module class (for instance,
+            // for a top-level `object A`, scalac creates two classfiles `A.class` and `A$.class`.).
+            // While, for an inner module, scalac creates only one classfile, i.e., the module class 
+            // classfile.
+            // Hence, in this branch we need to identify when `clazz` is an inner module. The implemented 
+            // heuristic assumes that only module classes ends with a '$'. `clazz` is an inner module iff
+            // the `clazz` decoded name ends with a '$' and if the `clazz` decoded name starts with one
+            // of the passed `prefix`.
+            val isInnerObject = {
+              clazz.decodedName.endsWith("$") && 
+              prefix.exists { outer =>
+                // this condition is to avoid looping indefinitely 
+                clazz.decodedName != outer.decodedName &&
+                // checks if `outer` is a prefix of `clazz` 
+                clazz.decodedName.startsWith(outer.decodedName)
+              }
+            }
+            isInnerObject ||
+            // class, trait, and top-level module go through this condition
+            (prefix exists (_.decodedName == clazz.decodedName.substring(0, idx))) // prefix before dollar is an accessible class detected previously
+          }
         }
       }
       clazz.isPublic && isReachable

reporter/functional-tests/src/test/class-removing-inner-object-nok/problems.txt

@@ -0,0 +1,2 @@
+object A#B does not have a correspondent in new version
+method B()A#B# in class A does not have a correspondent in new version

reporter/functional-tests/src/test/class-removing-inner-object-nok/v1/A.scala

@@ -0,0 +1,5 @@
+class A {
+  object B {
+    def foo: Int = 2
+  }
+}

reporter/functional-tests/src/test/class-removing-inner-object-nok/v2/A.scala

@@ -0,0 +1,2 @@
+class A {
+}

reporter/functional-tests/src/test/object-removing-inner-object-nok/problems.txt

@@ -0,0 +1 @@
+object A#B does not have a correspondent in new version

reporter/functional-tests/src/test/object-removing-inner-object-nok/v1/A.scala

@@ -0,0 +1,5 @@
+object A {
+  object B {
+    def foo: Int = 2
+  }
+}

reporter/functional-tests/src/test/object-removing-inner-object-nok/v2/A.scala

@@ -0,0 +1,2 @@
+object A {
+}