Report the loss of trait init methods

In the encoding of traits the presence of a method with a body (a
"concrete method") triggers the generation of a trait initialisation
method, "$init$", which will be invoked by subclasses during
construction.  The effect this has is that dropping the last concrete
method(s) from a trait _removes_ the trait init method, which is a
binary incompatible change to client's subclasses.

Fixes lightbend-labs#426

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

@@ -99,14 +99,15 @@ sealed abstract class ClassInfo(val owner: PackageInfo) extends InfoLike with Eq
     thisAndSuperClasses.flatMap(_.fields.get(field.bytecodeName))
 
   final def lookupClassMethods(method: MethodInfo): Iterator[MethodInfo] = {
-    method.bytecodeName match {
-      case MemberInfo.ConstructorName => methods.get(MemberInfo.ConstructorName) // constructors are not inherited
-      case name                       => thisAndSuperClasses.flatMap(_.methods.get(name))
-    }
+    val name = method.bytecodeName
+    if (name == MemberInfo.ConstructorName) methods.get(name) // constructors are not inherited
+    else if (method.isStatic) methods.get(name) // static methods are not inherited
+    else thisAndSuperClasses.flatMap(_.methods.get(name))
   }
 
   private def lookupInterfaceMethods(method: MethodInfo): Iterator[MethodInfo] =
-    allInterfaces.iterator.flatMap(_.methods.get(method.bytecodeName))
+    if (method.isStatic) Iterator.empty // static methods are not inherited
+    else allInterfaces.iterator.flatMap(_.methods.get(method.bytecodeName))
 
   final def lookupMethods(method: MethodInfo): Iterator[MethodInfo] =
     lookupClassMethods(method) ++ lookupInterfaceMethods(method)

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

@@ -54,13 +54,17 @@ private[mima] final class MethodInfo(owner: ClassInfo, bytecodeName: String, fla
   def matchesType(other: MethodInfo): Boolean = parametersDesc == other.parametersDesc
 
   private def isDefaultGetter: Boolean   = decodedName.contains("$default$")
+  private def isTraitInit: Boolean       = decodedName == "$init$"
   private def isExtensionMethod: Boolean = {
     var i = decodedName.length - 1
     while (i >= 0 && Character.isDigit(decodedName.charAt(i)))
       i -= 1
     decodedName.substring(0, i + 1).endsWith("$extension")
   }
-  def nonAccessible: Boolean = !isPublic || isSynthetic || (hasSyntheticName && !isExtensionMethod && !isDefaultGetter)
+  def nonAccessible: Boolean = {
+    !isPublic || isSynthetic ||
+      (hasSyntheticName && !(isExtensionMethod || isDefaultGetter || isTraitInit))
+  }
 
   override def toString = s"def $bytecodeName: $descriptor"
 }

functional-tests/src/test/trait-deleting-concrete-methods-is-nok/problems.txt

@@ -0,0 +1 @@
+synthetic static method $init$(B)Unit in interface B does not have a correspondent in new version

functional-tests/src/test/trait-pushing-up-concrete-methods-is-nok/problems.txt

@@ -0,0 +1 @@
+synthetic static method $init$(B)Unit in interface B does not have a correspondent in new version