SI-7455 Drop dummy param for synthetic access constructor

Java synthesizes public constructors in private classes to
allow access from inner classes. The signature of
that synthetic constructor (known as a "access constructor")
has a dummy parameter appended to avoid overloading clashes.
javac chooses the type "Enclosing$1" for the dummy parameter
(called the "access constructor tag") which is either an
existing anonymous class or a synthesized class for this purpose.

In OpenJDK, this transformation is performed in:

  langtools/src/share/classes/com/sun/tools/javac/comp/Lower.java

(Incidentally, scalac would just emits a byte-code public
constructor in this situation, rather than a private constructor /
access constructor pair.)

Scala parses the signature of the access contructor, and drops
the $outer parameter, but retains the dummy parameter. This causes
havoc when it tries to parse the bytecode for that anonymous class;
the class file parser doesn't have the enclosing type parameters
of Vector in scope and crash ensues.

In any case, we shouldn't allow user code to see that constructor;
it should only be called from within its own compilation unit.

This commit drops the dummy parameter from access constructor
signatures in class file parsing.

src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala

@@ -626,7 +626,7 @@ abstract class ClassfileParser {
         sawPrivateConstructor = true
       in.skip(2); skipAttributes()
     } else {
-      if ((sflags & PRIVATE) != 0L && global.settings.optimise.value) {
+      if ((sflags & PRIVATE) != 0L && global.settings.optimise.value) { // TODO this should be !optimize, no? See c4181f656d.
         in.skip(4); skipAttributes()
       } else {
         val name = pool.getName(in.nextChar)
@@ -636,7 +636,7 @@ abstract class ClassfileParser {
           info match {
             case MethodType(params, restpe) =>
               // if this is a non-static inner class, remove the explicit outer parameter
-              val newParams = innerClasses getEntry currentClass match {
+              val paramsNoOuter = innerClasses getEntry currentClass match {
                 case Some(entry) if !isScalaRaw && !isStatic(entry.jflags) =>
                   /* About `clazz.owner.isPackage` below: SI-5957
                    * For every nested java class A$B, there are two symbols in the scala compiler.
@@ -650,6 +650,15 @@ abstract class ClassfileParser {
                 case _ =>
                   params
               }
+              val newParams = paramsNoOuter match {
+                case (init :+ tail) if (jflags & JAVA_ACC_SYNTHETIC) != 0L =>
+                  // SI-7455 strip trailing dummy argument ("access constructor tag") from synthetic constructors which
+                  // are added when an inner class needs to access a private constructor.
+                  init
+                case _ =>
+                  paramsNoOuter
+              }
+
               info = MethodType(newParams, clazz.tpe)
           }
         sym.setInfo(info)

test/files/run/t7455.check

@@ -0,0 +1,4 @@
+private[package <empty>] def <init>(x$1: String): Outer[E]
+private[package <empty>] def <init>(): Outer$PrivateInner
+private[package <empty>] def <init>(): Outer$PrivateStaticInner
+private[package <empty>] def <init>(x$2: String): Outer$PublicInner

test/files/run/t7455/Outer.java

@@ -0,0 +1,31 @@
+public class Outer<E> {
+    public void elements() {
+        new C<E>() {
+        };
+    }
+
+    private Outer(String a) {}
+
+    static class SubSelf extends Outer<String> {
+        public SubSelf() { super(""); }
+    }
+
+    private class PrivateInner {
+    }
+    class SubPrivateInner extends PrivateInner {
+    }
+
+    private class PublicInner {
+        private PublicInner(String a) {}
+    }
+    class SubPublicInner extends PublicInner {
+        public SubPublicInner() { super(""); }
+    }
+
+    private static class PrivateStaticInner {
+    }
+    public static class SubPrivateStaticInner extends PrivateStaticInner {
+    }
+}
+
+class C<E> {}

test/files/run/t7455/Test.scala

@@ -0,0 +1,30 @@
+import scala.tools.partest._
+
+// javac adds dummy parameters of type Outer$1 to synthetic access constructors
+// This test shows that we strip them from the signatures. If we don't, we trigger
+// parsing of Outer$1 which can fail if it references type parameters of the Outer.
+//
+// OLD OUTPUT:
+//  private[package <empty>] def <init>(x$2: Outer$1): Outer$PrivateInner
+//  error: error while loading Outer$1, class file 't7455-run.obj/Outer$1.class' is broken
+//  (class java.util.NoSuchElementException/key not found: E)
+//  ...
+object Test extends DirectTest {
+  override def code = ""
+
+  def show {
+    val classpath = List(sys.props("partest.lib"), testOutput.path) mkString sys.props("path.separator")
+    val compiler = newCompiler("-cp", classpath, "-d", testOutput.path)
+    import compiler._, definitions._
+    new compiler.Run
+
+    for {
+      name <- Seq("Outer", "Outer$PrivateInner", "Outer$PrivateStaticInner", "Outer$PublicInner")
+      clazz = compiler.rootMirror.staticClass(name)
+      constr <- clazz.info.member(nme.CONSTRUCTOR).alternatives
+    } {
+      println(constr.defString)
+      fullyInitializeSymbol(constr)
+    }
+  }
+}