SI-7455 Drop dummy param for synthetic access constructor

Adapted from scalac commit 050b4c951c838699c2fe30cbf01b63942c63a299 by
Jason Zaugg:

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.

Author: smarter

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -194,13 +194,21 @@ class ClassfileParser(
         val name = pool.getName(in.nextChar)
         val isConstructor = name eq nme.CONSTRUCTOR
 
-        /** Strip leading outer param from constructor.
-         *  Todo: Also strip trailing access tag for private inner constructors?
+        /** Strip leading outer param from constructor and trailing access tag for
+         *  private inner constructors.
          */
-        def stripOuterParamFromConstructor() = innerClasses.get(currentClassName) match {
+        def normalizeConstructorParams() = innerClasses.get(currentClassName) match {
           case Some(entry) if !isStatic(entry.jflags) =>
             val mt @ MethodTpe(paramNames, paramTypes, resultType) = denot.info
-            denot.info = mt.derivedLambdaType(paramNames.tail, paramTypes.tail, resultType)
+            var normalizedParamNames = paramNames.tail
+            var normalizedParamTypes = paramTypes.tail
+            if ((jflags & JAVA_ACC_SYNTHETIC) != 0) {
+              // 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.
+              normalizedParamNames = paramNames.dropRight(1)
+              normalizedParamTypes = paramTypes.dropRight(1)
+            }
+            denot.info = mt.derivedLambdaType(normalizedParamNames, normalizedParamTypes, resultType)
           case _ =>
         }
 
@@ -216,7 +224,7 @@ class ClassfileParser(
 
         denot.info = pool.getType(in.nextChar)
         if (isEnum) denot.info = ConstantType(Constant(sym))
-        if (isConstructor) stripOuterParamFromConstructor()
+        if (isConstructor) normalizeConstructorParams()
         setPrivateWithin(denot, jflags)
         denot.info = translateTempPoly(parseAttributes(sym, denot.info))
         if (isConstructor) normalizeConstructorInfo()