Summary
-------

Add explicit constructors to API classes of `jdk.compiler` that have default constructors.

Problem
-------

Default constructors are not recommended for classes that are parts of a formal API.

Solution
--------

Add explicit no-arg constructors. For the `com.sun.source` classes, as those classes intend to allow instantiation of themselves or subclasses, equivalent public no-arg constructors are added. In contrast, for `com.sun.tools.javac.Main` the constructor seems unintended as all the members of the class are static. Therefore, the no-arg constructor is added and terminally deprecated.

Specification
-------------

    --- old/src/jdk.compiler/share/classes/com/sun/source/util/DocTreePathScanner.java	2020-07-27 09:20:26.556000000 -0700
    +++ new/src/jdk.compiler/share/classes/com/sun/source/util/DocTreePathScanner.java	2020-07-27 09:20:25.476000000 -0700
    @@ -39,6 +39,11 @@
      */
     public class DocTreePathScanner<R, P> extends DocTreeScanner<R, P> {
         /**
    +     * Constructs a {@code DocTreePathScanner}.
    +     */
    +    public DocTreePathScanner() {}
    +
    +    /**
          * Scans a tree from a position identified by a tree path.
          * @param path the path
          * @param p a value to be passed to visitor methods
    --- old/src/jdk.compiler/share/classes/com/sun/source/util/DocTreeScanner.java	2020-07-27 09:20:28.600000000 -0700
    +++ new/src/jdk.compiler/share/classes/com/sun/source/util/DocTreeScanner.java	2020-07-27 09:20:27.504000000 -0700
    @@ -68,6 +68,10 @@
      * @since 1.8
      */
     public class DocTreeScanner<R,P> implements DocTreeVisitor<R,P> {
    +    /**
    +     * Constructs a {@code DocTreeScanner}.
    +     */
    +    public DocTreeScanner() {}
     
         /**
          * Scans a single node.
    --- old/src/jdk.compiler/share/classes/com/sun/source/util/DocTrees.java	2020-07-27 09:20:30.312000000 -0700
    +++ new/src/jdk.compiler/share/classes/com/sun/source/util/DocTrees.java	2020-07-27 09:20:29.492000000 -0700
    @@ -47,6 +47,11 @@
      */
     public abstract class DocTrees extends Trees {
         /**
    +     * Constructor for subclasses to call.
    +     */
    +    public DocTrees() {}
    +
    +    /**
          * Returns a DocTrees object for a given CompilationTask.
          * @param task the compilation task for which to get the Trees object
          * @return the DocTrees object
    --- old/src/jdk.compiler/share/classes/com/sun/source/util/JavacTask.java	2020-07-27 09:20:32.088000000 -0700
    +++ new/src/jdk.compiler/share/classes/com/sun/source/util/JavacTask.java	2020-07-27 09:20:31.224000000 -0700
    @@ -50,6 +50,10 @@
      * @since 1.6
      */
     public abstract class JavacTask implements CompilationTask {
    +    /**
    +     * Constructor for subclasses to call.
    +     */
    +    public JavacTask() {}
     
         /**
          * Returns the {@code JavacTask} for a {@code ProcessingEnvironment}.
    --- old/src/jdk.compiler/share/classes/com/sun/source/util/TreePathScanner.java	2020-07-27 09:20:33.688000000 -0700
    +++ new/src/jdk.compiler/share/classes/com/sun/source/util/TreePathScanner.java	2020-07-27 09:20:32.804000000 -0700
    @@ -43,6 +43,10 @@
      * @since 1.6
      */
     public class TreePathScanner<R, P> extends TreeScanner<R, P> {
    +    /**
    +     * Constructs a {@code TreePathScanner}.
    +     */
    +    public TreePathScanner() {}
     
         /**
          * Scans a tree from a position identified by a TreePath.
    --- old/src/jdk.compiler/share/classes/com/sun/source/util/TreeScanner.java	2020-07-27 09:20:35.172000000 -0700
    +++ new/src/jdk.compiler/share/classes/com/sun/source/util/TreeScanner.java	2020-07-27 09:20:34.448000000 -0700
    @@ -75,6 +75,10 @@
      * @since 1.6
      */
     public class TreeScanner<R,P> implements TreeVisitor<R,P> {
    +    /**
    +     * Constructs a {@code TreeScanner}.
    +     */
    +    public TreeScanner() {}
     
         /**
          * Scans a single node.
    --- old/src/jdk.compiler/share/classes/com/sun/source/util/Trees.java	2020-07-27 09:20:36.912000000 -0700
    +++ new/src/jdk.compiler/share/classes/com/sun/source/util/Trees.java	2020-07-27 09:20:36.004000000 -0700
    @@ -53,6 +53,11 @@
      */
     public abstract class Trees {
         /**
    +     * Constructor for subclasses to call.
    +     */
    +    public Trees() {}
    +
    +    /**
          * Returns a Trees object for a given CompilationTask.
          * @param task the compilation task for which to get the Trees object
          * @throws IllegalArgumentException if the task does not support the Trees API.
    --- old/src/jdk.compiler/share/classes/com/sun/tools/javac/Main.java	2020-07-27 09:20:38.680000000 -0700
    +++ new/src/jdk.compiler/share/classes/com/sun/tools/javac/Main.java	2020-07-27 09:20:37.764000000 -0700
    @@ -34,6 +34,11 @@
      * module for details on replacement APIs.
      */
     public class Main {
    +    /**
    +     * Do not call.
    +     */
    +    @Deprecated(since="16", forRemoval=true)
    +    public Main(){}
     
         /** Main entry point for the launcher.
          *  Note: This method calls System.exit.