JDK 16 RFR of JDK-8251921: Expand default constructor warning to cover more cases

[Joe Darcy]

Hello,

As a follow-up to JDK-8071961: "Add javac lint warning when a default 
constructor is created," Phil observed that there are cases in the JDK 
where a *protected* class has a default constructor appearing in the 
JDK. Please review the changes and CSR to augment the warning to cover 
both public and protected classes. For nested classes, the enclosing 
types must be either public or protected all the way up for the warning 
to be issued.

     JDK-8251921: Expand default constructor warning to cover more cases
     CSR: https://bugs.openjdk.java.net/browse/JDK-8252348
     webrev: http://cr.openjdk.java.net/~darcy/8251921.0/

Patch below. Clean langtools test run with this change.

Thanks,

-Joe

--- 
old/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java 
2020-08-25 15:04:59.413000000 -0700
+++ 
new/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java 
2020-08-25 15:04:58.685000000 -0700
@@ -3835,7 +3835,7 @@
          if (lint.isEnabled(LintCategory.MISSING_EXPLICIT_CTOR) &&
              ((c.flags() & (ENUM | RECORD)) == 0) &&
              !c.isAnonymous() &&
-            ((c.flags() & PUBLIC) != 0) &&
+            ((c.flags() & (PUBLIC | PROTECTED)) != 0) &&
              Feature.MODULES.allowedInSource(source)) {
              NestingKind nestingKind = c.getNestingKind();
              switch (nestingKind) {
@@ -3844,10 +3844,10 @@
                  case TOP_LEVEL -> {;} // No additional checks needed
                  case MEMBER -> {
                      // For nested member classes, all the enclosing
-                    // classes must be public.
+                    // classes must be public or protected.
                      Symbol owner = c.owner;
                      while (owner != null && owner.kind == TYP) {
-                        if ((owner.flags() & PUBLIC) == 0)
+                        if ((owner.flags() & (PUBLIC | PROTECTED)) == 0)
                              return;
                          owner = owner.owner;
                      }
--- 
old/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/javac.properties 
2020-08-25 15:05:01.037000000 -0700
+++ 
new/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/javac.properties 
2020-08-25 15:05:00.189000000 -0700
@@ -183,7 +183,7 @@
      Warn about issues related to classfile contents.

  javac.opt.Xlint.desc.missing-explicit-ctor=\
-    Warn about missing explicit constructors in public classes in 
exported packages.
+    Warn about missing explicit constructors in public and protected 
classes in exported packages.

  javac.opt.Xlint.desc.deprecation=\
      Warn about use of deprecated items.
--- 
old/test/langtools/tools/javac/warnings/DefaultCtor/DefaultCtorWarningToolBox.java 
2020-08-25 15:05:02.313000000 -0700
+++ 
new/test/langtools/tools/javac/warnings/DefaultCtor/DefaultCtorWarningToolBox.java 
2020-08-25 15:05:01.597000000 -0700
@@ -97,7 +97,9 @@
              List.of("Foo.java:4:8: 
compiler.warn.missing-explicit-ctor: pkg1.Foo, pkg1, mod",
                      "Foo.java:12:12: 
compiler.warn.missing-explicit-ctor: pkg1.Foo.FooNest, pkg1, mod",
                      "Foo.java:16:19: 
compiler.warn.missing-explicit-ctor: pkg1.Foo.StaticFooNest, pkg1, mod",
-                    "3 warnings");
+            "Foo.java:25:15: compiler.warn.missing-explicit-ctor: 
pkg1.Foo.ProtectedFooNest, pkg1, mod",
+            "Foo.java:27:19: compiler.warn.missing-explicit-ctor: 
pkg1.Foo.ProtectedFooNest.ProtectedFooNestNest, pkg1, mod",
+                    "5 warnings");

          // Warning enable,
          log = new JavacTask(tb)
@@ -137,30 +139,34 @@
          class Bar {

              // No explicit constructor; use a default.
-            public class FooNest {
+            public class BarNest {
              }

              // No explicit constructor; use a default.
-            public static class StaticFooNest {
+            public static class StaticBaryNest {
+            }
+
+            // No explicit constructor; use a default.
+            protected class ProtectedBarNest {
              }

              // Package-access classes

              // No explicit constructor; use a default.
-            /*package*/ class PkgFooNest {
+            /*package*/ class PkgBarNest {
              }

              // No explicit constructor; use a default.
-            /*package*/ static class PkgStaticFooNest {
+            /*package*/ static class PkgStaticBarNest {
              }
              // Private classes

              // No explicit constructor; use a default.
-            private class PrvFooNest {
+            private class PrvBarNest {
              }

              // No explicit constructor; use a default.
-            private static class PrvStaticFooNest {
+            private static class PrvStaticBarNest {
              }
          }
          """;
@@ -190,10 +196,18 @@
              public static class SuppressedStaticFooNest {
              }

+            // No explicit constructor; use a default.
+            protected class ProtectedFooNest {
+                // No explicit constructor; use a default.
+                protected class ProtectedFooNestNest {}
+            }
+
              // Package-access classes

              // No explicit constructor; use a default.
              /*package*/ class PkgFooNest {
+                // No explicit constructor; use a default.
+                protected class PkgFooNestNest {}
              }

              // No explicit constructor; use a default.
@@ -203,6 +217,8 @@

              // No explicit constructor; use a default.
              private class PrvFooNest {
+                // No explicit constructor; use a default.
+                protected class PrvFooNestNest {}
              }

              // No explicit constructor; use a default.