8251921: Expand default constructor warning to cover more cases

Reviewed-by: jjg, abuckley

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Check.java

@@ -3835,7 +3835,7 @@ public class Check {
         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 @@ public class Check {
                 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;
                     }

src/jdk.compiler/share/classes/com/sun/tools/javac/resources/javac.properties

@@ -183,7 +183,7 @@ javac.opt.Xlint.desc.classfile=\
     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.

test/langtools/tools/javac/warnings/DefaultCtor/DefaultCtorWarningToolBox.java

@@ -97,7 +97,9 @@ public class DefaultCtorWarningToolBox extends TestRunner {
             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 @@ public class DefaultCtorWarningToolBox extends TestRunner {
         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 StaticBarNest {
+            }
+
+            // 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 class DefaultCtorWarningToolBox extends TestRunner {
             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 @@ public class DefaultCtorWarningToolBox extends TestRunner {
 
             // No explicit constructor; use a default.
             private class PrvFooNest {
+                // No explicit constructor; use a default.
+                protected class PrvFooNestNest {}
             }
 
             // No explicit constructor; use a default.