Make yield in singleton class definitions in methods a SyntaxError

This behavior was deprecated in 2.7 and scheduled to be removed
in 3.0.

Calling yield in a class definition outside a method is now a
SyntaxError instead of a LocalJumpError, as well.

NEWS.md

@@ -12,6 +12,8 @@ sufficient information, see the ChangeLog file or Redmine
 
 * $SAFE is now a normal global variable with no special behavior. [Feature #16131]
 
+* yield in singleton class definitions in methods is now a SyntaxError. [Feature #15575]
+
 ## Command line options
 
 ## Core classes updates (outstanding ones only)

bootstraptest/test_jump.rb

@@ -296,14 +296,19 @@ class << self
   s.return_eigenclass == class << s; self; end
 }, '[ruby-core:21379]'
 
-assert_equal "true", %q{
-  class Object
-    def yield_eigenclass
-      class << self
-        yield self
+assert_match %r{Invalid yield}, %q{
+STDERR.reopen(STDOUT)
+begin
+  eval %q{
+    class Object
+      def yield_eigenclass
+        class << self
+          yield self
+        end
       end
     end
-  end
-  s = "foo"
-  s.yield_eigenclass {|c| c == class << s; self; end }
-}, '[ruby-dev:40975]'
+  }
+rescue SyntaxError => e
+  e.message
+end
+}

compile.c

@@ -7096,20 +7096,13 @@ iseq_compile_each(rb_iseq_t *iseq, LINK_ANCHOR *ret, const NODE *node, int poppe
 }
 
 static int
-check_yield_place(const rb_iseq_t *iseq, int line)
+check_yield_place(const rb_iseq_t *iseq)
 {
-    VALUE file;
     switch (iseq->body->local_iseq->body->type) {
       case ISEQ_TYPE_TOP:
       case ISEQ_TYPE_MAIN:
-        return FALSE;
       case ISEQ_TYPE_CLASS:
-        file = rb_iseq_path(iseq);
-        if (rb_warning_category_enabled_p(RB_WARN_CATEGORY_DEPRECATED)) {
-            rb_compile_warn(RSTRING_PTR(file), line,
-                            "`yield' in class syntax will not be supported from Ruby 3.0. [Feature #15575]");
-        }
-        return TRUE;
+        return FALSE;
       default:
         return TRUE;
     }
@@ -7836,7 +7829,7 @@ iseq_compile_each0(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *node, in
 
 	INIT_ANCHOR(args);
 
-        if (check_yield_place(iseq, line) == FALSE) {
+        if (check_yield_place(iseq) == FALSE) {
 	    COMPILE_ERROR(ERROR_ARGS "Invalid yield");
             goto ng;
         }

spec/ruby/language/class_spec.rb

@@ -285,7 +285,7 @@ def xyz
     }.should raise_error(TypeError)
   end
 
-  ruby_version_is ""..."3.0" do
+  ruby_version_is ""..."2.8" do
     it "allows accessing the block of the original scope" do
       suppress_warning do
         ClassSpecs.sclass_with_block { 123 }.should == 123
@@ -293,6 +293,14 @@ def xyz
     end
   end
 
+  ruby_version_is "2.8" do
+    it "does not allow accessing the block of the original scope" do
+      -> {
+        ClassSpecs.sclass_with_block { 123 }
+      }.should raise_error(SyntaxError)
+    end
+  end
+
   it "can use return to cause the enclosing method to return" do
     ClassSpecs.sclass_with_return.should == :inner
   end

test/ruby/test_class.rb

@@ -312,7 +312,7 @@ def test_invalid_return_from_class_definition
   end
 
   def test_invalid_yield_from_class_definition
-    assert_raise(LocalJumpError) {
+    assert_raise(SyntaxError) {
       EnvUtil.suppress_warning {eval("class C; yield; end")}
     }
   end