Make circular argument reference a SyntaxError instead of a warning

Fixes [Bug #10314]

parse.y

@@ -9890,7 +9890,8 @@ gettable(struct parser_params *p, ID id, const YYLTYPE *loc)
 	if (dyna_in_block(p) && dvar_defined_ref(p, id, &vidp)) {
 	    if (NUMPARAM_ID_P(id) && numparam_nested_p(p)) return 0;
 	    if (id == p->cur_arg) {
-		rb_warn1("circular argument reference - %"PRIsWARN, rb_id2str(id));
+                compile_error(p, "circular argument reference - %"PRIsWARN, rb_id2str(id));
+                return 0;
 	    }
 	    if (vidp) *vidp |= LVAR_USED;
 	    node = NEW_DVAR(id, loc);
@@ -9898,7 +9899,8 @@ gettable(struct parser_params *p, ID id, const YYLTYPE *loc)
 	}
 	if (local_id_ref(p, id, &vidp)) {
 	    if (id == p->cur_arg) {
-		rb_warn1("circular argument reference - %"PRIsWARN, rb_id2str(id));
+                compile_error(p, "circular argument reference - %"PRIsWARN, rb_id2str(id));
+                return 0;
 	    }
 	    if (vidp) *vidp |= LVAR_USED;
 	    node = NEW_LVAR(id, loc);

spec/ruby/language/block_spec.rb

@@ -876,20 +876,38 @@ describe "Post-args" do
     end
 
     describe "with a circular argument reference" do
-      it "shadows an existing local with the same name as the argument" do
-        a = 1
-        -> {
-          @proc = eval "proc { |a=a| a }"
-        }.should complain(/circular argument reference/)
-        @proc.call.should == nil
+      ruby_version_is ''...'2.7' do
+        it "warns and uses a nil value when there is an existing local variable with same name" do
+          a = 1
+          -> {
+            @proc = eval "proc { |a=a| a }"
+          }.should complain(/circular argument reference/)
+          @proc.call.should == nil
+        end
+
+        it "warns and uses a nil value when there is an existing method with same name" do
+          def a; 1; end
+          -> {
+            @proc = eval "proc { |a=a| a }"
+          }.should complain(/circular argument reference/)
+          @proc.call.should == nil
+        end
       end
 
-      it "shadows an existing method with the same name as the argument" do
-        def a; 1; end
-        -> {
-          @proc = eval "proc { |a=a| a }"
-        }.should complain(/circular argument reference/)
-        @proc.call.should == nil
+      ruby_version_is '2.7' do
+        it "raises a SyntaxError if using an existing local with the same name as the argument" do
+          a = 1
+          -> {
+            @proc = eval "proc { |a=a| a }"
+          }.should raise_error(SyntaxError)
+        end
+
+        it "raises a SyntaxError if there is an existing method with the same name as the argument" do
+          def a; 1; end
+          -> {
+            @proc = eval "proc { |a=a| a }"
+          }.should raise_error(SyntaxError)
+        end
       end
 
       it "calls an existing method with the same name as the argument if explicitly using ()" do

spec/ruby/language/def_spec.rb

@@ -177,17 +177,32 @@ describe "An instance method with a default argument" do
     foo(2,3,3).should == [2,3,[3]]
   end
 
-  it "shadows an existing method with the same name as the local" do
-    def bar
-      1
+  ruby_version_is ''...'2.7' do
+    it "warns and uses a nil value when there is an existing local method with same name" do
+      def bar
+        1
+      end
+      -> {
+        eval "def foo(bar = bar)
+          bar
+        end"
+      }.should complain(/circular argument reference/)
+      foo.should == nil
+      foo(2).should == 2
+    end
+  end
+
+  ruby_version_is '2.7' do
+    it "raises a syntaxError an existing method with the same name as the local variable" do
+      def bar
+        1
+      end
+      -> {
+        eval "def foo(bar = bar)
+          bar
+        end"
+      }.should raise_error(SyntaxError)
     end
-    -> {
-      eval "def foo(bar = bar)
-        bar
-      end"
-    }.should complain(/circular argument reference/)
-    foo.should == nil
-    foo(2).should == 2
   end
 
   it "calls a method with the same name as the local when explicitly using ()" do

spec/ruby/language/lambda_spec.rb

@@ -267,20 +267,38 @@ describe "A lambda literal -> () { }" do
     end
 
     describe "with circular optional argument reference" do
-      it "shadows an existing local with the same name as the argument" do
-        a = 1
-        -> {
-          @proc = eval "-> (a=a) { a }"
-        }.should complain(/circular argument reference/)
-        @proc.call.should == nil
+      ruby_version_is ''...'2.7' do
+        it "warns and uses a nil value when there is an existing local variable with same name" do
+          a = 1
+          -> {
+            @proc = eval "-> (a=a) { a }"
+          }.should complain(/circular argument reference/)
+          @proc.call.should == nil
+        end
+
+        it "warns and uses a nil value when there is an existing method with same name" do
+          def a; 1; end
+          -> {
+            @proc = eval "-> (a=a) { a }"
+          }.should complain(/circular argument reference/)
+          @proc.call.should == nil
+        end
       end
 
-      it "shadows an existing method with the same name as the argument" do
-        def a; 1; end
-        -> {
-          @proc = eval "-> (a=a) { a }"
-        }.should complain(/circular argument reference/)
-        @proc.call.should == nil
+      ruby_version_is '2.7' do
+        it "raises a SyntaxError if using an existing local with the same name as the argument" do
+          a = 1
+          -> {
+            @proc = eval "-> (a=a) { a }"
+          }.should raise_error(SyntaxError)
+        end
+
+        it "raises a SyntaxError if there is an existing method with the same name as the argument" do
+          def a; 1; end
+          -> {
+            @proc = eval "-> (a=a) { a }"
+          }.should raise_error(SyntaxError)
+        end
       end
 
       it "calls an existing method with the same name as the argument if explicitly using ()" do

test/ruby/test_syntax.rb

@@ -230,27 +230,23 @@ class TestSyntax < Test::Unit::TestCase
   end
 
   def test_keyword_self_reference
-    bug9593 = '[ruby-core:61299] [Bug #9593]'
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var: defined?(var)) var end")
     end
-    assert_equal(42, o.foo(var: 42))
-    assert_equal("local-variable", o.foo, bug9593)
 
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var: var) var end")
     end
-    assert_nil(o.foo, bug9593)
 
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var: bar(var)) var end")
     end
 
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var: bar {var}) var end")
     end
 
@@ -302,37 +298,33 @@ class TestSyntax < Test::Unit::TestCase
   end
 
   def test_optional_self_reference
-    bug9593 = '[ruby-core:61299] [Bug #9593]'
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var = defined?(var)) var end")
     end
-    assert_equal(42, o.foo(42))
-    assert_equal("local-variable", o.foo, bug9593)
 
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var = var) var end")
     end
-    assert_nil(o.foo, bug9593)
 
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var = bar(var)) var end")
     end
 
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var = bar {var}) var end")
     end
 
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var = (def bar;end; var)) var end")
     end
 
     o = Object.new
-    assert_warn(/circular argument reference - var/) do
+    assert_raise(SyntaxError) do
       o.instance_eval("def foo(var = (def self.bar;end; var)) var end")
     end