Deprecate passing arguments to Set#to_set and Enumerable#to_set

Array#to_a, Hash#to_h, Enumerable#to_a, and Enumerable#to_h do not
allow you to specify subclasses.  This has undesired behavior when
passing non-Set subclasses.  All of these are currently allowed, and
none make sense:

```ruby
enum = [1,2,3].to_enum

enum.to_set(Hash)
enum.to_set(Struct.new("A", :a))
enum.to_set(ArgumentError)
enum.to_set(Thread){}
```

Users who want to create instances of a subclass of Set from an
enumerable should pass the enumerable to SetSubclass.new instead of
using to_set.

4 files changed, 27 insertions, 5 deletions

diff --git a/common.mk b/common.mk
index ad5ebac281..748e622beb 100644
--- a/common.mk
+++ b/common.mk
@@ -16792,6 +16792,7 @@ set.$(OBJEXT): $(top_srcdir)/internal/array.h
 set.$(OBJEXT): $(top_srcdir)/internal/basic_operators.h
 set.$(OBJEXT): $(top_srcdir)/internal/bits.h
 set.$(OBJEXT): $(top_srcdir)/internal/compilers.h
+set.$(OBJEXT): $(top_srcdir)/internal/error.h
 set.$(OBJEXT): $(top_srcdir)/internal/gc.h
 set.$(OBJEXT): $(top_srcdir)/internal/hash.h
 set.$(OBJEXT): $(top_srcdir)/internal/imemo.h
@@ -16801,6 +16802,7 @@ set.$(OBJEXT): $(top_srcdir)/internal/sanitizers.h
 set.$(OBJEXT): $(top_srcdir)/internal/serial.h
 set.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 set.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
+set.$(OBJEXT): $(top_srcdir)/internal/string.h
 set.$(OBJEXT): $(top_srcdir)/internal/symbol.h
 set.$(OBJEXT): $(top_srcdir)/internal/variable.h
 set.$(OBJEXT): $(top_srcdir)/internal/vm.h
diff --git a/prelude.rb b/prelude.rb
index 839b2bcc39..f49cada637 100644
--- a/prelude.rb
+++ b/prelude.rb
@@ -28,7 +28,14 @@ end
 
 module Enumerable
   # Makes a set from the enumerable object with given arguments.
-  def to_set(klass = Set, *args, &block)
+  # Passing arguments to this method is deprecated.
+  def to_set(*args, &block)
+    klass = if args.empty?
+      Set
+    else
+      warn "passing arguments to Enumerable#to_set is deprecated", uplevel: 1
+      args.shift
+    end
     klass.new(self, *args, &block)
   end
 end
diff --git a/set.c b/set.c
index 8676c62cd3..ed0ace4224 100644
--- a/set.c
+++ b/set.c
@@ -7,6 +7,7 @@
 #include "id.h"
 #include "internal.h"
 #include "internal/bits.h"
+#include "internal/error.h"
 #include "internal/hash.h"
 #include "internal/proc.h"
 #include "internal/sanitizers.h"
@@ -635,6 +636,7 @@ set_i_to_set(int argc, VALUE *argv, VALUE set)
         argc = 1;
     }
     else {
+        rb_warn_deprecated("passing arguments to Set#to_set", NULL);
         klass = argv[0];
         argv[0] = set;
     }
diff --git a/spec/ruby/core/enumerable/to_set_spec.rb b/spec/ruby/core/enumerable/to_set_spec.rb
index 966baae1d9..d0fecf6de4 100644
--- a/spec/ruby/core/enumerable/to_set_spec.rb
+++ b/spec/ruby/core/enumerable/to_set_spec.rb
@@ -11,10 +11,21 @@ describe "Enumerable#to_set" do
     [1, 2, 3].to_set { |x| x * x }.should == Set[1, 4, 9]
   end
 
-  it "instantiates an object of provided as the first argument set class" do
-    set = [1, 2, 3].to_set(EnumerableSpecs::SetSubclass)
-    set.should be_kind_of(EnumerableSpecs::SetSubclass)
-    set.to_a.sort.should == [1, 2, 3]
+  ruby_version_is "3.5" do
+    it "instantiates an object of provided as the first argument set class" do
+      set = nil
+      proc{set = [1, 2, 3].to_set(EnumerableSpecs::SetSubclass)}.should complain(/Enumerable#to_set/)
+      set.should be_kind_of(EnumerableSpecs::SetSubclass)
+      set.to_a.sort.should == [1, 2, 3]
+    end
+  end
+
+  ruby_version_is ""..."3.5" do
+    it "instantiates an object of provided as the first argument set class" do
+      set = [1, 2, 3].to_set(EnumerableSpecs::SetSubclass)
+      set.should be_kind_of(EnumerableSpecs::SetSubclass)
+      set.to_a.sort.should == [1, 2, 3]
+    end
   end
 
   it "does not need explicit `require 'set'`" do