Do not warn about extra `format` argument when passed as keywords

[Bug #20593]

It's fairly common to use `format` to interpolate a number of values
into a user provided strings.

The arguments not matching are a problem when they are positional,
but when they are named, it's absolutely fine and we shouldn't
emit a warning.

spec/ruby/core/kernel/format_spec.rb

@@ -12,4 +12,36 @@ describe "Kernel.format" do
   it "is accessible as a module function" do
     Kernel.format("%s", "hello").should == "hello"
   end
+
+  describe "when $VERBOSE is true" do
+    it "warns if too many arguments are passed" do
+      code = <<~RUBY
+        $VERBOSE = true
+        format("test", 1)
+      RUBY
+
+      ruby_exe(code, args: "2>&1").should include("warning: too many arguments for format string")
+    end
+
+    it "does not warns if too many keyword arguments are passed" do
+      code = <<~RUBY
+        $VERBOSE = true
+        format("test %{test}", test: 1, unused: 2)
+      RUBY
+
+      ruby_exe(code, args: "2>&1").should_not include("warning")
+    end
+
+    ruby_bug "#20593", ""..."3.4" do
+      it "doesn't warns if keyword arguments are passed and none are used" do
+        code = <<~RUBY
+          $VERBOSE = true
+          format("test", test: 1)
+          format("test", {})
+        RUBY
+
+        ruby_exe(code, args: "2>&1").should_not include("warning")
+      end
+    end
+  end
 end

sprintf.c

@@ -937,7 +937,7 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
     rb_str_tmp_frozen_release(orig, fmt);
     /* XXX - We cannot validate the number of arguments if (digit)$ style used.
      */
-    if (posarg >= 0 && nextarg < argc) {
+    if (posarg >= 0 && nextarg < argc && !(argc == 2 && RB_TYPE_P(argv[1], T_HASH))) {
         const char *mesg = "too many arguments for format string";
         if (RTEST(ruby_debug)) rb_raise(rb_eArgError, "%s", mesg);
         if (RTEST(ruby_verbose)) rb_warn("%s", mesg);