Reverse-sync from upstream

Author: kddnewton

src/prism.c

@@ -11043,7 +11043,10 @@ parser_lex(pm_parser_t *parser) {
                     }
 
                     if (lex_state_spcarg_p(parser, space_seen)) {
-                        pm_parser_warn_token(parser, &parser->current, PM_WARN_AMBIGUOUS_SLASH);
+                        // https://bugs.ruby-lang.org/issues/21994
+                        if (parser->version <= PM_OPTIONS_VERSION_CRUBY_4_0) {
+                            pm_parser_warn_token(parser, &parser->current, PM_WARN_AMBIGUOUS_SLASH);
+                        }
                         lex_mode_push_regexp(parser, '\0', '/');
                         LEX(PM_TOKEN_REGEXP_BEGIN);
                     }

test/prism/result/warnings_test.rb

@@ -19,7 +19,8 @@ def test_ambiguous_ustar
     end
 
     def test_ambiguous_regexp
-      assert_warning("a /b/", "wrap regexp in parentheses")
+      assert_warning("a /b/", "wrap regexp in parentheses", compare: false, version: "4.0")
+      refute_warning("a /b/", compare: false, version: "4.1")
     end
 
     def test_ambiguous_ampersand
@@ -408,8 +409,8 @@ def test_warnings_verbosity
       assert_equal "END in method; use at_exit", warning.message
       assert_equal :default, warning.level
 
-      warning = Prism.parse("foo /regexp/").warnings.first
-      assert_equal "ambiguous `/`; wrap regexp in parentheses or add a space after `/` operator", warning.message
+      warning = Prism.parse("foo +1").warnings.first
+      assert_equal "ambiguous first argument; put parentheses or a space even after `+` operator", warning.message
       assert_equal :verbose, warning.level
     end