Reject encodings determined at runtime as source code encodings

The encodings determined at runtime are affected by the runtime
environment, such as the OS and locale, while the file contents are
not.

parse.y

@@ -9477,11 +9477,20 @@ parser_encode_length(struct parser_params *p, const char *name, long len)
 static void
 parser_set_encode(struct parser_params *p, const char *name)
 {
-    int idx = rb_enc_find_index(name);
     rb_encoding *enc;
     VALUE excargs[3];
 
+    const char *wrong = 0;
+    switch (*name) {
+      case 'e': case 'E': wrong = "external"; break;
+      case 'i': case 'I': wrong = "internal"; break;
+      case 'f': case 'F': wrong = "filesystem"; break;
+      case 'l': case 'L': wrong = "locale"; break;
+    }
+    if (wrong && STRCASECMP(name, wrong) == 0) goto unknown;
+    int idx = rb_enc_find_index(name);
     if (idx < 0) {
+      unknown:
         excargs[1] = rb_sprintf("unknown encoding name: %s", name);
       error:
         excargs[0] = rb_eArgError;

test/ruby/test_parse.rb

@@ -767,6 +767,34 @@ def test_magic_comment
       END
     end
     assert_equal(__ENCODING__, x)
+
+    assert_raise(ArgumentError) do
+      EnvUtil.with_default_external(Encoding::US_ASCII) {eval <<-END, nil, __FILE__, __LINE__+1}
+# coding = external
+x = __ENCODING__
+      END
+    end
+
+    assert_raise(ArgumentError) do
+      EnvUtil.with_default_internal(Encoding::US_ASCII) {eval <<-END, nil, __FILE__, __LINE__+1}
+# coding = internal
+x = __ENCODING__
+      END
+    end
+
+    assert_raise(ArgumentError) do
+      eval <<-END, nil, __FILE__, __LINE__+1
+# coding = filesystem
+x = __ENCODING__
+      END
+    end
+
+    assert_raise(ArgumentError) do
+      eval <<-END, nil, __FILE__, __LINE__+1
+# coding = locale
+x = __ENCODING__
+      END
+    end
   end
 
   def test_utf8_bom