[ruby/strscan] Fix a bug that inconsistency of IndexError vs nil for

unknown capture group
(ruby/strscan#143)

Fix ruby/strscan#139

Reported by Benoit Daloze. Thanks!!!

ruby/strscan@bc8a0d2623

Author: naitoh

ext/strscan/strscan.c

@@ -1667,19 +1667,17 @@ strscan_matched_size(VALUE self)
 static int
 name_to_backref_number(struct re_registers *regs, VALUE regexp, const char* name, const char* name_end, rb_encoding *enc)
 {
-    int num;
-
-    num = onig_name_to_backref_number(RREGEXP_PTR(regexp),
-	(const unsigned char* )name, (const unsigned char* )name_end, regs);
-    if (num >= 1) {
-	return num;
-    }
-    else {
-	rb_enc_raise(enc, rb_eIndexError, "undefined group name reference: %.*s",
-					  rb_long2int(name_end - name), name);
+    if (RTEST(regexp)) {
+        int num = onig_name_to_backref_number(RREGEXP_PTR(regexp),
+                                              (const unsigned char* )name,
+                                              (const unsigned char* )name_end,
+                                              regs);
+        if (num >= 1) {
+	        return num;
+        }
     }
-
-    UNREACHABLE;
+    rb_enc_raise(enc, rb_eIndexError, "undefined group name reference: %.*s",
+                 rb_long2int(name_end - name), name);
 }
 
 /*
@@ -1768,7 +1766,6 @@ strscan_aref(VALUE self, VALUE idx)
             idx = rb_sym2str(idx);
             /* fall through */
         case T_STRING:
-            if (!RTEST(p->regex)) return Qnil;
             RSTRING_GETMEM(idx, name, i);
             i = name_to_backref_number(&(p->regs), p->regex, name, name + i, rb_enc_get(idx));
             break;

test/strscan/test_stringscanner.rb

@@ -458,7 +458,31 @@ def test_matched_string
   def test_AREF
     s = create_string_scanner('stra strb strc')
 
-    s.scan(/\w+/)
+    s.scan(/\s+/)
+    assert_nil(          s[-2])
+    assert_nil(          s[-1])
+    assert_nil(          s[0])
+    assert_nil(          s[1])
+    assert_nil(          s[:c])
+    assert_nil(          s['c'])
+
+    s.scan("not match")
+    assert_nil(          s[-2])
+    assert_nil(          s[-1])
+    assert_nil(          s[0])
+    assert_nil(          s[1])
+    assert_nil(          s[:c])
+    assert_nil(          s['c'])
+
+    s.check(/\w+/)
+    assert_nil(          s[-2])
+    assert_equal('stra', s[-1])
+    assert_equal('stra', s[0])
+    assert_nil(          s[1])
+    assert_raise(IndexError) { s[:c] }
+    assert_raise(IndexError) { s['c'] }
+
+    s.scan("stra")
     assert_nil(          s[-2])
     assert_equal('stra', s[-1])
     assert_equal('stra', s[0])
@@ -903,11 +927,11 @@ def test_aref_without_regex
 
     s = create_string_scanner('abc')
     s.get_byte
-    assert_nil(s[:c])
-    assert_nil(s["c"])
+    assert_raise(IndexError) { s[:c] }
+    assert_raise(IndexError) { s['c'] }
     s.getch
-    assert_nil(s[:c])
-    assert_nil(s["c"])
+    assert_raise(IndexError) { s[:c] }
+    assert_raise(IndexError) { s['c'] }
   end
 
   def test_size