Fixed #425 (False positive: usage of char variable.. c = c & 0x03)

Using char variables in bit operations are ok if:
 * the result is stored in a char
 * the variable is and'ed with a number that is less than 0x100

src/checkother.cpp

@@ -752,10 +752,26 @@ void CheckOther::CheckCharVariable()
                     break;
                 }
 
-                std::string tempFirst = "%var% [&|] " + tok->str();
+                if (Token::Match(tok2, "[;{}] %var% = %any% [&|] %any% ;"))
-                std::string tempSecond = tok->str() + " [&|]";
-                if (Token::Match(tok2, tempFirst.c_str()) || Token::Match(tok2, tempSecond.c_str()))
                 {
+                    // is the char variable used in the calculation?
+                    if (tok2->tokAt(3)->varId() != tok->varId() && tok2->tokAt(5)->varId() != tok->varId())
+                        continue;
+
+                    // it's ok with a bitwise and where the other operand is 0xff or less..
+                    if (std::string(tok2->strAt(4)) == "&")
+                    {
+                        if (tok2->tokAt(3)->isNumber() && MathLib::isGreater("0x100", tok2->strAt(3)))
+                            continue;
+                        if (tok2->tokAt(5)->isNumber() && MathLib::isGreater("0x100", tok2->strAt(5)))
+                            continue;
+                    }
+
+                    // is the result stored in a short|int|long?
+                    if (!Token::findmatch(_tokenizer->tokens(), "short|int|long %varid%", tok2->next()->varId()))
+                        continue;
+
+                    // This is an error..
                     charBitOpError(tok2);
                     break;
                 }

test/testcharvar.cpp

@@ -39,6 +39,8 @@ private:
         TEST_CASE(bitop1);
         TEST_CASE(bitop2);
         TEST_CASE(return1);
+        TEST_CASE(assignChar);
+        TEST_CASE(and03);
     }
 
     void check(const char code[])
@@ -93,6 +95,7 @@ private:
     {
         check("void foo()\n"
               "{\n"
+              "    int result = 0;\n"
               "    char ch;\n"
               "    result = a | ch;\n"
               "}\n");
@@ -118,6 +121,28 @@ private:
               "}\n");
         ASSERT_EQUALS("", errout.str());
     }
+
+
+    void assignChar()
+    {
+        check("void foo()\n"
+              "{\n"
+              "    char c;\n"
+              "    c = c & 0x123;\n"
+              "}\n");
+        ASSERT_EQUALS("", errout.str());
+    }
+
+    void and03()
+    {
+        check("void foo()\n"
+              "{\n"
+              "    char c;\n"
+              "    int i = c & 0x03;\n"
+              "}\n");
+        ASSERT_EQUALS("", errout.str());
+    }
+
 };
 
 REGISTER_TEST(TestCharVar)