Why same types are different?

Question

Why this (char is signed on my implementation):

cout << std::is_same< char,signed char>::value;

outputs false?

Answer 1

The three types were introduced at different times.

From the C99 Rational:

Three types of char are specified:
signed, plain, and unsigned. A plain char may be represented as either signed or unsigned depending upon the implementation, as in prior practice. The type signed char was introduced in C89 to make available a one-byte signed integer type on those systems which implement plain char as unsigned char.

They have to stay separate types in C++, to allow overloading on char to be portable.