Conversion from char* to signed char*

Question

I saw a piece of valid C code I tried to compile as C++ and I got an error I can't understand.

char* t;
signed char* v = t;

error: invalid conversion from char* to signed char*

From what I learned, char and signed char are semantically identical, but are still considered as different by the compiler.

I know that the error is caused by the difference between these two type, my question is: Why does this difference exists ?

As far as I know char is implemented either as a signed char or as a unsigned char so it should be identical to either one or the other.

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I consulted this question and it doesn't answer the point I want to know.

Answer 1

Actually I finally found the spec part talking about this:

3.9.1 Fundamental types

1. Objects declared as characters (char) shall be large enough to store any member of the implementation’s basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (3.11); that is, they have the same object representation. For character types, all bits of the object representation participate in the value representation. For unsigned character types, all possible bit patterns of the value representation represent numbers. These requirements do not hold for other types. In any particular implementation, a plain char object can take on either the same values as a signed char or an unsigned char; which one is implementation-defined.