Why are function pointers and data pointers incompatible in C/C++?

Solution 1:

An architecture doesn't have to store code and data in the same memory. With a Harvard architecture, code and data are stored in completely different memory. Most architectures are Von Neumann architectures with code and data in the same memory but C doesn't limit itself to only certain types of architectures if at all possible.

Solution 2:

Some computers have (had) separate address spaces for code and data. On such hardware it just doesn't work.

The language is designed not only for current desktop applications, but to allow it to be implemented on a large set of hardware.

It seems like the C language committee never intended void* to be a pointer to function, they just wanted a generic pointer to objects.

The C99 Rationale says:

6.3.2.3 Pointers
C has now been implemented on a wide range of architectures. While some of these architectures feature uniform pointers which are the size of some integer type, maximally portable code cannot assume any necessary correspondence between different pointer types and the integer types. On some implementations, pointers can even be wider than any integer type.

The use of void* (“pointer to void”) as a generic object pointer type is an invention of the C89 Committee. Adoption of this type was stimulated by the desire to specify function prototype arguments that either quietly convert arbitrary pointers (as in fread) or complain if the argument type does not exactly match (as in strcmp). Nothing is said about pointers to functions, which may be incommensurate with object pointers and/or integers.

Note Nothing is said about pointers to functions in the last paragraph. They might be different from other pointers, and the committee is aware of that.