Why can't a typedef of a function be used to define a function?

Question

From § 8.3.5.11 of ISO/IEC 14882:2011(E):

A typedef of function type may be used to declare a function but shall not be used to define a function

The standard goes on to give this example:

typedef void F();
F fv; // OK: equivalent to void fv();
F fv { } // ill-formed
void fv() { } // OK: definition of fv

What motivates this rule? It seems to limit the potential expressive usefulness of function typedefs.

Answer 1

Though this question is about C++, but since C++ inherits typedef and function pointer from C, so an explanation of the same question in C can be used in here. There's a formal explanation for C.

Rationale for International Standard - Programming Languages C §6.9.1 Function definitions

An argument list must be explicitly present in the declarator; it cannot be inherited from a typedef (see §6.7.5.3). That is to say, given the definition:

typedef int p(int q, int r);

the following fragment is invalid:

p funk // weird
{ return q + r ; }

Some current implementations rewrite the type of, for instance, a char parameter as if it were declared int, since the argument is known to be passed as an int in the absence of a prototype. The Standard requires, however, that the received argument be converted as if by assignment upon function entry. Type rewriting is thus no longer permissible.

Answer 2

It's probably mostly historical reasons. typedef was a relatively late addition to C, and was tacked onto the existing language (and caused a few problems for the parsing phase of compilers).

Also, a function definition has to define the names of the parameters, if any. A function type includes the function's return type and parameter types, but not its parameter names. For example, these:

void (int)
void (int x)
void (int y)

are three ways of writing the same function type. If you had:

typedef void func_t(int);

then this hypothetical definition:

func_t some_func { }

wouldn't define a name for its int parameter. I'm not sure how that could have been resolved in a reasonable manner. It would be possible, I suppose, but it was never done.

But the bottom line is probably just that Dennis Ritchie either didn't think it was worth the effort to define how a typedef could be used in a function definition, or he simply didn't think of it.