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

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Answer 1

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.