How is this typedef possible? [duplicate]

Question

In this SO question I've encountered a very weird typedef:

struct Date {     int day, month, year; } typedef date_s; 

I've always been seeing typedefs following this 'rule':

typedef <existing> <new>; 

For example:

typedef unsigned long long ull; typedef int kph; // speed typedef void (*alpm_cb_log)(alpm_loglevel_t, const char *, va_list);  typedef int int_t; typedef char char_t, *char_p, (*fp)(void); 

_{The 4th one is taken from here, the 5th and 6th are from cppreference}

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And this is how I would typedef a struct:

typedef struct {     int a, b, c; } data;  // and then use it data Something; 

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The question is how is this even possible to write such a typedef? It doesn't even make sense (at least to me).

clang doesn't give any errors or warnings, even with -Wall -Wextra.

Bonus question: should I advise the author of the question where this code could be found to avoid using such a typedef (because it's very unusual and may lead to confusion)?

Answer 1

It turns out that typedef can be placed after the existing type (in addition to before it). This little oddity, now obsolescent^*, is caused by the way in which the C standard "bundles" typedefs with storage class specifiers, such as static and auto:

A declaration is defined as follows:

<declaration> ::=  {<declaration-specifier>}+ {<init-declarator>}* 

This means that declaration specifiers can appear in any order. Now, declaration specifier is

<declaration-specifier> ::= <storage-class-specifier>                           | <type-specifier>                           | <type-qualifier> 

And storage class specifier is

<storage-class-specifier> ::= auto                         | register                         | static                         | extern                         | typedef 

The declaration of the struct's elements is a type specifier. Like typedef keyword, it is a declaration specifier. Since declaration specifiers can appear in any order, both placements of typedef (i.e., before and after the struct) are valid and identical to each other.

^*N1570, 6.11.5: "The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature." Thanks, Keith Thompson, for a great comment!