All struct identifiers are automatically forward declared

Question

While answer warning: assignment from incompatible pointer type for linklist array, I noticed any undeclared identifier perceded with struct keyword are considered as forward declared identifiers.

For instance the program below compiles well:

/* Compile with "gcc -std=c99 -W -Wall -O2 -pedantic %" */
#include <stdio.h>

struct foo 
{
    struct bar *next;  /* Linked list */
};


int main(void) {
    struct bar *a = 0;
    struct baz *b = 0;
    struct foo c = {0};

    printf("bar -> %p\n", (void *)a);
    printf("baz -> %p\n", (void *)b);
    printf("foo -> %p, %zu\n", (void *)&c, sizeof c); /* Remove %zu if compiling with -ansi flag */
    return 0;
}

My question: Which rule guides a C compiler to treat undeclared struct identifiers as forward declared incomplete struct types?

Answer 1

The Standard says (6.2.5.28)

All pointers to structure types shall have the same representation and alignment requirements as each other.

This means the compiler knows how to represent the pointers to any structure, even those that are (yet) undefined.
Your program deals only with pointers to such structures, so it's ok.

Answer 2

It is described in 6.2.5 Types and 6.7.2.3 Tags.

struct identifier is an object type.

6.2.5 Types

1. The meaning of a value stored in an object or returned by a function is determined by the type of the expression used to access it. (An identifier declared to be an object is the simplest such expression; the type is specified in the declaration of the identifier.) Types are partitioned into object types (types that describe objects) and function types (types that describe functions). At various points within a translation unit an object type may be incomplete (lacking sufficient information to determine the size of objects of that type) or complete (having sufficient information). 37)

37) A type may be incomplete or complete throughout an entire translation unit, or it may change states at different points within a translation unit.

22. An array type of unknown size is an incomplete type. It is completed, for an identifier of that type, by specifying the size in a later declaration (with internal or external linkage). A structure or union type of unknown content (as described in 6.7.2.3) is an incomplete type. It is completed, for all declarations of that type, by declaring the same structure or union tag with its defining content later in the same scope.

6.7.2.3 Tags

4. All declarations of structure, union, or enumerated types that have the same scope and use the same tag declare the same type. Irrespective of whether there is a tag or what other declarations of the type are in the same translation unit, the type is incomplete 129) until immediately after the closing brace of the list defining the content, and complete thereafter.

129) An incomplete type may only by used when the size of an object of that type is not needed. It is not needed, for example, when a typedef name is declared to be a specifier for a structure or union, or when a pointer to or a function returning a structure or union is being declared. (See incomplete types in 6.2.5.) The specification has to be complete before such a function is called or defined.