dereferencing pointer to incomplete type

I saw a question the other day where someone inadvertently used an incomplete type by specifying something like

struct a {
    int q; 
struct A *x; 
x->q = 3;

The compiler knew that struct A was a struct, despite A being totally undefined, by virtue of the struct keyword.

That was in C++, where such usage of struct is atypical (and, it turns out, can lead to foot-shooting). In C if you do

typedef struct a {
} a;

then you can use a as the typename and omit the struct later. This will lead the compiler to give you an undefined identifier error later, rather than incomplete type, if you mistype the name or forget a header.

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