Well, `a` could overload the "=" operator, in which case you would get `sizeof(a.operator=(12))`. But as far as pure C goes I believe you are right.
Edit: Via the C99 spec (http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf).
The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.
The type of an assignment expression is the type of the left operand unless the left operand has qualified type, in which case it is the unqualified version of the type of the left operand.
So it is indeed the case that `sizeof(a = 12)` is equivalent to `sizeof(a)` in the C world.
