On Thu, 9 Aug 2018, Martin Sebor wrote:

> But for a declaration at file scope and without an initializer,
> GCC warns that the array is assumed to have one element, but
> then gives an error when sizeof is applied to it:

That's how tentative definitions (C17 6.9.2) work.  There's an implicit 
initializer of { 0 }, but only at the end of the translation unit, so the 
type is incomplete until then and sizeof cannot be applied to it.

> even though the array does have a size of 1.  (I guess that's
> because of the [] syntax but the warning sure makes the sizeof
> error surprising.)  Auto declarations of arrays with no bound
> and with no intializer are rejected with an error:
> 
>   char a[];   // error: array size missing in ‘a’

Tentative definitions only exist at file scope (and in standard C they 
can't have an incomplete type if they have internal linkage).  See 6.7#7, 
"If an identifier for an object is declared with no linkage, the type for 
the object shall be complete by the end of its declarator, or by the end 
of its init-declarator if it has an initializer; in the case of function 
parameters (including in prototypes), it is the adjusted type (see 
6.7.6.3) that is required to be complete."

-- 
Joseph S. Myers
joseph@codesourcery.com