c - Why would GCC/Clang accept const qualified object as initializer for object with static storage duration?

Question

As far as I know, the ISO C standard is strict about initializers for object with static storage duration in C11 6.7.9 Initialization

All the expressions in an initializer for an object that has static or thread storage duration shall be constant expressions or string literals.

But both GCC/Clang accepted the following code:

const int i = 3; // const here should not make i a constant expression
static int j = i;

Even with -Wall -Wextra -Werror -pedantic-errors, the compilers above are not giving me any complaint.

However, these compilers do realize that i was not a constant expression. For example, Clang gave me:

error: size of static array must be an integer constant expression

for the following code:

const size_t sz = 3;
static int a[sz];

Am I getting anything wrong here?

Answer 1

You didn't miss anything, it is indeed a not a strictly conforming C program, and a non-conforming behavior on the compiler's part. This is very likely caused by the both GCC and Clang being compiler suites for C++ as well. In C++ i is a valid initializer for j.

It's worth noting however that while compilers are free to translate non-conforming C programs at their discretion, they must still issue a diagnostic about the non-conformance according to C11 5.1.1.3 p1:

A conforming implementation shall produce at least one diagnostic message (identified in an implementation-defined manner) if a preprocessing translation unit or translation unit contains a violation of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or implementation-defined. Diagnostic messages need not be produced in other circumstances.

Even if we accept the behavior as an extension, the fact GCC and Clang don't issue a diagnostic is conformance bug on both their parts.