GCC doesn't support simple integer constant expression?

Question

GCC 4.9 and 5.1 reject this simple C99 declaration at global scope. Clang accepts it.

const int a = 1, b = a; // error: initializer element is not constant
<         


        

Answer 1

C99¹ section 6.6 Constant expressions is the controlling section. It states in subsections 6 and 7:

6/ An integer constant expression shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, and floating constants that are the immediate operands of casts.

Cast operators in an integer constant expression shall only convert arithmetic types to integer types, except as part of an operand to the sizeof operator.

The definition of integer and floating point constants is specified in 6.4.4 of the standard, and it's restricted to actual values (literals) rather than variables.

7/ More latitude is permitted for constant expressions in initializers. Such a constant expression shall be, or evaluate to, one of the following (a) an arithmetic constant expression, (b) a null pointer constant, (c) an address constant, or (d) an address constant for an object type plus or minus an integer constant expression.

Since a is none of those things in either subsection 6 or 7, it is not considered a constant expression as per the standard.

The real question, therefore, is not why gcc rejects it but why clang accepts it, and that appears to be buried in subsection 10 of that same section:

10/ An implementation may accept other forms of constant expressions.

In other words, the standard states what an implementation must allow for constant expressions but doesn't limit implementations to allowing only that.

---

¹ C11 is much the same other than minor things like allowing _Alignof as well as sizeof.