Judging by the .cc extension, you are compiling this with a C++ compiler. Quoting from Annex C (which documents the incompatibilities between C++ and ISO C) of the C++ standard:
Change: C++ does not have “tentative definitions” as in C E.g., at
file scope,
int i;
int i;
is valid in C, invalid in C++. This makes it impossible to define
mutually referential file-local static objects, if initializers are
restricted to the syntactic forms of C. For example,
struct X { int i; struct X *next; };
static struct X a;
static struct X b = { 0, &a };
static struct X a = { 1, &b };
Rationale: This avoids having different initialization rules for
fundamental types and user-defined types.
Effect on original feature: Deletion of semantically well-defined
feature.
Difficulty of converting: Semantic transformation.
Rationale: In C++, the initializer for one of a set of
mutually-referential file-local static objects must invoke a
function call to achieve the initialization.
How widely used: Seldom.
