an ambiguity in c89 which will never be fixed

The event highlights a long-standing ambiguity in the C89 standard regarding implicit function declarations. The standard states that an undeclared function identifier in a call expression is implicitly declared as `extern int identifier();` in the 'innermost block.' However, it does not clarify whether 'innermost block' refers to block scope or function prototype scope. This lack of precision leads to divergent behavior between GCC and Clang when handling self-referential function declarations, such as `int f(int f[sizeof(f())]);`.

The practical consequence for engineers is that code relying on implicit function declarations may compile under one compiler but fail under another. For example, Clang accepts `int f(int f[sizeof(f())]);` while GCC rejects it, whereas the opposite is true for `int main(void) { int f(int [sizeof(x())]); int x; }`. This inconsistency complicates maintenance of legacy C89 codebases, particularly in environments where compiler choice is constrained by hardware or toolchain limitations.

Adopting a consistent behavior in legacy code requires either avoiding implicit function declarations entirely or explicitly declaring functions before use. The cost of this change is minimal for modern codebases, as the feature was deprecated in C99 and removed in later standards. However, for systems still bound to C89, such as those targeting obsolete hardware or proprietary compilers, the ambiguity remains a latent risk. Engineers must either accept the divergence or refactor code to eliminate reliance on implicit declarations.

The ambiguity stops being a concern in codebases that have migrated to C99 or later, where implicit function declarations are no longer allowed. However, for the subset of systems still using C89, the lack of resolution means this edge case will persist indefinitely. The disagreement between compilers also underscores the challenges of interpreting standards with underspecified behavior, particularly when the feature in question has been removed from subsequent revisions.