c – 我可以依靠我的编译器来诊断TU中的类型不匹配吗？

extern int a;
extern float a;

我以前认为我的编译器需要诊断它,但规范说(我强调添加)

After all adjustments of types (during which typedefs (7.1.3) are replaced by their definitions),the types speci?ed by all declarations referring to a given variable or function shall be identical,except that declarations for an array object can specify array types that differ by the presence or absence of a major array bound (8.3.4). A violation of this rule on type identity does not require a diagnostic.

事实上,我发现编译器不关心的情况.例如,GCC和clang接受以下内容

void g() { int f(); } 
void h() { float f(); }

由于违反了不需要诊断的规则意味着整个程序不再需要诊断,这意味着以下不正确的程序也不需要诊断(见1.4p2).幸运的是,GCC和Clang都诊断出它们.

int f();
float f();

此代码在转换时的行为实际上是未定义的.这是什么原因？为什么规范不要求拒绝此类案例并要求对其进行诊断？