Here are two simplified test files:

***header.h
inline int func1()
{
    return 1000;
}
inline int func2()
{
    return 10;
}
***main.cpp
#include "header.h"
int main()
{
    func1();
    return 0;
}

To reproduce the bug execute the following commands:
g++ -g main.cpp -o test_break
gdb ./test_break
(gdb) break header.h:7

The result of the last command is "No line 7 in file "header.h"."

GDB thinks the code is unused, because the func2 is not used in the current symbols. This might not be the case if a shared library which uses func2 is loaded in the future, therefore GDB should not refuse to set a pending breakpoint.