gcovr always shows 100% line coverage

Question

I run the following code with gcovr, and get 100% line-coverage, despite bar not being called. What am I doing wrong?

#include <iostream>

struct A{
    static void foo(){
        std::cout << "foo" << std::endl;
    }

    static void bar(){
        std::cout << "bar" << std::endl;
    }
};

int main() {
    A::foo();
    // A::bar();
}

The HTML report looks like this:

I use the following CMakeLists.txt:

cmake_minimum_required(VERSION 2.8)

set(PROJECT_NAME test_gcovr)
project(${PROJECT_NAME})

if(NOT CMAKE_COMPILER_IS_GNUCXX)
    message("must use gnu compiler")
    EXIT()
endif()

set(CMAKE_CXX_FLAGS "-g -Wall -O0")

add_executable(run_tests ${PROJECT_SOURCE_DIR}/test.cpp)

set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/CMakeModules)
include(CodeCoverage)
append_coverage_compiler_flags()

setup_target_for_coverage_gcovr_html(NAME run_tests_coverage
                                    EXECUTABLE run_tests
                                    DEPENDENCIES run_tests)

With this CodeCoverage.cmake.

I compile and run with a call to cmake, followed by make run_tests_coverage.

---

EDIT: It appears that in the example above, bar is not part of the executable, despite -O0... I fixed this with:

...
bool x = false;
if (x){
    A::bar();
};
...

and now I get a correct coverage report.

Now the question remains - how to check coverage of tests for header-only libraries (without going through the report in detail)?

---

EDIT #2:

see this Getting useful GCov results for header-only libraries