Following code fails to compile on any gcc version. Works on all clang versions.

It looks like inside template function get_templ_base the lookup of v.templ_base::a is done eagerly, and finds templ_base at the global scope, and hence errors out because the template parameters are missing.
But at this point qualified name "templ_base::a" should be type-dependent, so lookup should be delayed until template instantiation, where 'templ_base' is an injected class name, so template paremeters shouldn't be required.

g++ -std=c++11

struct base
{
    int a = 1;
};

template <typename T>
struct templ_base
{
    int a = 2;
};

namespace bar
{
    
struct base2
{
    int a = 3;
};

template <typename T>
struct templ_base2
{
    int a = 4;
};


}

struct foo : base, bar::templ_base2<char>, templ_base<int>, bar::base2
{
    int base = 5;
    int templ_base = 6;
    int base2 = 7;
    int templ_base2 = 8;
    int a = 9;
};

static_assert(foo{}.base::a == 1, "");
static_assert(foo{}.templ_base::a == 2, "");
static_assert(foo{}.base2::a == 3, "");
static_assert(foo{}.templ_base2::a == 4, "");


template <typename T>
int get_base(T&& v)
{
    return v.base::a;
}

template <typename T>
int get_templ_base(T&& v)
{
    return v.templ_base::a; // fails in all gcc versions
}

template <typename T>
int get_base2(T&& v)
{
    return v.base2::a; // fails for gcc <= 11
}

template <typename T>
int get_templ_base2(T&& v)
{
    return v.templ_base2::a;  // fails for gcc <= 11
}

int a = get_base(foo{});
int b = get_templ_base(foo{});
int c = get_base2(foo{});
int d = get_templ_base2(foo{});