https://godbolt.org/z/1TKz57K84

constexpr_map.h

#include <https://raw.githubusercontent.com/swansontec/map-macro/master/map.h>

#define CONST_MAP_CASE2(a, b) case a: return b;
#define CONST_MAP_CASE(pair) CONST_MAP_CASE2 pair

#define HEAD(h, ...) h
#define CONST_MAP_RET2(a, b) return decltype(b){};
#define CONST_MAP_RET(pair) CONST_MAP_RET2 pair

#define CONST_MAP(name, ...) \
constexpr auto name = [](auto v){ \
    switch (v) { \
    MAP(CONST_MAP_CASE, __VA_ARGS__) \
    } \
    CONST_MAP_RET(HEAD(__VA_ARGS__)) \
}

enum E
{
    A,
    B
};

CONST_MAP(mapEnum, (A, 1), (B, 2));

auto foo(E e)
{
    return mapEnum(e);
}

auto v = mapEnum(A);