User-defined Literal Operator on Macros

Question

How does one use a user-defined literal operator on a macro that expands to some literal expression?

e.g.:

std::string operator""_str(const char* sz, std::size_t len)
{
    return std::string(sz);
}

Where implementation is something like:

#define expr "expression"
auto str = expr _str;

Answer 1

Adjacent string literals are automatically concatenated ([lex.ext]/8), so

auto str = expr ""_str;

would work.

Answer 2

You need another macro that performs token pasting:

#define CONCAT2(A, B) A##B
#define CONCAT(A, B) CONCAT2(A, B)
auto str = CONCAT(expr, _str);

Demo