`constexpr` variable “used in its own initializer”: Clang vs. GCC

Question

This question seems related to an existing one, but I do not understand the \"portable workaround\" provided in the answer there (involving const auto this_ = this;

Answer 1

GCC is correct to reject the code (however the error message could use some work). You cannot use the address of a variable in a constant expression unless that variable has static storage duration.

foo is not static. If you move it outside of main, things will work. Demo

The line marked below is the problem:

constexpr Test(const Test& src) noexcept
: src_{&src} <--- That

Standard reference: (Emphasis mine)

[expr.const]

A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression (as defined below), or a prvalue core constant expression whose value satisfies the following constraints:

(5.2) — if the value is of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object (8.7), the address of a function, or a null pointer value,