gcc(g++) 12.1にて、inline関数に対して暗黙に constexpr 指定を行うコンパイルオプション -fimplicit-constexpr が導入された。同オプションはC++14/17/20言語仕様に対する独自拡張として機能する。

With each successive C++ standard the restrictions on the use of the constexpr keyword for functions get weaker and weaker; it recently occurred to me that it is heading toward the same fate as the C register keyword, which was once useful for optimization but became obsolete. Similarly, it seems to me that we should be able to just treat inlines as constexpr functions and not make people add the extra keyword everywhere.