How to get away with using designated initializers in C++17? Or, why is it seemingly safe to use them, if it's really not?

Question

C++20 introduced support for designated initializers.

In g++ with -std=c++17, one can use designated initializers and as long as you don't leave any out, it will compile without any errors or warnings:

struct Foo {
    int a;
    float b;
};

Foo f {
    .a = 7,
    .b = 42.1f,
};

Yet if I enable -Wpedantic (and -Werror) the C++17 compiler will spit out:

error: C++ designated initializers only available with '-std=c++20' or '-std=gnu++20' [-Werror=pedantic]

But there does not seem to be a way to disable or suppress this error without disabling -Wpedantic too, which is far too coarse in my case.

I find this very confounding because there's no warning emitted when using this feature with C++17 - code using it will compile and run, seemingly, with un-undefined behaviour, yet from everything I've found online it's not supposed to be used with C++17.

So why isn't there a warning or error when using this unsupported feature with C++17 without -Wpedantic? Surely it's not pedantic to warn the user of a non-supported language feature if it's technically UB? And if it's not UB, then it works, right?

Lastly, without recompiling g++, how can I trick the compiler into accepting -Wpedantic without generating such warnings if I choose to use designated initializers in my C++17 code?

Synergistic

Your understanding is backward. C++17 does not support designated initialisers at all but g++ (and other compilers) included experimental/provisional support for features before their eventual acceptance in the standardisation process. Due to historical lobbying by developers (from a time when their PAY was, say, tied to metrics like "compile without warnings") modern compilers do NOT diagnose non-standard features by default, and using -pedantic DIRECTS the compiler to diagnose them. Your options are (1) compile as C++20 (2) don't use -pedantic or (3) don't use designated initialisers – Inexplicable

The 'pedantic' warning and error are not there to tell you that your code isn't safe. They are there to tell you that your code is not fully ANSI compliant. – Basinger

@Caleth My comment didn't actually address the question as asked. It explained why the wrong question was being asked. Such things posted as answers are (at best) not particularly popular here and (at worst) attract down-votes that negate the effort of posting as an answer. – Inexplicable

Answer 1

I'll answer it myself based on the helpful comment from Peter:

"[My] understanding is backward. C++17 does not support designated initialisers at all but g++ (and other compilers) included experimental/provisional support for features before their eventual acceptance in the standardisation process. Due to historical lobbying by developers (from a time when their PAY was, say, tied to metrics like "compile without warnings") modern compilers do NOT diagnose non-standard features by default, and using -pedantic DIRECTS the compiler to diagnose them. Your options are (1) compile as C++20 (2) don't use -pedantic or (3) don't use designated initialisers."

There, a solid answer from Peter that explains my incorrect assumptions and provides suggestions for a course of action.

Synergistic

With the latest gcc version, you can turn c++20 extension-related warnings selectively with -Wno-c++20-extensions and related pragma directive. – Readiness