Spurious -Wpre-c++2b-compat warning with -std=c++2b -pedantic

[timsong-cpp]

struct A {
    void operator[](int, int);
};

with -std=c++2b -pedantic (https://gcc.godbolt.org/z/qnjd5YhPa):

<source>:2:10: warning: overloaded 'operator[]' with more than one parameter is a C++2b extension [-Wpre-c++2b-compat]
    void operator[](int, int);
         ^
1 warning generated.

I would not have expected -pedantic to warn me about constructs that are ill-formed in a previous standard.

[llvmbot]

@llvm/issue-subscribers-c-2b

[timsong-cpp]

It looks like static lambdas have a similar problem (https://gcc.godbolt.org/z/rWKdWvsv5):

auto x = [] static {};

<source>:1:13: warning: static lambdas are incompatible with C++ standards before C++2b [-Wpre-c++2b-compat]
auto x = [] static {};
            ^

[AaronBallman]

I looked through the rest of the C++2b precompat warnings to see if any more were also mistaken, and it seems like that's the last one. I'll get it fixed up.