[Clang] Allow auto() as an extension in all language modes

[philnik777]

Currently clang allows auto() casting only in C++23 and later. I don't think there is any reason to not support this as an extension all the way back to C++03, especially given that GCC does so already. This would allow us to replace _LIBCPP_AUTO_CAST with the appropriate language facility in a lot of places.

CC @AaronBallman

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Nikolas Klauser (philnik777)

Currently clang allows `auto()` casting only in C++23 and later. I don't think there is any reason to not support this as an extension all the way back to C++03, especially given that GCC does so already. This would allow us to replace `_LIBCPP_AUTO_CAST` with the appropriate language facility in a lot of places.

CC @AaronBallman

[shafik]

CC @cor3ntin I think this is area that needs to be updated right:

llvm-project/clang/lib/Parse/ParseTentative.cpp

Lines 1457 to 1464 in 9d85ba5

	case tok::kw_auto: {
	if (!getLangOpts().CPlusPlus23)
	return TPResult::True;
	if (NextToken().is(tok::l_brace))
	return TPResult::False;
	if (NextToken().is(tok::l_paren))
	return TPResult::Ambiguous;
	return TPResult::True;

good first issue?

[cor3ntin]

there are a bunch of places to update, this is only the tentative parse. it should still be relatively easy to do.

a few places to update in parsing + tests + feature test macro + languageextensions.rst.