Diagnostic enhancement: "auto" in lambda expression under -std=c++11

[haoxintu]

Bugzilla Link	46714
Version	trunk
OS	Linux
CC	@dwblaikie,@zygoloid

Extended Description

Hi, all.

"auto" used in lambda expression is a c++14 extension, but clang doesn't emit the appropriate diagnostic message about it.

For example,

$cat test.cc
int main(){
    auto var = [](auto){};
    return 0;
}

$clang++ -std=c++11 -c test.cc
test.cc:2:19: error: 'auto' not allowed in lambda parameter
    auto var = [](auto){};
                  ^~~~
1 error generated.

I guess clang should give a more specific diagnostic about this like GCC.

$g++ -std=c++11 -c test.cc
test.cc: In function ‘int main()’:
test.cc:2:19: error: use of ‘auto’ in lambda parameter declaration only available with ‘-std=c++14’ or ‘-std=gnu++14’
    2 |     auto var = [](auto){};
      |                   ^~~~

Just a small enhancement I guess, please understand if anything I stated is unsuitable.

Thanks,
Haoxin

[llvmbot]

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[cor3ntin]

Looks like a reasonable ask https://godbolt.org/z/d7aT6arnx

[Endilll]

@cor3ntin can you point out where this diagnostic is defined in sources? I haven't been able to find it.

[cor3ntin]

The code which determines there is an error is here
https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaType.cpp#L3584-L3594

The error is produced a bit below
https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaType.cpp#L3737

A solution might be to keep track of when the errors are conditioned on language versions, and modify the existing diagnostic to add "before C++20"

[Endilll]

llvm-project/clang/include/clang/Basic/DiagnosticSemaKinds.td

Lines 2345 to 2360 in fa2b038

	def err_auto_not_allowed : Error<
	"%select{'auto'|'decltype(auto)'|'__auto_type'|"
	"use of "
	"%select{class template|function template|variable template|alias template|"
	"template template parameter|concept|template}2 %3 requires template "
	"arguments; argument deduction}0 not allowed "
	"%select{in function prototype"
	"|in non-static struct member|in struct member"
	"|in non-static union member|in union member"
	"|in non-static class member|in interface member"
	"|in exception declaration|in template parameter until C++17|in block literal"
	"|in template argument|in typedef|in type alias|in function return type"
	"|in conversion function type|here|in lambda parameter"
	"|in type allocated by 'new'|in K&R-style function parameter"
	"|in template parameter|in friend declaration|in function prototype that is "
	"not a function declaration|in requires expression parameter}1">;

I've tried to get through this even before writing my previous comment, but I wasn't able to recognize the message, and find it hard still.

CC @AaronBallman