[clang-format] clang-format Fails on Lambda with Parameter Pack, Reference Capture and Constraint in .h Header

[kaigu1997]

Here is the minimal example:
.clang-format:

BasedOnStyle: Microsoft
---
Language: Cpp
UseTab: Always

test.h:

void f(void)
{
	int i = 3;
	auto impl = [&i]<int... I>()
	{
		(I, ...);
	};
}

when running clang-format --style=file -i test.h, Clang-Format would replace all the tab indentations with 4 spaces, even UseTab is Always; in fact, clang-format would neglect all given style option, except for BasedOnStyle (which I am not sure).

However, if change the extension to .cpp or .hpp, Clang-Format would work fine.
Without reference capture (say, with value capture only), this would also work fine.
Type parameter (typename... T) works fine too.

[llvmbot]

@llvm/issue-subscribers-clang-format

[EugeneZelenko]

Could you please try 16 or main branch?

[kaigu1997]

I tried 16.0.0rc3, and it seems to be fixed.
It inlines the lambda, but keeps the indents as tab.

Similar to #59522 and #57093, and fixed by 2476135?

So here is another demo of test.h that Clang-Format will fail again:

#include <type_traits>
#include <utility>

void f(void)
{
	int i = 3;
	auto impl = [&i]<std::size_t... I>(std::index_sequence<I...>)
		requires std::is_same_v<std::index_sequence<I...>, std::make_index_sequence<3>>
	{
		(I, ...);
	};
}

with the requires constraint, Clang-Format could not work properly. Removing the constraint shows good result.
With type template parameter pack, the issue arises as well:

#include <type_traits>
#include <utility>

void f(void)
{
	int i = 3;
	auto impl = [&i]<typename... T>(std::tuple<T...> t)
		requires (std::is_same_v<T, int> && ...)
	{
		std::get<0>(t);
	};
}

Clang-Format fails for this test.h as well.

Again, removing reference capture, or change the extension to .hpp or .cpp could fix the issue.

[rymiel]

The issue here is that because of the ambiguous .h extension, clang-format makes a guess on what the language is and somehow lands on... Objective-C

One possible workaround is to just not make the config being used language-specific, such as the following:

BasedOnStyle: Microsoft
UseTab: Always