misc-redundant-expression reports results in decltype expression

[LegalizeAdulthood]

Bugzilla Link	36509
Version	unspecified
OS	All
CC	@BatmanAoD,@EugeneZelenko,@Xazax-hun

Extended Description

The misc-redundant-expression check reports a warning for the following use of decltype:

int x;
int y;
decltype(x + y - (x + y)) z = 10;

It's unclear whether or not misc-redundant-expression should return results for expressions inside a decltype specifier.

[BatmanAoD]

I signed up to report this as well before seeing it was an existing issue.

Here's a use case from real code:

using sysclock_t = decltype(current_time());
using duration_t = decltype(std::declval<sysclock_t>() - std::declval<sysclock_t>());

The idea is simply to ensure that my time-point type alias is using the same underlying values and units as my duration type alias, and that both are using the same precision as current_time().

[llvmbot]

@llvm/issue-subscribers-bug

[LegalizeAdulthood]

Still an issue in clang-tidy 14

[PiotrZSL]

https://reviews.llvm.org/D157374