[cfe-dev] -Wunused-variable for user-defined types

Eugene Toder eltoder at gmail.com
Tue Aug 26 16:18:48 PDT 2014

Hi all,

I find clang's unused variable warning very useful to keep the code from
accumulating garbage over time. For built-in types it works great, but I've
noticed that for user-defined types it often misses unused variables. The
current logic for user-defined types is to warn on unused variable if 1)
the type has a trivial destructor 2) the constructor is either elided or
trivial, or the type is marked with warn_unused attribute [1].
Does anyone object to extending 2) to allow constexpr constructors? The
reasoning is that constexpr constructors are not supposed to have any
What about a check that constructor does not have side-effects, even if
it's not marked as constexpr? This can start by using the same code as the
one for constexpr checking. Is this the right thing in the compiler, or
should this go into clang-tidy?

[1] http://clang.llvm.org/doxygen/SemaDecl_8cpp_source.html#l01426

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