[PATCH] Warning on unevaluated expression contexts with side effects

[Richard Trieu]

After running some code through a Clang with this patch, a few issues came
up:

1) This warning should likely be disabled in macros.  Some macros will
perform a sizeof on the macro argument which will trigger this warning even
if argument is used elsewhere in the macro expansion.

2) This warning is counting a dereference of a volatile pointer as a side
effect, for instance:
  int* volatile x;
  return sizeof(*x);

3) This hits an llvm_unreachable:

template <typename T> void F(T t) {}
template <typename T> void G(T t) { decltype(&F<T>) x; }
template <typename T> class V {};
void foo() { G(V<int>()); }

shouldn't see dependent / unresolved nodes here
UNREACHABLE executed at ../tools/clang/lib/AST/Expr.cpp:2891!


On Tue, Nov 25, 2014 at 6:51 AM, Aaron Ballman <aaron at aaronballman.com>
wrote:

> The attached patch adds a -Wunused-value warning when an expression
> with side effects is used in an unevaluated expression context, such
> as sizeof(), or decltype(). It mostly reuses the logic from
> Expr::HasSideEffects, except with a flag that treats certain
> expressions as not having side effects -- mostly
> instantiation-dependent contexts, and function call-like operations.
>
> int f();
> int j = 0;
>
> (void)sizeof(f()); // Ok
> (void)sizeof(++j); // Warn
> (void)sizeof(int[++j]); // Ok
>
> I've added support for: sizeof, typeid, _Generic, _Alignof, noexcept,
> and decltype.
>
> ~Aaron
>
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