Reapplication of 7339c0f782d5c70e0928f8991b0c05338a90c84c with a fix
for a crash involving arrays without a size expression.
Clang supports VLAs in C++ as an extension, but we currently only warn
on their use when you pass -Wvla, -Wvla-extension, or -pedantic.
However, VLAs as they're expressed in C have been considered by WG21
and rejected, are easy to use accidentally to the surprise of users
(e.g., https://ddanilov.me/default-non-standard-features/), and they
have potential security implications beyond constant-size arrays
(https://wiki.sei.cmu.edu/confluence/display/c/ARR32-C.+Ensure+size+arguments+for+variable+length+arrays+are+in+a+valid+range).
C++ users should strongly consider using other functionality such as
std::vector instead.
This seems like sufficiently compelling evidence to warn users about
VLA use by default in C++ modes. This patch enables the -Wvla-extension
diagnostic group in C++ language modes by default, and adds the warning
group to -Wall in GNU++ language modes. The warning is still opt-in in
C language modes, where support for VLAs is somewhat less surprising to
users.
RFC: https://discourse.llvm.org/t/rfc-diagnosing-use-of-vlas-in-c/73109
Fixes https://github.com/llvm/llvm-project/issues/62836
Differential Revision: https://reviews.llvm.org/D156565
constant expressions.
We permit array-to-pointer decay on such arrays, but disallow pointer
arithmetic (since we do not know whether it will have defined behavior).
This is based on r311970 and r301822 (the former by me and the latter by Robert
Haberlach). Between then and now, two things have changed: we have committee
feedback indicating that this is indeed the right direction, and the code
broken by this change has been fixed.
This is necessary in C++17 to continue accepting certain forms of non-type
template argument involving arrays of unknown bound.
llvm-svn: 316245
handling of constexprs with unknown bounds.
This triggers a corner case of the language where it's not yet clear
whether this should be an error:
struct A {
static void *const a[];
static void *const b[];
};
constexpr void *A::a[] = {&b[0]};
constexpr void *A::b[] = {&a[0]};
When discovering the initializer for A::a, the bounds of A::b aren't known yet.
It is unclear whether warning about errors should be deferred until the end of
the translation unit, possibly resolving errors that can be resolved. In
practice, the compiler can know the bounds of all arrays in this example.
Credits for reproducers and explanation go to Richard Smith. Richard, please
add more info in case my explanation is wrong.
llvm-svn: 301963
Do not spuriously reject constexpr functions that access elements of an array
of unknown bound; this may later become valid once the bound is known. Permit
array-to-pointer decay on such arrays, but disallow pointer arithmetic (since
we do not know whether it will have defined behavior).
The standard is not clear on how this should work, but this seems to be a
decent answer.
Patch by Robert Haberlach!
llvm-svn: 301822
