[Clang] Invalid VLA is not rejected in constant evaluation

[Sirraide]

With -std=c++23, this compiles, even though it really shouldn’t (https://godbolt.org/z/xKjPEP34s):

consteval int foo(int n) {
    int x[n];
    return 12;
}

static_assert(foo(-1));

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Sirraide)

With `-std=c++23`, this compiles, even though it really shouldn’t (https://godbolt.org/z/xKjPEP34s): ```c++ consteval int foo(int n) { int x[n]; return 12; }

static_assert(foo(-1));

</details>

[zygoloid]

Generally it looks like VLA support during constant evaluation is pretty lacking. We also don't seem to support accessing VLA elements, and mostly treat them as arrays of unknown bound instead of as arrays of known bound (determined during evaluation). In addition to requiring the bound to be non-negative, we should probably also put an upper limit on it (maybe using the existing maximum size calculation in ConstantArrayType (getMaxSizeBits / getNumAddressingBits?)

[tbaederr]

In addition to requiring the bound to be non-negative, we should probably also put an upper limit on it (maybe using the existing maximum size calculation in ConstantArrayType (getMaxSizeBits / getNumAddressingBits?)

There's CheckArraySize in ExprConstant.cpp alreay, which should be used I guess. It only handles unsigned values at the moment though, so the negative case is special.