https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119180
Bug ID: 119180
Summary: GCC Accepts Non-Standard Variable-Length Arrays (VLAs)
in C++ Without Warnings
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: qurong at ios dot ac.cn
Target Milestone: ---
GCC compiles code containing variable-length arrays (VLAs) and lambda captures
of VLAs without diagnostics, while Clang/MSVC correctly reject it. This
violates C++ standard rules (C++11 §8.3.4) and may mislead developers into
writing non-portable code.
**Steps to Reproduce**:
1. Save the following code as `test.cpp`:
```cpp
template <typename T>
struct Container {
void f(int i) {
int ar[i]; // Non-standard VLA
[&ar] { (void)ar; }(); // Lambda capture of VLA
}
};
int main() {
Container<int> obj;
obj.f(5);
return 0;
}
2. Compile with GCC:
g++ -std=c++11 test.cpp
Expected Result:
Compilation fails with errors about VLAs being forbidden in ISO C++, or at
least issues a warning when extensions are enabled.
Actual Result:
GCC compiles the code silently (or with -pedantic warns but still succeeds).
Environment:
Compiler: GCC 13.2.0
Flags: -std=c++11
Proposal:
Enhance Warnings:
Emit a warning by default for VLAs in C++ mode (similar to -Wvla in C).
Reject lambda captures of VLAs explicitly.
Documentation Update:
Clarify VLA support as a GNU extension in C++ mode.
Additional Notes:
Standard References:
C++11 §8.3.4: Array dimensions must be constant expressions.
C++11 §5.1.2: Lambda captures require complete types.
Impact: Non-portable code may fail on other compilers.
Compiler Explorer link: https://godbolt.org/z/YdEn56Gq1
