Bogus warning: 'memset' will always overflow; destination buffer has size 1, but size argument is 2 [-Wfortify-source]

[stbergmann]

On recent LLVM trunk (towards LLVM 20), bisecting shows that 0a9c08c "[Clang] Implement P2280R4 Using unknown pointers and references in constant expressions (#95474)" started to cause

$ cat test.cc
#include <cstring>
struct S { char m1, m2; };
void f(S & x) { std::memset(&x, 0, sizeof (S)); }

$ clang++ -std=c++23 -fsyntax-only test.cc
test.cc:3:17: warning: 'memset' will always overflow; destination buffer has size 1, but size argument is 2 [-Wfortify-source]
    3 | void f(S & x) { std::memset(&x, 0, sizeof (S)); }
      |                 ^
1 warning generated.

[stbergmann]

ah, this has apparently already been fixed with c9637af "[Clang] Fix createConstexprUnknownAPValues to use zero offset when ceating APValue (#124478)"

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Stephan Bergmann (stbergmann)

On recent LLVM trunk (towards LLVM 20), bisecting shows that <https://github.com/llvm/llvm-project/commit/0a9c08c59ba61e727e9dee6d71883d9106963442> "[Clang] Implement P2280R4 Using unknown pointers and references in constant expressions (https://github.com//pull/95474)" started to cause ``` $ cat test.cc #include <cstring> struct S { char m1, m2; }; void f(S & x) { std::memset(&x, 0, sizeof (S)); } ``` ``` $ clang++ -std=c++23 -fsyntax-only test.cc test.cc:3:17: warning: 'memset' will always overflow; destination buffer has size 1, but size argument is 2 [-Wfortify-source] 3 | void f(S & x) { std::memset(&x, 0, sizeof (S)); } | ^ 1 warning generated. ```

[shafik]

Thank you for the bug report, I was pondering filing one just for tracking purposes :-)