[llvm-bugs] [Bug 50112] New: MemorySanitizer: use-of-uninitialized-value in std::string s = "x"; std::replace(s.begin(), s.end(), 'a', 'b'); on -O0

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50112

            Bug ID: 50112
           Summary: MemorySanitizer: use-of-uninitialized-value in
                    std::string s = "x"; std::replace(s.begin(), s.end(),
                    'a', 'b'); on -O0
           Product: libc++
           Version: 10.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: nok.raven at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

#include <string>

int main()
{
    std::string s = "x";
    std::replace(s.begin(), s.end(), 'a', 'b');
}

clang++-11 -fsanitize=memory -stdlib=libc++ -g z.cpp -o z && ./z

==62635==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x49a95b in std::__1::basic_string<char, std::__1::char_traits<char>,
std::__1::allocator<char> >::__get_pointer()
/usr/lib/llvm-11/bin/../include/c++/v1/string:1514:17
    #1 0x49a461 in std::__1::basic_string<char, std::__1::char_traits<char>,
std::__1::allocator<char> >::begin()
/usr/lib/llvm-11/bin/../include/c++/v1/string:909:26
    #2 0x499d79 in main /home/user/z.cpp:6:20
    #3 0x7fcd8bd35cb1 in __libc_start_main csu/../csu/libc-start.c:314:16
    #4 0x41d26d in _start (/home/user/z+0x41d26d)

SUMMARY: MemorySanitizer: use-of-uninitialized-value
/usr/lib/llvm-11/bin/../include/c++/v1/string:1514:17 in
std::__1::basic_string<char, std::__1::char_traits<char>,
std::__1::allocator<char> >::__get_pointer()
Exiting

Clang/libc++ 10+ are affected

https://godbolt.org/z/6E9aeWjjP

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