**Background**: `__clear_and_shrink()` (added in https://reviews.llvm.org/D41976) is used in the copy assignment of allocators. It was added as an optimization to prevent calling `__shrink_to_fit()`, which makes a new allocation when we have just called `clear()` on the string as it makes an additional allocation. Instead, it deallocates the allocated string and then sets the short string size to 0, making the active member of the union in `__rep`, __s I`__s` I.e. a short string. It changes the representation to a short string to satisfy the invariant `data()[size()] != value_type()` and to prevent deallocation of the heap allocated long string a second time in the string destructor.
**Issue**: The problem with this function is that during constant evaluation, short string optimisation is not used so any read operations on __rep will try to access __l I.e. the long string. This will fail as it is illegal to change the active member of a union during constant evaluation.
**Issue**: Since C++20, it is legal to change the active member of a union within a constant expression. However, it is not legal to read from a non-active member of a union. During constant evaluation, short string optimisation is not used so the string is always a long string. After calling `__clear_and_shrink`, the active member is changed to a short string. Any operations which attempt to read from `__rep` will then cause a compiler error. This will happen at the latest when the string is destructed.
**Reproducing**: The following code demonstrates the issue:
```
#include <cassert>
#include <string>
constexpr bool test()
{
std::string l = "Long string so that allocation definitely, for sure, absolutely happens. Probably.";
l.__clear_and_shrink();
return true;
}
int main(int, char**)
{
static_assert(test());
return 0;
}
```
This code fails with the error:
```
../src/clear_and_shrink_test.cpp:14:19: error: static assertion expression is not an integral constant expression
static_assert(test());
^~~~~~
/home/brendanemery/work_esr/llvm-project/build/include/c++/v1/string:1618:17: note: read of member '__l' of union with active member '__s' is not allowed in a constant expression
{return __r_.first().__l.__data_;}
^
/home/brendanemery/work_esr/llvm-project/build/include/c++/v1/string:2347:47: note: in call to '&l->__get_long_pointer()'
__alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
^
../src/clear_and_shrink_test.cpp:6:17: note: in call to '&l->~basic_string()'
std::string l = "Long string so that allocation definitely, for sure, absolutely happens. Probably.";
^
../src/clear_and_shrink_test.cpp:14:19: note: in call to 'test()'
static_assert(test());
```