38b948bd4b
Tldr; We can't unconditionally trivially copy empty classes because that would clobber the stored entries in the object that the optimized empty class overlaps with. This regression was introduced by #115918, which introduced other clobbering issues, like the handling of `[[no_unique_address]]` fields in #137252. Read issue #157467 for the detailed explanation, but in short, I'd propose reverting the original patch because these was a lot of problems with it for arguably not much gain. In particular, that patch was motivated by unifying the handling of classes so that copy events would be triggered for a class no matter if it had data members or not. So in hindsight, it was not worth it. I plan to backport this to clang-21 as well, and mention in the release notes that this should fix the regression from clang-20. PS: Also an interesting read [D43714](https://reviews.llvm.org/D43714) in hindsight. Fixes #157467 CPP-6574
40 lines
1.2 KiB
C++
40 lines
1.2 KiB
C++
// RUN: %clang_analyze_cc1 -analyzer-checker=core -verify %s
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// expected-no-diagnostics
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template <class T, int Idx, bool CanBeEmptyBase = __is_empty(T) && (!__is_final(T))>
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struct compressed_pair_elem {
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explicit compressed_pair_elem(T u) : value(u) {}
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T value;
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};
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template <class T, int Idx>
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struct compressed_pair_elem<T, Idx, /*CanBeEmptyBase=*/true> : T {
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explicit compressed_pair_elem(T u) : T(u) {}
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};
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template <class T1, class T2, class Base1 = compressed_pair_elem<T1, 0>, class Base2 = compressed_pair_elem<T2, 1>>
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struct compressed_pair : Base1, Base2 {
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explicit compressed_pair(T1 t1, T2 t2) : Base1(t1), Base2(t2) {}
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};
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// empty deleter object
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template <class T>
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struct default_delete {
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void operator()(T* p) {
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delete p;
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}
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};
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template <class T, class Deleter = default_delete<T> >
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struct some_unique_ptr {
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// compressed_pair will employ the empty base class optimization, thus overlapping
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// the `int*` and the empty `Deleter` object, clobbering the pointer.
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compressed_pair<int*, Deleter> ptr;
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some_unique_ptr(int* p, Deleter d) : ptr(p, d) {}
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~some_unique_ptr();
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};
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void entry_point() {
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some_unique_ptr<int, default_delete<int> > u3(new int(12), default_delete<int>());
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}
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