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bad example in P0593R6 [diff.cpp17] #3742

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zygoloid opened this issue Feb 19, 2020 · 0 comments
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bad example in P0593R6 [diff.cpp17] #3742

zygoloid opened this issue Feb 19, 2020 · 0 comments
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P0593R6 adds this example to Annex C:

constexpr int f() {
  int a = 123;
  using T = int;
  a.~T();
  return a;  // undefined behavior; previously returned 123
}
static_assert(f() == 123);  // ill-formed; previously valid

... but the comments here are wrong: in C++17, pseudo-destruction expressions weren't permitted in constant evaluation at all. This can be fixed by using a runtime example instead of a compile-time one, such as:

int main() {
  int a = 123;
  using T = int;
  a.~T();
  return a;  // undefined behavior; previously returned 123
}
@burblebee burblebee mentioned this issue Feb 19, 2020
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@zygoloid zygoloid self-assigned this Feb 20, 2020
zygoloid added a commit that referenced this issue Feb 20, 2020
@zygoloid
[diff.cpp17.basic] Fix example added by P0593R6.
3e877f3 
Pseudo-destructors weren't permitted in constant expressions before
C++20.

Fixes #3742.
zygoloid added a commit that referenced this issue Feb 25, 2020
@zygoloid
[diff.cpp17.basic] Fix example added by P0593R6.
e8cda79 
Pseudo-destructors weren't permitted in constant expressions before
C++20.

Fixes #3742.
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