[clang] C++98 implicit moves are back with a vengeance

Authored by mizvekov on Jul 9 2021, 5:34 PM.


[clang] C++98 implicit moves are back with a vengeance

After taking C++98 implicit moves out in D104500,
we put it back in, but now in a new form which preserves
compatibility with pure C++98 programs, while at the same time
giving almost all the goodies from P1825.

  • We use the exact same rules as C++20 with regards to which id-expressions are move eligible. The previous incarnation would only benefit from the proper subset which is copy ellidable. This means we can implicit move, in addition:
    • Parameters.
    • RValue references.
    • Exception variables.
    • Variables with higher-than-natural required alignment.
    • Objects with different type from the function return type.
  • We preserve the two-overload resolution, with one small tweak to the first one: If we either pick a (possibly converting) constructor which does not take an rvalue reference, or a user conversion operator which is not ref-qualified, we abort into the second overload resolution.

This gives C++98 almost all the implicit move patterns which we had created test
cases for, while at the same time preserving the meaning of these
three patterns, which are found in pure C++98 programs:

  • Classes with both const and non-const copy constructors, but no move constructors, continue to have their non-const copy constructor selected.
  • We continue to reject as ambiguous the following pattern:
struct A { A(B &); };
struct B { operator A(); };
A foo(B x) { return x; }
  • We continue to pick the copy constructor in the following pattern:
class AutoPtrRef { };
struct AutoPtr {
  AutoPtr(AutoPtr &);

  operator AutoPtrRef();
AutoPtr test_auto_ptr() {
  AutoPtr p;
  return p;

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: Quuxplusone

Differential Revision: https://reviews.llvm.org/D105756


mizvekovJul 13 2021, 10:16 AM
Differential Revision
D105756: [clang] C++98 implicit moves are back with a vengeance
rG14f77576c9c4: [llvm-mca] [NFC] Formatting code