[c++20] Implement semantic restrictions for C++20 designated initializers.

Authored by Richard Smith <richard-llvm@metafoo.co.uk> on Aug 30 2019, 3:52 PM.


[c++20] Implement semantic restrictions for C++20 designated initializers.

This has some interesting interactions with our existing extensions to
support C99 designated initializers as an extension in C++. Those are
resolved as follows:

  • We continue to permit the full breadth of C99 designated initializers in C++, with the exception that we disallow a partial overwrite of an initializer with a non-trivially-destructible type. (Full overwrite is OK, because we won't run the first initializer at all.)
  • The C99 extensions are disallowed in SFINAE contexts and during overload resolution, where they could change the meaning of valid programs.
  • C++20 disallows reordering of initializers. We only check for that for the simple cases that the C++20 rules permit (designators of the form '.field_name =' and continue to allow reordering in other cases). It would be nice to improve this behavior in future.
  • All C99 designated initializer extensions produce a warning by default in C++20 mode. People are going to learn the C++ rules based on what Clang diagnoses, so it's important we diagnose these properly by default.
  • In C++ <= 17, we apply the C++20 rules rather than the C99 rules, and so still diagnose C99 extensions as described above. We continue to accept designated C++20-compatible initializers in C++ <= 17 silently by default (but naturally still reject under -pedantic-errors).

This is not a complete implementation of P0329R4. In particular, that
paper introduces new non-C99-compatible syntax { .field { init } }, and
we do not support that yet.

This is based on a previous patch by Don Hinton, though I've made
substantial changes when addressing the above interactions.

Differential Revision: https://reviews.llvm.org/D59754

llvm-svn: 370544

Event Timeline

Richard Smith <richard-llvm@metafoo.co.uk> committed rG5030928d60a1: [c++20] Implement semantic restrictions for C++20 designated initializers. (authored by Richard Smith <richard-llvm@metafoo.co.uk>).Aug 30 2019, 3:52 PM