[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

1 files changed, 1 insertions, 5 deletions

diff --git a/clang/test/SemaCXX/constexpr-printing.cpp b/clang/test/SemaCXX/constexpr-printing.cpp
index 7f6a9c6a82f..ebd91b8a3ca 100644
--- a/clang/test/SemaCXX/constexpr-printing.cpp
+++ b/clang/test/SemaCXX/constexpr-printing.cpp
@@ -41,11 +41,7 @@ expected-error {{constant expression}} \
 expected-note {{in call to 'test_printing(12, 3.976200e+01, 3+4i, 1.290000e+01+3.600000e+00i, &u2.T::arr[4], u2.another.arr[2], {5, 1, 2, 3}, {{{}}, {{}}, &u1.T::arr[2]})'}}
 
 struct V {
-  // FIXME: when we can generate these as constexpr constructors, remove the
-  // explicit definitions.
-  constexpr V() : arr{[255] = 42} {}
-  constexpr V(const V &v) : arr{[255] = 42} {}
-  int arr[256];
+  int arr[256] = {[255] = 42}; // expected-warning {{C99}}
 };
 constexpr V v;
 constexpr int get(const int *p) { return *p; } // expected-note {{read of dereferenced one-past-the-end pointer}}