[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, 2 insertions, 2 deletions

diff --git a/clang/test/SemaTemplate/instantiate-init.cpp b/clang/test/SemaTemplate/instantiate-init.cpp
index b58ad3a1576..99b29c77d55 100644
--- a/clang/test/SemaTemplate/instantiate-init.cpp
+++ b/clang/test/SemaTemplate/instantiate-init.cpp
@@ -151,8 +151,8 @@ namespace InitListUpdate {
 
   // Check that an init list update doesn't "lose" the pack-ness of an expression.
   template <int... N> void f() {
-    g(AA{0, [0].n = N} ...); // expected-warning 3{{overrides prior init}} expected-note 3{{previous init}}
-    g(AA{N, [0].n = 0} ...); // expected-warning 3{{overrides prior init}} expected-note 3{{previous init}}
+    g(AA{0, [0].n = N} ...); // expected-warning 3{{extension}} expected-note {{here}} expected-warning 3{{overrides prior init}} expected-note 3{{previous init}}
+    g(AA{N, [0].n = 0} ...); // expected-warning 3{{extension}} expected-note {{here}} expected-warning 3{{overrides prior init}} expected-note 3{{previous init}}
   };
 
   void g(AA, AA);