[PATCH] D149550: [clang][Interp] Fix compound assign operator evaluation order

[Timm Bäder via Phabricator via cfe-commits]

tbaeder added inline comments.


================
Comment at: clang/lib/AST/Interp/ByteCodeExprGen.cpp:683-685
+  // C++17 onwards require that we evaluate the RHS first.
+  // Compute RHS and save it in a temporary variable so we can
+  // load it again later.
----------------
tbaeder wrote:
> aaron.ballman wrote:
> > In C, the evaluation of the operands are unsequenced. C doesn't currently have constexpr functions, only constexpr objects, but that eliminates mutating operations like compound assignment... for the moment. Perhaps a FIXME comment for figuring out how to handle C?
> > 
> > (The situation I'm worried about in C is with UB dealing with unsequenced operations, like rejecting: https://godbolt.org/z/W11jchrKc)
> Could C make them sequenced when introducing constexpr functions? :)
Since we already emit a warning for this, we could in the future just check if the statement is in a constexpr function and emit an error instead? We're emitting the warning for c++ pre-17 as well but we don't make it an error, I guess because it's not UB there?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D149550/new/

https://reviews.llvm.org/D149550