ilya-biryukov added a comment. It seems wrong to change semantics of initialization when instantiating concept requirements. It implies that semantic checking may behave differently inside requires expressions, this is a red flag. Clang has a mechanism <https://github.com/llvm/llvm-project/blob/84d07d021333f7b5716f0444d5c09105557272e0/clang/lib/Sema/SemaOverload.cpp#L7429> to prevent recursion when considering user-defined conversion for copy initialization.
Currently the intention of the Clang code path that handles this case is to: 1. Deduce `TO` to be `Deferred` 2. Try to check `Numeric<Deferred>`, 3. Check conversions during overload resolution for `foo(a)`. Go to step 1, <infinite recursion, so we never get to the next step> 4. <never reached> Check <https://github.com/llvm/llvm-project/blob/84d07d021333f7b5716f0444d5c09105557272e0/clang/lib/Sema/SemaOverload.cpp#L7429> that conversion operator converts the type to itself and mark the candidate as failed. If move the check in step 4 to happen before step 3 (even if we need to duplicate the check), we will get the desired semantics. Does that look reasonable? I also agree there is a more general issue with recursive concept instantiations at play here, e.g. for code like template <class T> concept A = requires (T a) { foo(a); }; template <class T> concept B = requires (T a) { bar(a); }; struct X {}; template <A T> void bar(T); template <B T> void foo(T); void test() { foo(X()); } Clang will currently cut this out because the template instantiation depth is too high, whereas GCC will provide a useful diagnostics that says concept satisfaction computation is recursive. We probably want a more informative error message too. Probably worth investigating separately from that particular change. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D133052/new/ https://reviews.llvm.org/D133052 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
