Otherwise we get invalid results for ODR checks. See changed test for an
example: despite the fact that we merge the first concept, its **uses**
were considered different by `Profile`, leading to redefinition errors.
After this change, canonical decl for a concept can come from a
different module and may not be visible. This behavior looks suspicious,
but does not break any tests. We might want to add a mechanism to make
the canonical concept declaration visible if we find code that relies on
this invariant.
Additionally make sure we always merge with the canonical declaration to
avoid chains of merged concepts being reported as redefinitions. An
example was added to the test.
Also change the order of includes in the test. Importing a moduralized
header before its textual part causes the include guard macro to be
exported and the corresponding `#include` becomes a no-op.
Reviewed By: ChuanqiXu
Differential Revision: https://reviews.llvm.org/D130585
Currently the C++20 concepts are only merged in `ASTReader`, i.e. when
coming from different TU. This can causes ambiguious reference errors when
trying to access the same concept that should otherwise be merged.
Please see the added test for an example.
Note that we currently use `ASTContext::isSameEntity` to check for ODR
violations. However, it will not check that concept requirements match.
The same issue holds for mering concepts from different TUs, I added a
FIXME and filed a GH issue to track this:
https://github.com/llvm/llvm-project/issues/56310
Reviewed By: ChuanqiXu
Differential Revision: https://reviews.llvm.org/D128921
Ilya Biryukov