[clang] [C++20] [Modules] Introduce -fskip-odr-check-in-gmf (PR #79959)

[Matheus Izvekov via cfe-commits]

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@@ -457,6 +457,28 @@ Note that **currently** the compiler doesn't consider inconsistent macro definit
 Currently Clang would accept the above example. But it may produce surprising results if the
 debugging code depends on consistent use of ``NDEBUG`` also in other translation units.
 
+Definitions consistency
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The C++ language defines that same declarations in different translation units should have
+the same definition, as known as ODR (One Definition Rule). Prior to modules, the translation
+units don't dependent on each other and the compiler itself don't and can't perform a strong
+ODR violation check. Sometimes it is the linker does some jobs related to ODR, where the
+higher level semantics are missing. With the introduction of modules, now the compiler have
+the chance to perform ODR violations with language semantics across translation units.
+
+However, in the practice we found the existing ODR checking mechanism may be too aggressive.
+In the many issue reports about ODR violation diagnostics, most of them are false positive
+ODR violations and the true positive ODR violations are rarely reported. Also MSVC don't
+perform ODR check for declarations in the global module fragment.
+
+So in order to get better user experience, save the time checking ODR and keep consistent
+behavior with MSVC, we disabled the ODR check for the declarations in the global module
+fragment by default. Users who want more strict check can still use the
+``-Xclang -fno-skip-odr-check-in-gmf`` flag to get the ODR check enabled. It is also
+encouraged to report issues if users find false positive ODR violations or false negative ODR
+violations with the flag enabled.
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mizvekov wrote:

Again I find this paragraph concerning 3). This jusitifies in terms of MSVC conformance, but doesn't limit the behavior to that environment. Also I don't think we offer compiler switches that decrease standards conformance in order to save build time.

https://github.com/llvm/llvm-project/pull/79959