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

[Chuanqi Xu 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.
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ChuanqiXu9 wrote:

For 1, I don't understand why it is relevant here. I know what you're saying, but I just feel it is not related here. We're talking about ODR checker, right?

For 2, I thought the term `false positive` implies 2. But maybe it is not impressive. So I tried to rewrite this paragraph to make it more explicit.

For MSVC, it is a simple supporting argument here. I mean, the document is for users, and what I want to say or express is, while all of us know and understand it is not good, but you (the users) don't need to be too panic since MSVC did the same thing.

BTW, for the delayed template parsing, I remember one of the supporting reason to disable that in C++20 is that MSVC disable that in C++20 too. But this is not relevant here.

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