[clang] [clang][Modules] Complete the implementation of P2615: Meaningful exports (PR #194201)

[Victor Chernyakin via cfe-commits]

================
@@ -3,9 +3,135 @@
 
 
 // RUN: %clang_cc1 -std=c++20 -verify -fsyntax-only %t/A.cpp
+// RUN: %clang_cc1 -std=c++20 -verify -fsyntax-only %t/B.cpp
+// RUN: not %clang_cc1 -std=c++20 -fsyntax-only -fdiagnostics-parseable-fixits %t/B.cpp 2>&1 | FileCheck %t/B.cpp
+// RUN: %clang_cc1 -std=c++20 -verify -fsyntax-only %t/msvc-stl-exception-1.cpp
----------------
localspook wrote:

That's factually true, but I don't see how it leads to the conclusion that we should check `-fms-extensions` in addition to `_MSVC_STL_UPDATE`. Checking for `-fms-extensions` is strictly more complexity, so something must justify adding that complexity. But I don't see what justifies it. Does checking `_MSVC_STL_UPDATE` but not `-fms-extensions` do the wrong thing under some circumstances?

https://github.com/llvm/llvm-project/pull/194201