[clang] [Clang] strengthen checks for 'main' function to meet [basic.start.main] p3 requirements (PR #101853)

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@@ -990,6 +990,12 @@ def warn_main_redefined : Warning<"variable named 'main' with external linkage "
     "has undefined behavior">, InGroup<Main>;
 def ext_main_used : Extension<
     "referring to 'main' within an expression is a Clang extension">, InGroup<Main>;
+def ext_main_invalid_linkage_specification : ExtWarn<
+  "'main' should not be "
+  "'extern \"%select{C|C++}0\"'">, InGroup<Main>;
+def err_main_invalid_linkage_specification : Error<
+  "'main' should not be "
+  "'extern \"%select{C|C++}0\"'">;
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Sirraide wrote:

Thinking about this again, I think making these an error by default is a bit too strong; no-one else does that by default; the most you’ll get is a warning from EDG and a pedantic warning from GCC. I think making this a single `ExtWarn` is fine.

https://github.com/llvm/llvm-project/pull/101853