[clang] [clang] Fix conflicting declaration error with using_if_exists (PR #167646)

[Mariya Podchishchaeva via cfe-commits]

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@@ -12832,6 +12833,15 @@ bool Sema::CheckUsingShadowDecl(BaseUsingDecl *BUD, NamedDecl *Orig,
       (isa_and_nonnull<UnresolvedUsingIfExistsDecl>(NonTag))) {
     if (!NonTag && !Tag)
       return false;
+
+    // Only report the error if this using_if_exists decl can be a
+    // substitute for the original decl. LookupResult will find things with
+    // the same name but we also want to take into account namespaces and
+    // other scopes. GlobalDecl helps take care of that.
+    NamedDecl *UsedTag = NonTag ? NonTag : Tag;
+    if (GlobalDecl(Target) != GlobalDecl(UsedTag))
----------------
Fznamznon wrote:

Hmm, how exactly `GlobalDecl` helps here?
I see it simply stores a pointer, say for a `FunctionDecl`. It is not asking for a canonical decl and then its operator `==` which is used by `!=` operator compares the stored pointers.

https://github.com/llvm/llvm-project/pull/167646