[clang] fix merging of UsingShadowDecl

Previously, when deciding if two UsingShadowDecls where mergeable,
we would incorrectly only look for both pointing to the exact redecla
ration, whereas the correct thing is to look for declarations to the
same entity.

This problem has existed as far back as 2013, introduced in commit
fd8634a.

This problem could manifest itself as ODR check false positives
when importing modules.

Fixes: #80252

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -813,6 +813,9 @@ Bug Fixes to C++ Support
 - Clang now allows ``@$``` in raw string literals. Fixes (#GH93130).
 - Fix an assertion failure when checking invalid ``this`` usage in the wrong context. (Fixes #GH91536).
 - Clang no longer models dependent NTTP arguments as ``TemplateParamObjectDecl`` s. Fixes (#GH84052).
+- Fix incorrect merging of modules which contain using declarations which shadow
+  other declarations. This could manifest as ODR checker false positives.
+  Fixes (`#80252 <https://github.com/llvm/llvm-project/issues/80252>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ASTContext.cpp

@@ -6794,7 +6794,7 @@ bool ASTContext::isSameEntity(const NamedDecl *X, const NamedDecl *Y) const {
   // Using shadow declarations with the same target match.
   if (const auto *USX = dyn_cast<UsingShadowDecl>(X)) {
     const auto *USY = cast<UsingShadowDecl>(Y);
-    return USX->getTargetDecl() == USY->getTargetDecl();
+    return declaresSameEntity(USX->getTargetDecl(), USY->getTargetDecl());
   }
 
   // Using declarations with the same qualifier match. (We already know that

clang/test/Modules/cxx20-decls.cppm

@@ -28,8 +28,8 @@ using xxx = baz::foo;
 // CHECK-NEXT: NamespaceDecl 0x[[BAZ_REDECL_ADDR:[^ ]*]] prev 0x[[BAZ_ADDR:[^ ]*]]
 // CHECK:      TypeAliasDecl 0x[[ALIAS_REDECL_ADDR:[^ ]*]] prev 0x[[ALIAS_ADDR:[^ ]*]]
 // FIXME: UsingShadowDecl should have been merged
-// CHECK:      UsingShadowDecl 0x{{[^ ]*}} <{{.*}}> col:{{.*}} imported in A.<global> hidden implicit TypeAlias 0x[[ALIAS_REDECL_ADDR]] 'foo'
+// CHECK:      UsingShadowDecl 0x{{[^ ]*}} prev 0x[[SHADOW_ADDR:[^ ]*]] {{.*}} imported in A.<global> {{.*}} 'foo'
 
 // CHECK-LABEL: Dumping baz:
 // CHECK-NEXT: NamespaceDecl 0x[[BAZ_ADDR]] <{{.*}}> line:{{.*}} baz
-// CHECK:      UsingShadowDecl 0x[[SHADOW_ADDR:[^ ]*]] <{{.*}}> col:{{.*}} implicit TypeAlias 0x[[ALIAS_ADDR]] 'foo'
+// CHECK:      UsingShadowDecl 0x[[SHADOW_ADDR]] {{.*}} 'foo'