[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

clang/docs/ReleaseNotes.rst

@@ -188,6 +188,9 @@ Bug Fixes to C++ Support
   and (`#79745 <https://github.com/llvm/llvm-project/issues/79745>`_)
 - Fix incorrect code generation caused by the object argument of ``static operator()`` and ``static operator[]`` calls not being evaluated.
   Fixes (`#67976 <https://github.com/llvm/llvm-project/issues/67976>`_)
+- 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

@@ -6733,7 +6733,7 @@ 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/GH80252.cppm

@@ -37,6 +37,4 @@ import A;
 import B;
 // Since modules are loaded lazily, force loading by performing a lookup.
 using xxx = bar;
-// FIXME: This is a false positive ODR violation.
-// expected-error@bar.h:2 {{'bar' has different definitions in different modules; first difference is defined here found constructor with 1st parameter of type 'baz::foo' (aka 'char')}}
-// expected-note@bar.h:2 {{but in 'B.<global>' found constructor with 1st parameter of type 'baz::foo' (aka 'char')}}
+// expected-no-diagnostics

clang/test/Modules/cxx20-decls.cppm

@@ -31,14 +31,12 @@ using xxx = baz::foo;
 // CHECK-NEXT: | `-BuiltinType {{.*}} 'char'
 // CHECK-NEXT: |-UsingDecl {{.*}} baz::foo
 // CHECK-NEXT: | `-NestedNameSpecifier Namespace {{.*}} 'baz'
-// FIXME: UsingShadowDecl should have been merged
-// CHECK-NOT:  `-UsingShadowDecl 0x{{[^ ]*}} prev 0x{{[^ ]*}} {{.*}} imported in A.<global> {{.*}} 'foo'
-// CHECK-NEXT: `-UsingShadowDecl 0x{{[^ ]*}} {{.*}} imported in A.<global> {{.*}} 'foo'
+// CHECK-NEXT: `-UsingShadowDecl 0x{{[^ ]*}} prev 0x[[SHADOW_ADDR:[^ ]*]] {{.*}} imported in A.<global> {{.*}} 'foo'
 
 // CHECK-LABEL: Dumping baz:
 // CHECK-NEXT: NamespaceDecl {{.*}} baz
 // CHECK-NEXT: |-TypeAliasDecl {{.*}} foo 'char'
 // CHECK-NEXT: | `-BuiltinType {{.*}} 'char'
 // CHECK-NEXT: |-UsingDecl {{.*}} baz::foo
 // CHECK-NEXT: | `-NestedNameSpecifier Namespace {{.*}} 'baz'
-// CHECK-NEXT:  `-UsingShadowDecl 0x[[SHADOW_ADDR:[^ ]*]] {{.*}} 'foo'
+// CHECK-NEXT:  `-UsingShadowDecl 0x[[SHADOW_ADDR]] {{.*}} 'foo'