[clang] Fix conflicting declaration error with using_if_exists

This fixes an issue with using_if_exists where we would hit
`conflicts with target of using declaration already in scope` with a
using_if_exists attribute referring to a declaration which did not
exist. That is, if we have `using ::bar
__attribute__((using_if_exists))` but `bar` is not in the global
namespace, then nothing should actually be declared here.

This PR contains the following changes:
1. Ensure we only diagnose this error if the target decl and [Non]Tag
   decl can be substitutes for each other.
2. Prevent LookupResult from considering UnresolvedUsingIfExistsDecls in
   the event of ambiguous results.
3. Update tests. This includes the minimal repo for a regression test,
   and changes to existing tests which also seem to exhibit this bug.

Fixes #85335

Author: PiJoules

clang/lib/Sema/SemaDeclCXX.cpp

@@ -23,6 +23,7 @@
 #include "clang/AST/EvaluatedExprVisitor.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
+#include "clang/AST/GlobalDecl.h"
 #include "clang/AST/RecordLayout.h"
 #include "clang/AST/StmtVisitor.h"
 #include "clang/AST/TypeLoc.h"
@@ -12832,6 +12833,15 @@ bool Sema::CheckUsingShadowDecl(BaseUsingDecl *BUD, NamedDecl *Orig,
       (isa_and_nonnull<UnresolvedUsingIfExistsDecl>(NonTag))) {
     if (!NonTag && !Tag)
       return false;
+
+    // Only check 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))
+      return false;
+
     Diag(BUD->getLocation(), diag::err_using_decl_conflict);
     Diag(Target->getLocation(), diag::note_using_decl_target);
     Diag((NonTag ? NonTag : Tag)->getLocation(),

clang/lib/Sema/SemaLookup.cpp

@@ -521,11 +521,15 @@ void LookupResult::resolveKind() {
 
   llvm::SmallVector<const NamedDecl *, 4> EquivalentNonFunctions;
   llvm::BitVector RemovedDecls(N);
+  llvm::BitVector UnresolvedUsingDecls(N);
 
   for (unsigned I = 0; I < N; I++) {
     const NamedDecl *D = Decls[I]->getUnderlyingDecl();
     D = cast<NamedDecl>(D->getCanonicalDecl());
 
+    if (isa<UnresolvedUsingIfExistsDecl>(D))
+      UnresolvedUsingDecls.set(I);
+
     // Ignore an invalid declaration unless it's the only one left.
     // Also ignore HLSLBufferDecl which not have name conflict with other Decls.
     if ((D->isInvalidDecl() || isa<HLSLBufferDecl>(D)) &&
@@ -633,17 +637,24 @@ void LookupResult::resolveKind() {
     getSema().diagnoseEquivalentInternalLinkageDeclarations(
         getNameLoc(), HasNonFunction, EquivalentNonFunctions);
 
+  if ((HasNonFunction && (HasFunction || HasUnresolved)) ||
+      (HideTags && HasTag && (HasFunction || HasNonFunction || HasUnresolved)))
+    Ambiguous = true;
+
+  if (Ambiguous && UnresolvedUsingDecls.count()) {
+    // If we would have an ambiguous reference but any of them are
+    // using_if_exist decls, ignore them since they are unresolved.
+    RemovedDecls |= UnresolvedUsingDecls;
+    Ambiguous = false;
+  }
+
   // Remove decls by replacing them with decls from the end (which
   // means that we need to iterate from the end) and then truncating
   // to the new size.
   for (int I = RemovedDecls.find_last(); I >= 0; I = RemovedDecls.find_prev(I))
     Decls[I] = Decls[--N];
   Decls.truncate(N);
 
-  if ((HasNonFunction && (HasFunction || HasUnresolved)) ||
-      (HideTags && HasTag && (HasFunction || HasNonFunction || HasUnresolved)))
-    Ambiguous = true;
-
   if (Ambiguous && ReferenceToPlaceHolderVariable)
     setAmbiguous(LookupAmbiguityKind::AmbiguousReferenceToPlaceholderVariable);
   else if (Ambiguous)

clang/test/SemaCXX/using-if-exists.cpp

@@ -22,28 +22,28 @@ using NS::x UIE;
 namespace NS1 {}
 namespace NS2 {}
 namespace NS3 {
-int A();     // expected-note{{target of using declaration}}
-struct B {}; // expected-note{{target of using declaration}}
-int C();     // expected-note{{conflicting declaration}}
-struct D {}; // expected-note{{conflicting declaration}}
+int A();
+struct B {};
+int C();
+struct D {};
 } // namespace NS3
 
-using NS1::A UIE;
-using NS2::A UIE; // expected-note{{using declaration annotated with 'using_if_exists' here}} expected-note{{conflicting declaration}}
-using NS3::A UIE; // expected-error{{target of using declaration conflicts with declaration already in scope}}
-int i = A();      // expected-error{{reference to unresolved using declaration}}
+using NS1::A UIE; // OK since this declaration shouldn't exist since `A` is not in `NS1` 
+using NS2::A UIE; // OK since this declaration shouldn't exist since `A` is not in `NS2` 
+using NS3::A UIE; // OK since prior UIEs of `A` shouldn't have declare anything since they don't exist
+int i = A();      // OK since `A` resolved to the single UIE in the previous line
 
-using NS1::B UIE;
-using NS2::B UIE; // expected-note{{conflicting declaration}} expected-note{{using declaration annotated with 'using_if_exists' here}}
-using NS3::B UIE; // expected-error{{target of using declaration conflicts with declaration already in scope}}
-B myB;            // expected-error{{reference to unresolved using declaration}}
+using NS1::B UIE; // OK since this declaration shouldn't exist since `B` is not in `NS1`
+using NS2::B UIE; // OK since this declaration shouldn't exist since `B` is not in `NS2 
+using NS3::B UIE; // OK since prior UIEs of `B` shouldn't have declare anything since they don't exist
+B myB;            // OK since `B` resolved to the single UIE in the previous lin
 
 using NS3::C UIE;
-using NS2::C UIE; // expected-error{{target of using declaration conflicts with declaration already in scope}} expected-note{{target of using declaration}}
+using NS2::C UIE; // OK since NS2::C doesn't exist
 int j = C();
 
 using NS3::D UIE;
-using NS2::D UIE; // expected-error{{target of using declaration conflicts with declaration already in scope}} expected-note{{target of using declaration}}
+using NS2::D UIE; // OK since NS2::D doesn't exist
 D myD;
 } // namespace test_redecl
 
@@ -113,29 +113,36 @@ struct NonDep : BaseEmpty {
 namespace test_using_pack {
 template <class... Ts>
 struct S : Ts... {
-  using typename Ts::x... UIE; // expected-error 2 {{target of using declaration conflicts with declaration already in scope}} expected-note{{conflicting declaration}} expected-note{{target of using declaration}}
+  // We don't expect any errors with conflicting targets for variables `a`, `b`, `c`,
+  // and `d` below this. For `a`, `x` will not be declared because neither E1 nor E2
+  // defines it. For `b`, `x` is the same type so there won't be any conflicts. For
+  // `c` and `d`, only one of the template parameters has a class that defines it,
+  // so there's no conflict.
+  using typename Ts::x... UIE;
 };
 
 struct E1 {};
 struct E2 {};
 S<E1, E2> a;
 
 struct F1 {
-  typedef int x; // expected-note 2 {{conflicting declaration}}
+  typedef int x;
 };
 struct F2 {
-  typedef int x; // expected-note 2 {{target of using declaration}}
+  typedef int x;
 };
 S<F1, F2> b;
 
-S<E1, F2> c; // expected-note{{in instantiation of template class}}
-S<F1, E2> d; // expected-note{{in instantiation of template class}}
+S<E1, F2> c;
+S<F1, E2> d;
 
 template <class... Ts>
 struct S2 : Ts... {
-  using typename Ts::x... UIE; // expected-error 2 {{target of using declaration conflicts with declaration already in scope}} expected-note 3 {{using declaration annotated with 'using_if_exists' here}} expected-note{{conflicting declaration}} expected-note{{target of using declaration}}
+  // OK for the same reasons listed in `struct S` above. We don't expect any conflicts w.r.t
+  // redefinitions of `x` but we still expect errors when using `x` for cases it's not available.
+  using typename Ts::x... UIE; // expected-note 4 {{using declaration annotated with 'using_if_exists' here}}
 
-  x mem(); // expected-error 3 {{reference to unresolved using declaration}}
+  x mem(); // expected-error 4 {{reference to unresolved using declaration}}
 };
 
 S2<E1, E2> e; // expected-note{{in instantiation of template class}}
@@ -145,14 +152,15 @@ S2<F1, E2> h; // expected-note{{in instantiation of template class}}
 
 template <class... Ts>
 struct S3 : protected Ts... {
-  using Ts::m... UIE; // expected-error{{target of using declaration conflicts with declaration already in scope}} expected-note{{target of using declaration}}
+  // No errors for conflicting declarations because only one of the parent classes declares `m`.
+  using Ts::m... UIE;
 };
 struct B1 {
-  enum { m }; // expected-note{{conflicting declaration}}
+  enum { m };
 };
 struct B2 {};
 
-S3<B1, B2> i; // expected-note{{in instantiation of template}}
+S3<B1, B2> i;
 S<B2, B1> j;
 
 } // namespace test_using_pack
@@ -170,23 +178,23 @@ NS2::x y; // expected-error {{reference to unresolved using declaration}}
 } // namespace test_nested
 
 namespace test_scope {
-int x; // expected-note{{conflicting declaration}}
+int x;
 void f() {
-  int x; // expected-note{{conflicting declaration}}
+  int x;
   {
     using ::x UIE; // expected-note {{using declaration annotated with 'using_if_exists' here}}
     (void)x;       // expected-error {{reference to unresolved using declaration}}
   }
 
   {
     using test_scope::x;
-    using ::x UIE; // expected-error{{target of using declaration conflicts with declaration already in scope}} expected-note{{target of using declaration}}
+    using ::x UIE; // OK since there's no `x` in the global namespace, so this wouldn't be any declaration
     (void)x;
   }
 
   (void)x;
 
-  using ::x UIE; // expected-error{{target of using declaration conflicts with declaration already in scope}} expected-note{{target of using declaration}}
+  using ::x UIE; // OK since there's no `x` in the global namespace, so this wouldn't be any declaration
   (void)x;
 }
 } // namespace test_scope
@@ -224,3 +232,20 @@ int main() {
   size = fake_printf();
   size = std::fake_printf();
 }
+
+// Regression test for https://github.com/llvm/llvm-project/issues/85335.
+// No errors should be reported here.
+namespace PR85335 {
+void foo();
+
+namespace N {
+  void bar();
+
+  using ::foo __attribute__((__using_if_exists__));
+  using ::bar __attribute__((__using_if_exists__));
+}
+
+void baz() {
+  N::bar();
+}
+}  // namespace PR85335