[SemaCXX] Fixed defaulted equality operator deletion issue (#97087)[SemaCXX] Fix defaulted equality operator being incorrectly deleted when using namespace (llvm#97087)

[suhas-1012]

Summary

Fixed an issue where defaulted equality comparison operators (operator==) were incorrectly treated as implicitly deleted when the required operator== for a member subobject (such as X inside Y) was only made visible through a using namespace directive.

---

Changes Made

• Implemented a fix in clang/lib/Sema/SemaDeclCXX.cpp to ensure that unqualified lookup correctly accounts for using namespace directives when generating defaulted comparison operators.
• Added two new test cases in clang/test/SemaCXX/:
  • default-op-using-namespace.cpp – verifies that the code now compiles successfully and no longer treats the operator as implicitly deleted when using namespace makes it visible.
  • default-op-notusing-namespace.cpp – verifies that the operator remains correctly deleted when no using namespace directive is present.

---

Explanation

Previously, Clang sometimes performed lookup for defaulted comparison operators without a valid scope, which caused it to miss operators introduced via using namespace directives.
The fix ensures that when the lookup Scope* is nullptr, Clang now explicitly retrieves the appropriate lexical scope (via getScopeForContext(FD->getLexicalDeclContext())) instead of ignoring the case.
This change allows unqualified lookup to correctly include using namespace directives and ensures consistent behavior during operator synthesis.

---

Fixes: llvm/llvm-project#97087

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Muvvala Sairam Suhas (suhas-1012)

Changes

Summary

Fixed an issue where defaulted equality comparison operators (operator==) were incorrectly treated as implicitly deleted when the required operator== for a member subobject (such as X inside Y) was only made visible through a using namespace directive.

---

Changes Made

• Implemented a fix in clang/lib/Sema/SemaDeclCXX.cpp to ensure that unqualified lookup correctly accounts for using namespace directives when generating defaulted comparison operators.
• Added two new test cases in clang/test/SemaCXX/:
  • default-op-using-namespace.cpp – verifies that the code now compiles successfully and no longer treats the operator as implicitly deleted when using namespace makes it visible.
  • default-op-notusing-namespace.cpp – verifies that the operator remains correctly deleted when no using namespace directive is present.

---

Explanation

Previously, Clang sometimes performed lookup for defaulted comparison operators without a valid scope, which caused it to miss operators introduced via using namespace directives.
The fix ensures that when the lookup Scope* is nullptr, Clang now explicitly retrieves the appropriate lexical scope (via getScopeForContext(FD->getLexicalDeclContext())) instead of ignoring the case.
This change allows unqualified lookup to correctly include using namespace directives and ensures consistent behavior during operator synthesis.

---

Fixes: llvm/llvm-project#97087

---

Full diff: https://github.com/llvm/llvm-project/pull/166996.diff

3 Files Affected:

• (modified) clang/lib/Sema/SemaDeclCXX.cpp (+5)
• (added) clang/test/SemaCXX/default-op-notusing-namespace.cpp (+17)
• (added) clang/test/SemaCXX/default-op-using-namespace.cpp (+13)

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index d41ab126c426f..2331774804c89 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -8262,6 +8262,8 @@ class DefaultedComparisonAnalyzer
     ///   resolution [...]
     CandidateSet.exclude(FD);
 
+   auto &Fns = this->Fns;
+
     if (Args[0]->getType()->isOverloadableType())
       S.LookupOverloadedBinOp(CandidateSet, OO, Fns, Args);
     else
@@ -8913,6 +8915,9 @@ bool Sema::CheckExplicitlyDefaultedComparison(Scope *S, FunctionDecl *FD,
 
   // Perform any unqualified lookups we're going to need to default this
   // function.
+  if (!S)
+    S = getScopeForContext(FD->getLexicalDeclContext());
+
   if (S) {
     UnresolvedSet<32> Operators;
     lookupOperatorsForDefaultedComparison(*this, S, Operators,
diff --git a/clang/test/SemaCXX/default-op-notusing-namespace.cpp b/clang/test/SemaCXX/default-op-notusing-namespace.cpp
new file mode 100644
index 0000000000000..48515fb28366f
--- /dev/null
+++ b/clang/test/SemaCXX/default-op-notusing-namespace.cpp
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+struct X {};
+namespace NS {
+  bool operator==(X, X);
+}
+
+struct Y {
+  X x;
+  friend bool operator==(Y, Y);
+};
+
+// There is no `using namespace NS;` here, so this operator==
+// will be implicitly deleted due to missing viable operator== for X.
+bool operator==(Y, Y) = default;
+// expected-error@-1 {{defaulting this equality comparison operator would delete it after its first declaration}}
+// expected-note@9 {{defaulted 'operator==' is implicitly deleted because there is no viable 'operator==' for member 'x'}}
diff --git a/clang/test/SemaCXX/default-op-using-namespace.cpp b/clang/test/SemaCXX/default-op-using-namespace.cpp
new file mode 100644
index 0000000000000..89003ae0a42ca
--- /dev/null
+++ b/clang/test/SemaCXX/default-op-using-namespace.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+// expected-no-diagnostics
+
+struct X {};
+namespace NS {
+    bool operator==(X, X);
+}
+using namespace NS;
+struct Y {
+    X x;
+    friend bool operator==(Y, Y);
+};
+bool operator==(Y, Y) = default;
\ No newline at end of file

[suhas-1012]

Please let me know if any more test cases or changes are to be done.

[erichkeane]

This has 2 problems: First, it is horrifyingly subtle (with the shadowing). Second, you have to use the type here.

[Sirraide]

What does this even do? Before this line, Fns is an lvalue equivalent to this->Fns; after this line, Fns is ... an lvalue that aliases this->Fns?

[erichkeane]

This doesn't align right with the comment. This probably needs a comment itself as well.

[erichkeane]

newline needed here.

[erichkeane]

These two should just be in the same test file, there is no reason to have two separate tests for this. Perhaps find an existing test file for this to live in?

[shafik]

Based on @zygoloid comment:

#97087 (comment)

here it looks like the fix is the pass Scope instead of nullptr here:

llvm-project/clang/lib/Sema/SemaDeclCXX.cpp

Line 17990 in 4c63672

if (CheckExplicitlyDefaultedComparison(nullptr, FD, DefKind.asComparison()))

Did you try this?