[clang] Correctly compute conversion seq for args to fn with reversed param order (PR #68999)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Utkarsh Saxena (usx95)

<details>
<summary>Changes</summary>

We associated conversion seq for args (when reversed) to the wrong index.
This lead to clang believing reversed `operator==` a worse overload candidate than the `operator==` without reversed args when both these candidate were ambiguous.

Fixes https://github.com/llvm/llvm-project/issues/53954

---
Full diff: https://github.com/llvm/llvm-project/pull/68999.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+2) 
- (modified) clang/lib/Sema/SemaOverload.cpp (+2-2) 
- (modified) clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp (+35) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 2d918967e7f0b02..07cedeabcdbd9b7 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -113,6 +113,8 @@ C++ Language Changes
 
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
+- Fix a bug in conversion sequence of arguments to a function with reversed parameter order.
+  Fixes `GH <https://github.com/llvm/llvm-project/issues/53954>`_.
 
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index ce78994e6553814..b00adb00eab01aa 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -7688,7 +7688,7 @@ bool Sema::CheckNonDependentConversions(
     QualType ParamType = ParamTypes[I + Offset];
     if (!ParamType->isDependentType()) {
       unsigned ConvIdx = PO == OverloadCandidateParamOrder::Reversed
-                             ? 0
+                             ? Args.size() - 1 - (ThisConversions + I)
                              : (ThisConversions + I);
       Conversions[ConvIdx]
         = TryCopyInitialization(*this, Args[I], ParamType,
@@ -10631,7 +10631,7 @@ OverloadCandidateSet::BestViableFunction(Sema &S, SourceLocation Loc,
   // Find the best viable function.
   Best = end();
   for (auto *Cand : Candidates) {
-    Cand->Best = false;
+        Cand->Best = false;
     if (Cand->Viable) {
       if (Best == end() ||
           isBetterOverloadCandidate(S, *Cand, *Best, Loc, Kind))
diff --git a/clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp b/clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp
index 5c6804eb7726b5f..8975706e721cff1 100644
--- a/clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp
+++ b/clang/test/CXX/over/over.match/over.match.funcs/over.match.oper/p3-2a.cpp
@@ -324,6 +324,41 @@ bool x = X() == X(); // expected-warning {{ambiguous}}
 }
 } // namespace P2468R2
 
+namespace GH53954{
+namespace test1 {
+struct P {
+    template <class S>
+    friend bool operator==(const P &, const S &); // expected-note {{candidate}} \
+                                                  // expected-note {{reversed parameter order}}
+};
+struct A : public P {};
+struct B : public P {};
+bool check(A a, B b) { return a == b; } // expected-error {{ '==' is ambiguous}}
+}
+
+namespace test2 {
+struct P {
+    template <class S>
+    friend bool operator==(const S &, const P &); // expected-note {{candidate}} \
+                                                  // expected-note {{reversed parameter order}}
+};
+struct A : public P {};
+struct B : public P {};
+bool check(A a, B b) { return a == b; } // expected-error {{ '==' is ambiguous}}
+}
+
+namespace test3 {
+struct P {
+  template<class S>
+  bool operator==(const S &) const; // expected-note {{candidate}} \
+                                    // expected-note {{reversed parameter order}}
+};
+struct A : public P {};
+struct B : public P {};
+bool check(A a, B b) { return a == b; } // expected-error {{ '==' is ambiguous}}
+}
+}
+
 #else // NO_ERRORS
 
 namespace problem_cases {

``````````

</details>


https://github.com/llvm/llvm-project/pull/68999