[clang] [Clang][Sema] Fix last argument not being used when comparing function template specializations when one has an explicit object argument (PR #92263)

[Mital Ashok via cfe-commits]

https://github.com/MitalAshok created https://github.com/llvm/llvm-project/pull/92263

Fixes #92188

>From 6496dbfc8100812c8d3ea5a668af0593b532e4fe Mon Sep 17 00:00:00 2001
From: Mital Ashok <mital at mitalashok.co.uk>
Date: Wed, 15 May 2024 14:23:17 +0100
Subject: [PATCH] [Clang][Sema] Fix last argument not being used when comparing
 function template specializations when one has an explicit object argument

Fixes #92188
---
 clang/docs/ReleaseNotes.rst                |  2 +
 clang/lib/Sema/SemaTemplateDeduction.cpp   |  9 +++-
 clang/test/SemaCXX/cxx2b-deducing-this.cpp | 49 ++++++++++++++++++++++
 3 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ae699ebfc6038..9850fc9f8e0b4 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -713,6 +713,8 @@ Bug Fixes to C++ Support
 - Correctly treat the compound statement of an ``if consteval`` as an immediate context. Fixes (#GH91509).
 - When partial ordering alias templates against template template parameters,
   allow pack expansions when the alias has a fixed-size parameter list. Fixes (#GH62529).
+- Fix a bug where the last argument was not considered when considering the most viable function for
+  explicit object argument member functions. Fixes (#GH92188).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 853c0e1b50619..094f767399e2a 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -5522,7 +5522,8 @@ static bool isAtLeastAsSpecializedAs(Sema &S, SourceLocation Loc,
 /// function templates.
 ///
 /// \param NumCallArguments1 The number of arguments in the call to FT1, used
-/// only when \c TPOC is \c TPOC_Call.
+/// only when \c TPOC is \c TPOC_Call. Does not include the object argument when
+/// calling a member function.
 ///
 /// \param RawObj1Ty The type of the object parameter of FT1 if a member
 /// function only used if \c TPOC is \c TPOC_Call and FT1 is a Function
@@ -5591,7 +5592,11 @@ FunctionTemplateDecl *Sema::getMoreSpecializedTemplate(
                                               IsRValRef1);
       Args2.push_back(Obj2Ty);
     }
-    size_t NumComparedArguments = NumCallArguments1 + ShouldConvert1;
+    size_t NumComparedArguments = NumCallArguments1;
+    // Either added an argument above or the prototype includes an explicit
+    // object argument we need to count
+    if (Method1)
+      ++NumComparedArguments;
 
     Args1.insert(Args1.end(), Proto1->param_type_begin(),
                  Proto1->param_type_end());
diff --git a/clang/test/SemaCXX/cxx2b-deducing-this.cpp b/clang/test/SemaCXX/cxx2b-deducing-this.cpp
index 5f29a955e053c..55792e1448ac5 100644
--- a/clang/test/SemaCXX/cxx2b-deducing-this.cpp
+++ b/clang/test/SemaCXX/cxx2b-deducing-this.cpp
@@ -838,3 +838,52 @@ int h() {
   }();
 }
 }
+
+namespace GH92188 {
+struct A {
+  template<auto N>
+  void operator+=(this auto &&, const char (&)[N]);
+  void operator+=(this auto &&, auto &&) = delete;
+
+  void f1(this A &, auto &);
+  void f1(this A &, auto &&) = delete;
+
+  void f2(this auto&);
+  void f2(this auto&&) = delete;
+
+  void f3(auto&) &;
+  void f3(this A&, auto&&) = delete;
+
+  void f4(auto&&) & = delete;
+  void f4(this A&, auto&);
+
+  static void f5(auto&);
+  void f5(this A&, auto&&) = delete;
+
+  static void f6(auto&&) = delete;
+  void f6(this A&, auto&);
+
+  void implicit_this() {
+    int lval;
+    operator+=("123");
+    f1(lval);
+    f2();
+    f3(lval);
+    f4(lval);
+    f5(lval);
+    f6(lval);
+  }
+};
+
+void g() {
+  A a;
+  int lval;
+  a += "123";
+  a.f1(lval);
+  a.f2();
+  a.f3(lval);
+  a.f4(lval);
+  a.f5(lval);
+  a.f6(lval);
+}
+}