[clang] [Clang] Fix confusing diagnositcs related to explicit this parameters (PR #100351)

[Braden Helmer via cfe-commits]

https://github.com/bradenhelmer updated https://github.com/llvm/llvm-project/pull/100351

>From 4e0845195d6593b907bbf65d436ededee9c2aacd Mon Sep 17 00:00:00 2001
From: Braden Helmer <bradenhelmeraus at gmail.com>
Date: Tue, 23 Jul 2024 11:04:06 -0400
Subject: [PATCH] Fixed confusing diagnostics / add release notes

---
 clang/docs/ReleaseNotes.rst                | 2 ++
 clang/lib/Sema/SemaOverload.cpp            | 2 +-
 clang/test/SemaCXX/cxx2b-deducing-this.cpp | 7 +++++++
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ac1de0db9ce48..81624902c4f43 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -120,6 +120,8 @@ Improvements to Clang's diagnostics
       template <typename> int i; // error: non-static data member 'i' cannot be declared as a template
      };
 
+- Clang now has improved diagnostics for functions with explicit 'this' parameters. Fixes #GH97878
+
 Improvements to Clang's time-trace
 ----------------------------------
 
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index a8d250fbabfed..ac90ccd5643c1 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -11149,7 +11149,7 @@ static void DiagnoseBadConversion(Sema &S, OverloadCandidate *Cand,
   if (isa<CXXMethodDecl>(Fn) && !isa<CXXConstructorDecl>(Fn)) {
     if (I == 0)
       isObjectArgument = true;
-    else
+    else if (!Fn->hasCXXExplicitFunctionObjectParameter())
       I--;
   }
 
diff --git a/clang/test/SemaCXX/cxx2b-deducing-this.cpp b/clang/test/SemaCXX/cxx2b-deducing-this.cpp
index 5cbc1f735383b..f5cae2ecb5f08 100644
--- a/clang/test/SemaCXX/cxx2b-deducing-this.cpp
+++ b/clang/test/SemaCXX/cxx2b-deducing-this.cpp
@@ -959,3 +959,10 @@ void f();
 };
 void a::f(this auto) {} // expected-error {{an explicit object parameter cannot appear in a non-member function}}
 }
+
+struct R {
+  void f(this auto &&self, int &&r_value_ref) {} // expected-note {{candidate function template not viable: expects an rvalue for 2nd argument}}
+  void g(int &&r_value_ref) {
+	f(r_value_ref); // expected-error {{no matching member function for call to 'f'}}
+  }
+};