[Clang] Fix confusing diagnositcs related to explicit this parameters

[bradenhelmer]

Fixes #97878.

This PR improves diagnostics related to explicit 'this' parameters. Previously, the 'this' parameter would be incorrectly underlined when diagnosing a bad conversion.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Braden Helmer (bradenhelmer)

Changes

New PR for #97878.

---

Full diff: https://github.com/llvm/llvm-project/pull/100351.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/Sema/SemaOverload.cpp (+1-1)
• (modified) clang/test/SemaCXX/cxx2b-deducing-this.cpp (+7)

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'}}
+  }
+};

[shafik]

Thank you for this PR, could please put a little more detail in the summary since that is what will be seen by folks using git log.

[cor3ntin]

LGTM

[bradenhelmer]

I'll need the merge button clicked, thanks