[clang][AST] Handle implicit first argument in CallExpr::getBeginLoc() (#135757)

Fixes #135522

Author: HighCommander4

clang/docs/ReleaseNotes.rst

@@ -411,6 +411,8 @@ Bug Fixes in This Version
 - ``#embed`` directive now diagnoses use of a non-character file (device file)
   such as ``/dev/urandom`` as an error. This restriction may be relaxed in the
   future. See (#GH126629).
+- Fixed a clang 20 regression where diagnostics attached to some calls to member functions
+  using C++23 "deducing this" did not have a diagnostic location (#GH135522)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/Expr.cpp

@@ -1652,8 +1652,11 @@ SourceLocation CallExpr::getBeginLoc() const {
   if (!isTypeDependent()) {
     if (const auto *Method =
             dyn_cast_if_present<const CXXMethodDecl>(getCalleeDecl());
-        Method && Method->isExplicitObjectMemberFunction())
-      return getArg(0)->getBeginLoc();
+        Method && Method->isExplicitObjectMemberFunction()) {
+      if (auto FirstArgLoc = getArg(0)->getBeginLoc(); FirstArgLoc.isValid()) {
+        return FirstArgLoc;
+      }
+    }
   }
 
   SourceLocation begin = getCallee()->getBeginLoc();

clang/test/SemaCXX/cxx2b-deducing-this.cpp

@@ -1134,3 +1134,10 @@ struct S {
 static_assert((S{} << 11) == a);
 // expected-error@-1 {{use of undeclared identifier 'a'}}
 }
+
+namespace GH135522 {
+struct S {
+  auto f(this auto) -> S;
+  bool g() { return f(); } // expected-error {{no viable conversion from returned value of type 'S' to function return type 'bool'}}
+};
+}