[clang][AST] Remove HasFirstArg assertion in CallExpr::getBeginLoc() (#130725)

There are cases where the assertion legitimately does not hold (e.g.
CallExpr::CreateTemporary()), and there's no readily available way to
tell such cases apart.

Fixes #130272

Author: HighCommander4

clang/docs/ReleaseNotes.rst

@@ -308,6 +308,7 @@ Bug Fixes to C++ Support
 - Correctly diagnoses template template paramters which have a pack parameter
   not in the last position.
 - Clang now correctly parses ``if constexpr`` expressions in immediate function context. (#GH123524)
+- Fixed an assertion failure affecting code that uses C++23 "deducing this". (#GH130272)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/Expr.cpp

@@ -1656,9 +1656,11 @@ SourceLocation CallExpr::getBeginLoc() const {
     if (const auto *Method =
             dyn_cast_if_present<const CXXMethodDecl>(getCalleeDecl());
         Method && Method->isExplicitObjectMemberFunction()) {
-      bool HasFirstArg = getNumArgs() > 0 && getArg(0);
-      assert(HasFirstArg);
-      if (HasFirstArg)
+      // Note: while we typically expect the call to have a first argument
+      // here, we can't assert it because in some cases it does not, e.g.
+      // calls created with CallExpr::CreateTemporary() during overload
+      // resolution.
+      if (getNumArgs() > 0 && getArg(0))
         return getArg(0)->getBeginLoc();
     }
   }

clang/test/AST/ast-dump-cxx2b-deducing-this.cpp

@@ -26,3 +26,12 @@ struct S {
   // CHECK-NEXT:   `-DeclRefExpr 0x{{[^ ]*}} <col:5> 'S<T>' lvalue ParmVar 0x{{[^ ]*}} 's' 'S<T>'
 };
 }
+
+namespace GH130272 {
+struct A {};
+struct B {
+  operator A(this B);
+};
+A a = A(B{});
+// CHECK: CallExpr 0x{{[^ ]*}} <col:9, col:11> 'A':'GH130272::A'
+}