[clang][AST] Handle dependent representation of call to function with explicit object parameter in CallExpr::getBeginLoc()

Fixes #126720

Author: HighCommander4

clang/lib/AST/Expr.cpp

@@ -1648,8 +1648,11 @@ SourceLocation CallExpr::getBeginLoc() const {
   if (const auto *Method =
           dyn_cast_if_present<const CXXMethodDecl>(getCalleeDecl());
       Method && Method->isExplicitObjectMemberFunction()) {
-    assert(getNumArgs() > 0 && getArg(0));
-    return getArg(0)->getBeginLoc();
+    if (!isTypeDependent()) {
+      assert(getNumArgs() > 0 && getArg(0));
+      if (getNumArgs() > 0 && getArg(0))
+        return getArg(0)->getBeginLoc();
+    }
   }
 
   SourceLocation begin = getCallee()->getBeginLoc();

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

@@ -13,3 +13,16 @@ void main() {
   // CHECK-NEXT: | `-DeclRefExpr 0x{{[^ ]*}} <col:13> 'int (S &)' lvalue CXXMethod 0x{{[^ ]*}} 'f' 'int (S &)'
 }
 }
+
+namespace GH1269720 {
+template <typename T>
+struct S {
+  void f(this S&);
+  void g(S s) {
+    s.f();
+  }
+  // CHECK: CallExpr 0x{{[^ ]*}} <line:22:5, col:9> '<dependent type>'
+  // CHECK-NEXT: `-MemberExpr 0x{{[^ ]*}} <col:5, col:7> '<bound member function type>' .f
+  // CHECK-NEXT:   `-DeclRefExpr 0x{{[^ ]*}} <col:5> 'S<T>' lvalue ParmVar 0x{{[^ ]*}} 's' 'S<T>'
+};
+}