Fix loop-convert crash.

[angelgarcia]

Summary

loop-convert no longer crashes when calling a member function using a member pointer which is a member of another record.

[klimek]

lg

clang-tidy/modernize/LoopConvertUtils.cpp

if (OpCall->getOperator() == OO_Subscript) {
       assert(OpCall->getNumArgs() == 2);
       return isIndexInSubscriptExpr(OpCall->getArg(1), IndexVar);
     }
     break;
   }
 
   case Stmt::CXXMemberCallExprClass: {
     const auto *MemCall = cast<CXXMemberCallExpr>(Init);
-    if (MemCall->getMethodDecl()->getName() == "at") {
+    // This check is needed because getMethodDecl can return nullptr if the
+    // callee is a member function pointer.
+    if (MemCall->getMethodDecl() &&
+        MemCall->getMethodDecl()->getName() == "at") {
       assert(MemCall->getNumArgs() == 1);
       return isIndexInSubscriptExpr(MemCall->getArg(0), IndexVar);
     }
     return false;
   }
 
   default:
     break;

test/clang-tidy/modernize-loop-convert-basic.cpp

 void memberFunctionPointer() {
   Val v;
   void (Val::*mfpArr[N])(void) = {&Val::g};
   for (int i = 0; i < N; ++i)
     (v.*mfpArr[i])();
   // CHECK-MESSAGES: :[[@LINE-2]]:3: warning: use range-based for loop instead
   // CHECK-FIXES: for (auto & elem : mfpArr)
   // CHECK-FIXES-NEXT: (v.*elem)();
+
+  struct Foo {
+    int (Val::*f)();
+  } foo[N];
+
+  for (int i = 0; i < N; ++i)
+    int r = (v.*(foo[i].f))();
+  // CHECK-MESSAGES: :[[@LINE-2]]:3: warning: use range-based for loop instead
+  // CHECK-FIXES: for (auto & elem : foo)
+  // CHECK-FIXES-NEXT: int r = (v.*(elem.f))();
+
 }
 
 } // namespace Array
 
 namespace Iterator {
 
 void f() {
   /// begin()/end() - based for loops here: