Index: clang-reorder-fields/ReorderFieldsAction.cpp
===================================================================
--- clang-reorder-fields/ReorderFieldsAction.cpp
+++ clang-reorder-fields/ReorderFieldsAction.cpp
@@ -22,12 +22,14 @@
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/Lex/Lexer.h"
 #include "clang/Tooling/Refactoring.h"
+#include "llvm/ADT/SetVector.h"
 #include <algorithm>
 #include <string>
 
 namespace clang {
 namespace reorder_fields {
 using namespace clang::ast_matchers;
+using llvm::SmallSetVector;
 
 /// \brief Finds the definition of a record by name.
 ///
@@ -91,6 +93,24 @@
   consumeError(Replacements[R.getFilePath()].add(R));
 }
 
+/// \brief Find all member fields used in the given init-list initializer expr
+/// that belong to the same record
+///
+/// \returns a set of field declarations, empty if none were present
+static SmallSetVector<FieldDecl *, 1>
+findMembersUsedInInitExpr(const CXXCtorInitializer *Initializer,
+                          ASTContext &Context) {
+  SmallSetVector<FieldDecl *, 1> Results;
+  auto FoundExprs =
+      match(findAll(memberExpr(hasObjectExpression(cxxThisExpr())).bind("ME")),
+            *Initializer->getInit(), Context);
+  for (BoundNodes &BN : FoundExprs)
+    if (auto *MemExpr = BN.getNodeAs<MemberExpr>("ME"))
+      if (auto *FD = dyn_cast<FieldDecl>(MemExpr->getMemberDecl()))
+        Results.insert(FD);
+  return Results;
+}
+
 /// \brief Reorders fields in the definition of a struct/class.
 ///
 /// At the moment reodering of fields with
@@ -133,7 +153,7 @@
 /// Thus, we need to ensure that we reorder just the initializers that are present.
 static void reorderFieldsInConstructor(
     const CXXConstructorDecl *CtorDecl, ArrayRef<unsigned> NewFieldsOrder,
-    const ASTContext &Context,
+    ASTContext &Context,
     std::map<std::string, tooling::Replacements> &Replacements) {
   assert(CtorDecl && "Constructor declaration is null");
   if (CtorDecl->isImplicit() || CtorDecl->getNumCtorInitializers() <= 1)
@@ -153,6 +173,22 @@
   for (const auto *Initializer : CtorDecl->inits()) {
     if (!Initializer->isWritten())
       continue;
+
+    // Warn if this reordering violates initialization expr dependencies
+    const auto UsedMembers = findMembersUsedInInitExpr(Initializer, Context);
+    const FieldDecl *ThisM = Initializer->getMember();
+    for (const FieldDecl *UM : UsedMembers) {
+      if (NewFieldsPositions[UM->getFieldIndex()] >
+          NewFieldsPositions[ThisM->getFieldIndex()]) {
+        llvm::errs() << Context.getSourceManager().getSpellingLineNumber(
+                            Initializer->getSourceLocation())
+                     << ": Warning: reordering field " << UM->getName()
+                     << " after " << ThisM->getName() << " makes "
+                     << UM->getName()
+                     << " uninitialized when used in init expression\n";
+      }
+    }
+
     OldWrittenInitializersOrder.push_back(Initializer);
     NewWrittenInitializersOrder.push_back(Initializer);
   }
Index: test/clang-reorder-fields/FieldDependencyWarning.cpp
===================================================================
--- test/clang-reorder-fields/FieldDependencyWarning.cpp
+++ test/clang-reorder-fields/FieldDependencyWarning.cpp
@@ -0,0 +1,42 @@
+// RUN: clang-reorder-fields -record-name bar::Foo -fields-order y,z,c,x %s -- 2>&1 | FileCheck %s
+
+#include <string>
+
+namespace bar {
+class Foo { 
+public:
+  Foo(int x, double y, char cin);
+  Foo(int nx);
+  Foo();
+  int x;
+  double y;
+  char c;
+  std::string z;
+};
+
+static char bar(char c) {
+  return c + 1;
+}
+
+Foo::Foo(int x, double y, char cin) :  
+  x(x),                 
+  y(y),                 
+  c(cin),               
+  z(this->x, bar(c))    // CHECK:      [[@LINE]]: Warning: reordering field x after z makes x uninitialized when used in init expression
+                        // CHECK-NEXT: [[@LINE-1]]: Warning: reordering field c after z makes c uninitialized when used in init expression
+{}
+
+Foo::Foo(int nx) :
+  x(nx),              
+  y(x),                 // CHECK-NEXT: [[@LINE]]: Warning: reordering field x after y makes x uninitialized when used in init expression
+  c(0),               
+  z(bar(bar(x)), c)     // CHECK-NEXT: [[@LINE]]: Warning: reordering field x after z makes x uninitialized when used in init expression
+                        // CHECK-NEXT: [[@LINE-1]]: Warning: reordering field c after z makes c uninitialized when used in init expression
+{}
+
+} // namespace bar
+
+int main() {
+  bar::Foo F(5, 12.8, 'c');
+  return 0;
+}