Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -369,6 +369,10 @@
           "static data member of %2|"
           "field of %2}1">,
   InGroup<Shadow>, DefaultIgnore;
+def warn_modifying_shadowed_field :
+  Warning<"modifying constructor parameter %0 that shadows field of %1">,
+  InGroup<Shadow>, DefaultIgnore;
+
 
 // C++ using declarations
 def err_using_requires_qualname : Error<
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -6389,11 +6389,16 @@
   if (!isa<VarDecl>(ShadowedDecl) && !isa<FieldDecl>(ShadowedDecl))
     return;
 
-  // Fields are not shadowed by variables in C++ static methods.
-  if (isa<FieldDecl>(ShadowedDecl))
+  if (isa<FieldDecl>(ShadowedDecl)) {
+    // Fields are not shadowed by variables in C++ static methods.
     if (CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(NewDC))
       if (MD->isStatic())
         return;
+    // Fields are not shadowed by constructor parameters. This allows a popular
+    // pattern where constructor parameters are named after fields.
+    if (isa<CXXConstructorDecl>(NewDC) && isa<ParmVarDecl>(D))
+      return;
+  }
 
   if (VarDecl *shadowedVar = dyn_cast<VarDecl>(ShadowedDecl))
     if (shadowedVar->isExternC()) {
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -9654,10 +9654,42 @@
   S.Diag(Loc, diag::err_typecheck_assign_const) << ExprRange << ConstUnknown;
 }
 
+/// Check if 'E', which is an expression that is about to be modified, refers
+/// to a constructor parameter which shadows a field.
+static void checkForShadowedCtorParameter(Sema &S, Expr *E, SourceLocation Loc) {
+  // Return early before doing lookup if the warning can't happen or would be
+  // ignored.
+  if (!S.getLangOpts().CPlusPlus ||
+      S.Diags.isIgnored(diag::warn_decl_shadow, Loc))
+    return;
+  auto *DRE = dyn_cast<DeclRefExpr>(E);
+  const NamedDecl *D = DRE->getDecl();
+  if (!isa<ParmVarDecl>(D))
+    return;
+  const DeclContext *DC = D->getDeclContext();
+  auto *CD = dyn_cast<CXXConstructorDecl>(DC);
+  if (!CD)
+    return;
+  const CXXRecordDecl *Record = CD->getParent();
+  DeclContextLookupResult Lookup = Record->lookup(D->getDeclName());
+  const NamedDecl *ShadowedDecl =
+      Lookup.size() == 1 ? Lookup.front() : nullptr;
+  // If there is no shadowing or the shadowed decl is not a field, we don't
+  // care. It has already been diagnosed.
+  if (!ShadowedDecl || !isa<FieldDecl>(ShadowedDecl))
+    return;
+  S.Diag(Loc, diag::warn_modifying_shadowed_field) << D << Record;
+  S.Diag(ShadowedDecl->getLocation(), diag::note_member_declared_here)
+      << ShadowedDecl;
+}
+
 /// CheckForModifiableLvalue - Verify that E is a modifiable lvalue.  If not,
 /// emit an error and return true.  If so, return false.
 static bool CheckForModifiableLvalue(Expr *E, SourceLocation Loc, Sema &S) {
   assert(!E->hasPlaceholderType(BuiltinType::PseudoObject));
+
+  checkForShadowedCtorParameter(S, E, Loc);
+
   SourceLocation OrigLoc = Loc;
   Expr::isModifiableLvalueResult IsLV = E->isModifiableLvalue(S.Context,
                                                               &Loc);
@@ -9852,12 +9884,12 @@
     }
 
     if (ConvTy == Compatible) {
+      const Expr *InnerLHS = LHSExpr->IgnoreParenCasts();
+      const DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(InnerLHS);
       if (LHSType.getObjCLifetime() == Qualifiers::OCL_Strong) {
         // Warn about retain cycles where a block captures the LHS, but
         // not if the LHS is a simple variable into which the block is
         // being stored...unless that variable can be captured by reference!
-        const Expr *InnerLHS = LHSExpr->IgnoreParenCasts();
-        const DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(InnerLHS);
         if (!DRE || DRE->getDecl()->hasAttr<BlocksAttr>())
           checkRetainCycles(LHSExpr, RHS.get());
 
Index: test/SemaCXX/warn-shadow.cpp
===================================================================
--- test/SemaCXX/warn-shadow.cpp
+++ test/SemaCXX/warn-shadow.cpp
@@ -29,7 +29,16 @@
 
 class A {
   static int data; // expected-note {{previous declaration}}
-  int field; // expected-note {{previous declaration}}
+  // expected-note@+1 {{previous declaration}}
+  int field; // expected-note 4 {{member 'field' is declared here}}
+
+  // The initialization is safe, but the modifications are not.
+  A(int field) : field(field) {
+    field = 3; // expected-warning {{modifying constructor parameter 'field' that shadows field of 'A'}}
+    field++; // expected-warning {{modifying constructor parameter 'field' that shadows field of 'A'}}
+    --field; // expected-warning {{modifying constructor parameter 'field' that shadows field of 'A'}}
+    field += 2; // expected-warning {{modifying constructor parameter 'field' that shadows field of 'A'}}
+  }
 
   void test() {
     char *field; // expected-warning {{declaration shadows a field of 'A'}}