diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -8743,6 +8743,10 @@
 def note_inequality_comparison_to_or_assign : Note<
   "use '|=' to turn this inequality comparison into an or-assignment">;
 
+def warn_access_member_of_vector_of_incomplete_type : Warning<
+  "ISO C++ forbids access to member %0 of 'std::vector' with incomplete type %1">,
+  InGroup<DiagGroup<"access-vector-incomplete-member">>, DefaultIgnore;
+
 def err_incomplete_type_used_in_type_trait_expr : Error<
   "incomplete type %0 used in type trait expression">;
 
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -206,6 +206,24 @@
   }
 }
 
+namespace {
+bool IsStdVector(Decl *D) {
+  auto *ND = dyn_cast_or_null<NamedDecl>(D);
+  auto *Name = ND ? ND->getIdentifier() : nullptr;
+  auto *Parent = ND ? ND->getDeclContext() : nullptr;
+  return Name && Name->isStr("vector") && Parent && Parent->isStdNamespace();
+}
+
+QualType ExtractStdVectorTemplateArg(Decl* D) {
+  auto *SD = dyn_cast<ClassTemplateSpecializationDecl>(D);
+  if (!SD) return QualType();
+  if (SD->getTemplateArgs().size() < 1) return QualType();
+  auto &A = SD->getTemplateArgs().get(0);
+  if (A.getKind() != TemplateArgument::Type) return QualType();
+  return A.getAsType();
+}
+} // namespace
+
 /// Determine whether the use of this declaration is valid, and
 /// emit any corresponding diagnostics.
 ///
@@ -392,6 +410,18 @@
     return true;
   }
 
+  // Diagnose access to members of `vector<T>` with incomplete `T`.
+  // [vector.overview]p4
+  // An incomplete type T may be used when instantiating vector if the allocator meets the allocator completeness
+  // requirements (16.5.3.5.1). T shall be complete before any member of the resulting specialization of vector is
+  // referenced.
+  if (auto *Parent = dyn_cast<Decl>(D->getDeclContext()); IsStdVector(Parent)) {
+    auto T = ExtractStdVectorTemplateArg(Parent);
+    if (!T.isNull() && T->isIncompleteType())
+      Diag(Loc, diag::warn_access_member_of_vector_of_incomplete_type)
+          << D << T;
+  }
+
   return false;
 }
 
diff --git a/clang/test/Sema/warn-vector-of-incomplete-access.cpp b/clang/test/Sema/warn-vector-of-incomplete-access.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Sema/warn-vector-of-incomplete-access.cpp
@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 %s -verify -Waccess-vector-incomplete-member
+
+namespace std {
+template <class T> struct vector {
+  using size_type = unsigned;
+
+  vector();
+  ~vector();
+
+  unsigned size();
+};
+}
+
+struct Incomplete;
+std::vector<Incomplete>::size_type x; // expected-warning{{ISO C++ forbids access to member 'size_type' of 'std::vector' with incomplete type 'Incomplete'}}
+std::vector<Incomplete> y; // expected-warning{{ISO C++ forbids access to member 'vector' of 'std::vector' with incomplete type 'Incomplete'}} \
+                              expected-warning{{ISO C++ forbids access to member '~vector' of 'std::vector' with incomplete type 'Incomplete'}} \
+
+// Some tricky usages that should be ok.
+struct Recursive {
+  std::vector<Recursive> x;
+};
+
+struct WithSubtype {
+  struct Subtype {
+    std::vector<WithSubtype> x;
+  };
+  Subtype y;
+  unsigned count = y.x.size();
+
+};