Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -6778,6 +6778,13 @@
   "taking address of function is not allowed">;
 
 def err_invalid_conversion_between_vector_and_scalar : Error<
+def warn_member_call_in_ctor_init : Warning<
+  "member function call %0 in ctor-initializer for base class %1 results in undefined behavior">,
+  InGroup<DiagGroup<"member-call-in-ctor-init">>, DefaultIgnore;
+def warn_dynamic_cast_in_ctor_init : Warning<
+  "dynamic_cast with this as operand in ctor-initializer for base class %0 results in undefined behavior">,
+  InGroup<DiagGroup<"dynamic-cast-in-ctor-init">>, DefaultIgnore;
+
   "invalid conversion between vector type %0 and scalar type %1">;
 
 // C++ member initializers.
Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -2290,6 +2290,9 @@
     bool InitList;
     FieldDecl *InitListFieldDecl;
     llvm::SmallVector<unsigned, 4> InitFieldIndex;
+    // The base class that is initialized by the current CXXCtorInitializer
+    // or 0 if the current CXXCtorInitializer isn't initializing a base class.
+    const Type *CurrentBaseClass;
 
   public:
     typedef EvaluatedExprVisitor<UninitializedFieldVisitor> Inherited;
@@ -2488,6 +2491,7 @@
 
       Constructor = FieldConstructor;
       InitListExpr *ILE = dyn_cast<InitListExpr>(E);
+      CurrentBaseClass = BaseClass;
 
       if (ILE && Field) {
         InitList = true;
@@ -2534,7 +2538,54 @@
       Inherited::VisitCXXConstructExpr(E);
     }
 
+    void VisitCXXDynamicCastExpr(CXXDynamicCastExpr *E) {
+      // Calling dynamic_cast on the current object before
+      // all base classes are initialized is undefined behavior.
+      // [C++14/17 12.6.2 p16] [C++11 12.6.2 p13]
+
+      // Check that we are currently initializing a base class (which means
+      // that not all base classes are initialized at the moment).
+      if (CurrentBaseClass) {
+        // Simple check if the operand of the dynamic_cast is a this expression
+        // and therefore results in undefined behavior.
+        if (isa<CXXThisExpr>(E->getSubExpr())) {
+          S.Diag(E->getExprLoc(), diag::warn_dynamic_cast_in_ctor_init)
+              << CurrentBaseClass->getAsCXXRecordDecl();
+        }
+      }
+      Inherited::VisitCXXDynamicCastExpr(E);
+    }
+
     void VisitCXXMemberCallExpr(CXXMemberCallExpr *E) {
+      // Calling a member function of the current object before
+      // all base classes are initialized is undefined behavior.
+      // [C++14/17 12.6.2 p16] [C++11 12.6.2 p13]
+
+      // Check that we are currently initializing a base class (which means
+      // that not all base classes are initialized at the moment).
+      if (CurrentBaseClass) {
+
+        // Calling a member function of the current object in this context
+        // is undefined behavior.
+        // As getImplicitObjectArgument() returns 0 for member pointer
+        // calls we use dyn_cast_or_null instead of isa.
+        if (dyn_cast_or_null<CXXThisExpr>(E->getImplicitObjectArgument())) {
+          S.Diag(E->getExprLoc(), diag::warn_member_call_in_ctor_init)
+              << E << CurrentBaseClass->getAsCXXRecordDecl();
+        }
+
+        // Same as above but taking the implicit cast when calling
+        // a member function of a parent class into account.
+        auto ImplicitCast =
+            dyn_cast_or_null<ImplicitCastExpr>(E->getImplicitObjectArgument());
+        if (ImplicitCast) {
+          if (isa<CXXThisExpr>(ImplicitCast->getSubExpr())) {
+            S.Diag(E->getExprLoc(), diag::warn_member_call_in_ctor_init)
+                << E << CurrentBaseClass->getAsCXXRecordDecl();
+          }
+        }
+      }
+
       Expr *Callee = E->getCallee();
       if (isa<MemberExpr>(Callee)) {
         HandleValue(Callee, false /*AddressOf*/);
Index: test/SemaCXX/ctor-init-with-dynamic-cast.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/ctor-init-with-dynamic-cast.cpp
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11 -Wdynamic-cast-in-ctor-init
+
+// Helper class for the following test case.
+class A {
+public:
+  A(A *a) {}
+};
+
+// Calling dynamic cast on this before base class is initialized is undefined
+// behavior.
+class B : public A {
+
+  A *j;
+
+public:
+  B()
+      : A(dynamic_cast<A *>(this) + 1), j(nullptr) { // expected-warning {{dynamic_cast with this as operand in ctor-initializer for base class 'A' results in undefined behavior}}
+  }
+  B(int var) : A(nullptr), j(dynamic_cast<A *>(this)) {} // no-warning
+};
Index: test/SemaCXX/ctor-init-with-member-call-std.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/ctor-init-with-member-call-std.cpp
@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -Wmember-call-in-ctor-init
+
+// Test UB in ctor-initializer with the example from [C++11 12.6.2 p13]
+class A {
+public:
+  A(int);
+};
+
+class B : public A {
+  int j;
+
+public:
+  int f();
+  B()
+      : A(f()),   // expected-warning {{member function call this->f() in ctor-initializer for base class 'A' results in undefined behavior}}
+        j(f()) {} // no-warning
+};
+
+class C {
+public:
+  C(int);
+};
+class D : public B, C {
+  int i;
+
+public:
+  D()
+      : C(f()),   // expected-warning {{member function call this->f() in ctor-initializer for base class 'C' results in undefined behavior}}
+        i(f()) {} // no-warning
+};
Index: test/SemaCXX/ctor-init-with-member-call.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/ctor-init-with-member-call.cpp
@@ -0,0 +1,68 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -Wmember-call-in-ctor-init
+
+// Helper class for the following test cases.
+class A {
+public:
+  A(int i) {}
+};
+
+// Calling member functions before bass class initialized is undefined behavior.
+class B : public A {
+  int member_;
+
+public:
+  B()
+      : A(1 + get_i()) { // expected-warning {{member function call this->get_i() in ctor-initializer for base class 'A' results in undefined behavior}}
+  }
+  B(int var) : A(0), member_(get_i()) {} // no-warning
+
+  int get_i() { return 2; }
+};
+
+// Same as previous test but with explicit this.
+class C : public A {
+public:
+  C()
+      : A(this->get_i() + 1) { // expected-warning {{member function call this->get_i() in ctor-initializer for base class 'A' results in undefined behavior}}
+  }
+
+  int get_i() { return 2; }
+};
+
+// Check that the whole ctor-initializer is checked for member calls.
+class OtherA {
+public:
+  OtherA(int i) {}
+};
+
+class D : public OtherA, public A {
+public:
+  D()
+      : OtherA(this->get_i() + 1), A(this->get_i() + 1) { // expected-warning {{member function call this->get_i() in ctor-initializer for base class 'OtherA' results in undefined behavior}} \
+                                                          // expected-warning {{member function call this->get_i() in ctor-initializer for base class 'A' results in undefined behavior}}
+  }
+
+  int get_i() { return 2; }
+};
+
+// Calling static functions of this class is not undefined behavior.
+class E : public A {
+public:
+  E() : A(this->get_i() + 1) { // no-warning
+  }
+
+  static int get_i() { return 2; }
+};
+
+// Calling other functions of this class is not undefined behavior.
+int other_foo() { return 2; }
+class F : public A {
+public:
+  F() : A(other_foo()) {} // no-warning
+};
+
+// Calling member functions of other classes is not undefined behavior.
+class G : public A {
+public:
+  G(B &b) : A(b.get_i()) {} // no-warning
+};