Index: clang/lib/AST/Interp/FunctionPointer.h
===================================================================
--- clang/lib/AST/Interp/FunctionPointer.h
+++ clang/lib/AST/Interp/FunctionPointer.h
@@ -6,6 +6,7 @@
 #include "Function.h"
 #include "Primitives.h"
 #include "clang/AST/APValue.h"
+#include "clang/AST/ASTContext.h"
 
 namespace clang {
 namespace interp {
@@ -38,6 +39,13 @@
     OS << ")";
   }
 
+  QualType getType(const ASTContext &Ctx) const {
+    if (!Func)
+      return Ctx.NullPtrTy;
+
+    return Func->getDecl()->getType();
+  }
+
   ComparisonCategoryResult compare(const FunctionPointer &RHS) const {
     if (Func == RHS.Func)
       return ComparisonCategoryResult::Equal;
Index: clang/lib/AST/Interp/Interp.h
===================================================================
--- clang/lib/AST/Interp/Interp.h
+++ clang/lib/AST/Interp/Interp.h
@@ -637,6 +637,32 @@
   return CmpHelper<T>(S, OpPC, Fn);
 }
 
+/// Function pointers cannot be compared in an ordered way.
+template <>
+inline bool CmpHelper<FunctionPointer>(InterpState &S, CodePtr OpPC,
+                                       CompareFn Fn) {
+  const auto &RHS = S.Stk.pop<FunctionPointer>();
+  const auto &LHS = S.Stk.pop<FunctionPointer>();
+
+  std::string LS =
+      LHS.toAPValue().getAsString(S.getCtx(), LHS.getType(S.getCtx()));
+  std::string RS =
+      RHS.toAPValue().getAsString(S.getCtx(), RHS.getType(S.getCtx()));
+  const SourceInfo &Loc = S.Current->getSource(OpPC);
+  S.FFDiag(Loc, diag::note_constexpr_pointer_comparison_unspecified)
+      << LS << RS;
+  return false;
+}
+
+template <>
+inline bool CmpHelperEQ<FunctionPointer>(InterpState &S, CodePtr OpPC,
+                                         CompareFn Fn) {
+  const auto &RHS = S.Stk.pop<FunctionPointer>();
+  const auto &LHS = S.Stk.pop<FunctionPointer>();
+  S.Stk.push<Boolean>(Boolean::from(Fn(LHS.compare(RHS))));
+  return true;
+}
+
 template <>
 inline bool CmpHelper<Pointer>(InterpState &S, CodePtr OpPC, CompareFn Fn) {
   using BoolT = PrimConv<PT_Bool>::T;
Index: clang/lib/AST/Interp/Opcodes.td
===================================================================
--- clang/lib/AST/Interp/Opcodes.td
+++ clang/lib/AST/Interp/Opcodes.td
@@ -94,7 +94,7 @@
 }
 
 def ComparableTypeClass : TypeClass {
-  let Types = !listconcat(AluTypeClass.Types, [Ptr], [Float]);
+  let Types = !listconcat(AluTypeClass.Types, [Ptr], [Float], [FnPtr]);
 }
 
 class SingletonTypeClass<Type Ty> : TypeClass {
Index: clang/test/AST/Interp/functions.cpp
===================================================================
--- clang/test/AST/Interp/functions.cpp
+++ clang/test/AST/Interp/functions.cpp
@@ -178,6 +178,31 @@
   static_assert(s.fp == nullptr, ""); // zero-initialized function pointer.
 }
 
+namespace Comparison {
+  void f(), g();
+  constexpr void (*pf)() = &f, (*pg)() = &g;
+
+  constexpr bool u13 = pf < pg; // ref-warning {{ordered comparison of function pointers}} \
+                                // ref-error {{must be initialized by a constant expression}} \
+                                // ref-note {{comparison between '&f' and '&g' has unspecified value}} \
+                                // expected-warning {{ordered comparison of function pointers}} \
+                                // expected-error {{must be initialized by a constant expression}} \
+                                // expected-note {{comparison between '&f' and '&g' has unspecified value}}
+
+  constexpr bool u14 = pf < (void(*)())nullptr; // ref-warning {{ordered comparison of function pointers}} \
+                                                // ref-error {{must be initialized by a constant expression}} \
+                                                // ref-note {{comparison between '&f' and 'nullptr' has unspecified value}} \
+                                                // expected-warning {{ordered comparison of function pointers}} \
+                                                // expected-error {{must be initialized by a constant expression}} \
+                                                // expected-note {{comparison between '&f' and 'nullptr' has unspecified value}}
+
+
+
+  static_assert(pf != pg, "");
+  static_assert(pf == &f, "");
+  static_assert(pg == &g, "");
+}
+
 }
 
 struct F {