[clang][Interp] Handle null function pointers

We were instead asserting that they are non-null before.

Author: tbaederr

clang/lib/AST/Interp/FunctionPointer.h

@@ -26,6 +26,7 @@ class FunctionPointer final {
   FunctionPointer(const Function *Func) : Func(Func) { assert(Func); }
 
   const Function *getFunction() const { return Func; }
+  bool isZero() const { return !Func; }
 
   APValue toAPValue() const {
     if (!Func)

clang/lib/AST/Interp/Interp.h

@@ -2087,6 +2087,12 @@ inline bool CallPtr(InterpState &S, CodePtr OpPC, uint32_t ArgSize) {
   const FunctionPointer &FuncPtr = S.Stk.pop<FunctionPointer>();
 
   const Function *F = FuncPtr.getFunction();
+  if (!F) {
+    const Expr *E = S.Current->getExpr(OpPC);
+    S.FFDiag(E, diag::note_constexpr_null_callee)
+        << const_cast<Expr *>(E) << E->getSourceRange();
+    return false;
+  }
   assert(F);
 
   assert(ArgSize >= F->getWrittenArgSize());

clang/test/AST/Interp/functions.cpp

@@ -123,9 +123,12 @@ namespace FunctionPointers {
   }
 
   constexpr int applyBinOp(int a, int b, int (*op)(int, int)) {
-    return op(a, b);
+    return op(a, b); // both-note {{evaluates to a null function pointer}}
   }
   static_assert(applyBinOp(1, 2, add) == 3, "");
+  static_assert(applyBinOp(1, 2, nullptr) == 3, ""); // both-error {{not an integral constant expression}} \
+                                                     // both-note {{in call to}}
+
 
   constexpr int ignoreReturnValue() {
     int (*foo)(int, int) = add;