[clang][Interp] Fix calling invalid function pointers

Checking for isConstexpr() is wrong; we need to (try to) call
the function and let later code diagnose the failure accordingly.

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -2071,8 +2071,7 @@ inline bool CallPtr(InterpState &S, CodePtr OpPC, uint32_t ArgSize) {
   const FunctionPointer &FuncPtr = S.Stk.pop<FunctionPointer>();
 
   const Function *F = FuncPtr.getFunction();
-  if (!F || !F->isConstexpr())
-    return false;
+  assert(F);
 
   assert(ArgSize >= F->getWrittenArgSize());
   uint32_t VarArgSize = ArgSize - F->getWrittenArgSize();

clang/test/AST/Interp/functions.cpp

@@ -187,6 +187,14 @@ namespace FunctionReturnType {
   static_assert(!!op, "");
   constexpr int (*op2)(int, int) = nullptr;
   static_assert(!op2, "");
+
+  int m() { return 5;} // ref-note {{declared here}} \
+                       // expected-note {{declared here}}
+  constexpr int (*invalidFnPtr)() = m;
+  static_assert(invalidFnPtr() == 5, ""); // ref-error {{not an integral constant expression}} \
+                                 // ref-note {{non-constexpr function 'm'}} \
+                                 // expected-error {{not an integral constant expression}} \
+                                 // expected-note {{non-constexpr function 'm'}}
 }
 
 namespace Comparison {