PR c++/93324 - ICE with -Wall on constexpr if.

This is a crash with constexpr if, when trying to see if the call in
the if-statement is std::is_constant_evaluated.

cp_get_callee_fndecl_nofold can return NULL_TREE and fndecl_built_in_p
doesn't expect to get a null tree, so check FNDECL first.

gcc/cp/ChangeLog

+2020-01-22  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93324 - ICE with -Wall on constexpr if.
+	* semantics.c (is_std_constant_evaluated_p): Check fndecl.
+
 2020-01-22  Patrick Palka  <ppalka@redhat.com>
 
 	* constraint.cc (get_mapped_args): Avoid using auto_vec

gcc/cp/semantics.c

@@ -734,6 +734,9 @@ is_std_constant_evaluated_p (tree fn)
     return false;
 
   tree fndecl = cp_get_callee_fndecl_nofold (fn);
+  if (fndecl == NULL_TREE)
+    return false;
+
   if (fndecl_built_in_p (fndecl, CP_BUILT_IN_IS_CONSTANT_EVALUATED,
 			 BUILT_IN_FRONTEND))
     return true;

gcc/testsuite/ChangeLog

+2020-01-22  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93324 - ICE with -Wall on constexpr if.
+	* g++.dg/cpp1z/constexpr-if33.C: New test.
+
 2020-01-22  Richard Sandiford  <richard.sandiford@arm.com>
 
 	* gcc.target/aarch64/sve/acle/general/stack_vars_1.c: New test.

gcc/testsuite/g++.dg/cpp1z/constexpr-if33.C

+// PR c++/93324 - ICE with -Wall on constexpr if.
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wall" }
+
+struct {
+  template <int>
+  static constexpr bool a() { return 0; }
+} e;
+
+template <typename>
+void d()
+{
+  auto c(e);
+  using b = decltype(c);
+  if constexpr (b::a<2>());
+}