diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 3d5823832279..d9946997ba09 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,8 @@
+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
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 3669b247e346..3b88f1520bc8 100644
--- a/gcc/cp/semantics.c
+++ b/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;
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index d22747b1dfed..70d7e8869e18 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+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.
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if33.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-if33.C
new file mode 100644
index 000000000000..e5ef659932b3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if33.C
@@ -0,0 +1,16 @@
+// 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>());
+}