diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9122,6 +9122,10 @@
   "%select{|member|base class}0 %1 declared here">;
 def note_defaulted_comparison_cannot_deduce_callee : Note<
   "selected 'operator<=>' for %select{|member|base class}0 %1 declared here">;
+def err_defaulted_comparison_cannot_deduce_unsupported_builtin_type : Error<
+  "return type of defaulted 'operator<=>' cannot be deduced because "
+  "three-way comparison for %select{|member|base class}0 %1 "
+  "would compare as builtin type %2 which is not currently supported by clang">;
 def err_incorrect_defaulted_comparison_constexpr : Error<
   "defaulted definition of %select{%sub{select_defaulted_comparison_kind}1|"
   "three-way comparison operator}0 "
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -7868,7 +7868,15 @@
                  "invalid builtin comparison");
           Optional<ComparisonCategoryType> Cat =
               getComparisonCategoryForBuiltinCmp(T);
-          assert(Cat && "no category for builtin comparison?");
+          if (!Cat) {
+            if (Diagnose == NoDiagnostics)
+              S.Diag(
+                  FD->getLocation(),
+                  diag::
+                      err_defaulted_comparison_cannot_deduce_unsupported_builtin_type)
+                  << Subobj.Kind << Subobj.Decl << T;
+            return Result::deleted();
+          }
           R.Category = *Cat;
         }
       }
diff --git a/clang/test/CXX/class/class.compare/class.spaceship/p2.cpp b/clang/test/CXX/class/class.compare/class.spaceship/p2.cpp
--- a/clang/test/CXX/class/class.compare/class.spaceship/p2.cpp
+++ b/clang/test/CXX/class/class.compare/class.spaceship/p2.cpp
@@ -191,4 +191,14 @@
     a2 f;
   };
   std::partial_ordering cmp_b2 = b2() <=> b2();
+
+  struct a3 {
+    using fp = void (*)();
+    operator fp() const;
+  };
+  struct b3 {
+    auto operator<=>(b3 const &) const = default; // expected-error {{cannot be deduced because three-way comparison for member 'f' would compare as builtin type 'void (*)()' which is not currently supported}}
+    // expected-warning@-1 {{implicitly deleted}}
+    a3 f;
+  };
 }