c++: Poor diagnostic for dynamic_cast in constexpr context [PR93414]

I neglected to add a proper diagnostic for the reference dynamic_cast
case when the operand of a dynamic_cast doesn't refer to a public base
of Derived, resulting in suboptimal error message

   error: call to non-'constexpr' function 'void* __cxa_bad_cast()'

2020-01-25  Marek Polacek  <polacek@redhat.com>

	PR c++/93414 - poor diagnostic for dynamic_cast in constexpr context.
	* constexpr.c (cxx_eval_dynamic_cast_fn): Add a reference
	dynamic_cast diagnostic.

	* g++.dg/cpp2a/constexpr-dynamic18.C: New test.

gcc/cp/ChangeLog

+2020-01-25  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93414 - poor diagnostic for dynamic_cast in constexpr context.
+	* constexpr.c (cxx_eval_dynamic_cast_fn): Add a reference
+	dynamic_cast diagnostic.
+
 2020-01-24  Jason Merrill  <jason@redhat.com>
 
 	PR c++/93400 - ICE with constrained friend.

gcc/cp/constexpr.c

@@ -1888,7 +1888,20 @@ cxx_eval_dynamic_cast_fn (const constexpr_ctx *ctx, tree call,
   if (tree t = (TREE_CODE (obj) == COMPONENT_REF
 		? TREE_OPERAND (obj, 1) : obj))
     if (TREE_CODE (t) != FIELD_DECL || !DECL_FIELD_IS_BASE (t))
-      return integer_zero_node;
+      {
+	if (reference_p)
+	  {
+	    if (!ctx->quiet)
+	      {
+		error_at (loc, "reference %<dynamic_cast%> failed");
+		inform (loc, "dynamic type %qT of its operand does "
+			"not have a base class of type %qT",
+			objtype, type);
+	      }
+	    *non_constant_p = true;
+	  }
+	return integer_zero_node;
+      }
 
   /* [class.cdtor] When a dynamic_cast is used in a constructor ...
      or in a destructor ... if the operand of the dynamic_cast refers

gcc/testsuite/ChangeLog

+2020-01-25  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93414 - poor diagnostic for dynamic_cast in constexpr context.
+	* g++.dg/cpp2a/constexpr-dynamic18.C: New test.
+
 2020-01-25  Feng Xue  <fxue@os.amperecomputing.com>
 
 	PR ipa/93166

gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic18.C

+// PR c++/93414 - poor diagnostic for dynamic_cast in constexpr context.
+// { dg-do compile { target c++2a } }
+// Here 'b' doesn't point/refer to a public base of Derived.
+
+struct Base {
+    constexpr virtual ~Base(){}
+};
+
+struct Derived: Base {
+    constexpr ~Derived(){}
+};
+
+constexpr const Derived& cast(const Base& b) {
+    return dynamic_cast<const Derived&>(b); // { dg-error "reference .dynamic_cast. failed" }
+// { dg-message "dynamic type .const Base. of its operand does not have a base class of type .Derived." "" { target *-*-* } .-1 }
+}
+
+auto test() {
+    static constexpr Base b;
+    constexpr auto res = cast(b);
+    return res;
+}