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

[Marek Polacek]

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()'

Tested x86_64-linux, ok for trunk?

2020-01-24  Marek Polacek  <pola...@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/constexpr.c                            | 15 ++++++++++++-
 .../g++.dg/cpp2a/constexpr-dynamic18.C        | 22 +++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic18.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 8e8806345c1..577022e9b9a 100644
--- a/gcc/cp/constexpr.c
+++ b/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
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic18.C 
b/gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic18.C
new file mode 100644
index 00000000000..346f9f56470
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic18.C
@@ -0,0 +1,22 @@
+// 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;
+}

base-commit: 55dd44535d2e4e5703c0103c26e7c51ab8c502c4
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA