[gcc(refs/vendors/ARM/heads/morello)] cp: Fix ICEs evaluating dynamic_cast in constexpr context

[gcc(refs/vendors/ARM/heads/morello)] cp: Fix ICEs evaluating dynamic_cast in constexpr context

[Alex Coplan]

https://gcc.gnu.org/g:a54686a53cfd2ea7017d3b583877be5c2b91d19d

commit a54686a53cfd2ea7017d3b583877be5c2b91d19d
Author: Alex Coplan <alex.coplan@arm.com>
Date:   Tue Mar 22 11:42:07 2022 +0000

    cp: Fix ICEs evaluating dynamic_cast in constexpr context
    
    cxx_eval_dynamic_cast_fn was returning integer_zero_node in failure
    cases, but __dynamic_cast is required to return a pointer type. The
    calling code was trying to NOP_EXPR the resulting integer to a pointer,
    which is invalid with capabilities. Fixed by changing
    cxx_eval_dynamic_cast_fn to return null_pointer_node in failure cases.
    
    We also need to adjust the CONVERT_EXPR case of
    cxx_eval_constant_expression: with the above change, we now trip a
    diagnostic here that complains we're doing an invalid conversion between
    null pointer types. We suppress this diagnostic in the case where the
    operand being converted was originally a dynamic_cast, since the
    diagnostic we actually want in this case is the one that will later be
    produced by cxx_eval_dynamic_cast_fn.

Diff:
---
 gcc/cp/constexpr.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 5294dc452e9..5fa5d433c5d 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1973,7 +1973,7 @@ cxx_eval_dynamic_cast_fn (const constexpr_ctx *ctx, tree call,
 	      }
 	    *non_constant_p = true;
 	  }
-	return integer_zero_node;
+	return null_pointer_node;
       }
 
   /* [class.cdtor] When a dynamic_cast is used in a constructor ...
@@ -2020,7 +2020,7 @@ cxx_eval_dynamic_cast_fn (const constexpr_ctx *ctx, tree call,
 		}
 	      *non_constant_p = true;
 	    }
-	  return integer_zero_node;
+	  return null_pointer_node;
 	}
       else if (t)
 	/* The result points to the TYPE object.  */
@@ -2050,7 +2050,7 @@ cxx_eval_dynamic_cast_fn (const constexpr_ctx *ctx, tree call,
 	    }
 	  *non_constant_p = true;
 	}
-      return integer_zero_node;
+      return null_pointer_node;
     }
   else
     gcc_assert (obj);
@@ -2076,7 +2076,7 @@ cxx_eval_dynamic_cast_fn (const constexpr_ctx *ctx, tree call,
 	    }
 	  *non_constant_p = true;
 	}
-      return integer_zero_node;
+      return null_pointer_node;
     }
   /* If so, return the TYPE subobject of the most derived object.  */
   obj = convert_to_base_statically (obj, binfo);
@@ -2175,6 +2175,19 @@ cxx_eval_thunk_call (const constexpr_ctx *ctx, tree t, tree thunk_fndecl,
 				       non_constant_p, overflow_p);
 }
 
+static bool
+cxx_dynamic_cast_call_p (tree t)
+{
+  if (TREE_CODE (t) != CALL_EXPR)
+    return false;
+
+  tree fn = get_function_named_in_call (t);
+  if (!fn)
+    return false;
+
+  return cxx_dynamic_cast_fn_p (fn);
+}
+
 /* Subroutine of cxx_eval_constant_expression.
    Evaluate the call expression tree T in the context of OLD_CALL expression
    evaluation.  */
@@ -6318,7 +6331,11 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 		  {
 		    tree from = TREE_TYPE (op);
 
-		    if (!can_convert (type, from, tf_none))
+		    /* Don't diagnose the conversion of a null pointer from
+		       evaluating calls to dynamic_cast: null pointer means the
+		       dynamic_cast failed; this will be diagnosed later.  */
+		    if (!cxx_dynamic_cast_call_p (oldop)
+			&& !can_convert (type, from, tf_none))
 		      {
 			if (!ctx->quiet)
 			  error_at (loc,