From: Jason Merrill <jason@redhat.com>
Date: Fri, 5 Nov 2021 04:08:53 +0000 (-0400)
Subject: c++: don't do constexpr folding in unevaluated context
X-Git-Tag: basepoints/gcc-13~3060
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4df7f8c79835d56928f51f9e674d326300936e8e;p=thirdparty%2Fgcc.git

c++: don't do constexpr folding in unevaluated context

The implicit constexpr patch revealed that we were doing constant evaluation
of arbitrary expressions in unevaluated contexts, leading to failure when we
tried to evaluate e.g. a call to declval.  This is wrong more generally;
only manifestly-constant-evaluated expressions should be evaluated within
an unevaluated operand.

Making this change revealed a case we were failing to mark as manifestly
constant-evaluated.

gcc/cp/ChangeLog:

	* constexpr.c (maybe_constant_value): Don't evaluate
	in an unevaluated operand unless manifestly const-evaluated.
	(fold_non_dependent_expr_template): Likewise.
	* decl.c (compute_array_index_type_loc): This context is
	manifestly constant-evaluated.
---

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 7c27131f5063..82a597d7bad6 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -7696,6 +7696,10 @@ maybe_constant_value (tree t, tree decl, bool manifestly_const_eval)
       return r;
     }
 
+  /* Don't evaluate an unevaluated operand.  */
+  if (cp_unevaluated_operand)
+    return t;
+
   uid_sensitive_constexpr_evaluation_checker c;
   r = cxx_eval_outermost_constant_expr (t, true, true, false, false, decl);
   gcc_checking_assert (r == t
@@ -7759,6 +7763,9 @@ fold_non_dependent_expr_template (tree t, tsubst_flags_t complain,
 	  return t;
 	}
 
+      if (cp_unevaluated_operand && !manifestly_const_eval)
+	return t;
+
       tree r = cxx_eval_outermost_constant_expr (t, true, true,
 						 manifestly_const_eval,
 						 false, object);
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 947bbfc66377..eed478199ea5 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -11000,7 +11000,7 @@ compute_array_index_type_loc (location_t name_loc, tree name, tree size,
 				    cp_convert (ssizetype, integer_one_node,
 						complain),
 				    complain);
-	itype = maybe_constant_value (itype);
+	itype = maybe_constant_value (itype, NULL_TREE, true);
       }
 
       if (!TREE_CONSTANT (itype))