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

[Jason Merrill via Gcc-patches]

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.

Tested x86_64-pc-linux-gnu, applying to trunk.

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.
---
 gcc/cp/constexpr.c | 7 +++++++
 gcc/cp/decl.c      | 2 +-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 7c27131f506..82a597d7bad 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 947bbfc6637..eed478199ea 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))

base-commit: 267318a2857a42922c3ca033dac4690172b17683
-- 
2.27.0