Re: [PR c++/84596] implicit conv in static assert

[Marek Polacek <polacek at redhat dot com>]

On Wed, Feb 28, 2018 at 02:01:06PM -0500, Jason Merrill wrote:
> On Wed, Feb 28, 2018 at 8:57 AM, Marek Polacek <polacek@redhat.com> wrote:
> > On Wed, Feb 28, 2018 at 09:11:24AM -0300, Alexandre Oliva wrote:
> >> Evaluation of constant expressions, such as those passed to
> >> static_assert, ICEd when encountering IMPLICIT_CONV_EXPRs.
> >>
> >> Handle them like CONVERT_EXPR and NOP_EXPR.
> >>
> >> Regstrapped on x86_64- and i686-linux-gnu.  Ok to install?
> >>
> >> for  gcc/cp/ChangeLog
> >>
> >>       PR c++/84596
> >>       * constexpr.c (cxx_eval_constant_expression): Handle
> >>       IMPLICIT_CONV_EXPR.
> >>
> >> for  gcc/testsuite/ChangeLog
> >>
> >>       PR c++/84596
> >>       * g++.dg/cpp0x/pr84596.C: New.
> >> ---
> >>  gcc/cp/constexpr.c                   |    1 +
> >>  gcc/testsuite/g++.dg/cpp0x/pr84596.C |    7 +++++++
> >>  2 files changed, 8 insertions(+)
> >>  create mode 100644 gcc/testsuite/g++.dg/cpp0x/pr84596.C
> >>
> >> diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> >> index 4bbdbf434877..d38e2d83ae8c 100644
> >> --- a/gcc/cp/constexpr.c
> >> +++ b/gcc/cp/constexpr.c
> >> @@ -4549,6 +4549,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
> >>                                      non_constant_p, overflow_p);
> >>        break;
> >>
> >> +    case IMPLICIT_CONV_EXPR:
> >>      case CONVERT_EXPR:
> >>      case VIEW_CONVERT_EXPR:
> >>      case NOP_EXPR:
> >
> > I don't think it's correct to handle template codes here; they shouldn't
> > have gotten to cxx_eval_constant_expression in the first place.  Usually
> > they're substituted in instantiate_non_dependent_expr_internal e.g. via
> > calling fold_non_dependent_expr.  So I guess we need to find out how
> > IMPLICIT_CONV_EXPR has gotten there.
> 
> Agreed.

Here's my take on this; Alex, sorry for interfering.

The problem is that perform_implicit_conversion_flags in finish_static_assert
produces "implicit_conv_expr <c>" where "c" is a PARM_DECL.  Subsequent
fold_non_dependent_expr is supposed to substitute that implicit_conv_expr but
doesn't because the expression is not is_nondependent_constant_expression --
a PARM_DECL is only considered a potentially constant expression if NOW is
false, which it isn't.  But the require_potential_rvalue_constant_expression
call that follows uses NOW = false.  So I think we need the following.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-03-01  Marek Polacek  <polacek@redhat.com>

	PR c++/84596
	* constexpr.c (require_rvalue_constant_expression): New function.
	* cp-tree.h: Declare it.
	* semantics.c (finish_static_assert): Use it instead of
	require_potential_rvalue_constant_expression.

	* g++.dg/cpp0x/static_assert14.C: New test.

diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 4bbdbf43487..39e6cdfb33d 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -6047,7 +6047,8 @@ potential_rvalue_constant_expression (tree t)
 bool
 require_potential_constant_expression (tree t)
 {
-  return potential_constant_expression_1 (t, false, true, false, tf_warning_or_error);
+  return potential_constant_expression_1 (t, false, true, false,
+					  tf_warning_or_error);
 }
 
 /* Cross product of the above.  */
@@ -6055,7 +6056,17 @@ require_potential_constant_expression (tree t)
 bool
 require_potential_rvalue_constant_expression (tree t)
 {
-  return potential_constant_expression_1 (t, true, true, false, tf_warning_or_error);
+  return potential_constant_expression_1 (t, true, true, false,
+					  tf_warning_or_error);
+}
+
+/* Like above, but don't consider PARM_DECL a potential_constant_expression.  */
+
+bool
+require_rvalue_constant_expression (tree t)
+{
+  return potential_constant_expression_1 (t, true, true, true,
+					  tf_warning_or_error);
 }
 
 /* Like potential_constant_expression, but don't consider possible constexpr
diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
index 743dd340245..17d8c6d2650 100644
--- gcc/cp/cp-tree.h
+++ gcc/cp/cp-tree.h
@@ -7409,6 +7409,7 @@ extern bool is_static_init_expression    (tree);
 extern bool potential_rvalue_constant_expression (tree);
 extern bool require_potential_constant_expression (tree);
 extern bool require_constant_expression (tree);
+extern bool require_rvalue_constant_expression (tree);
 extern bool require_potential_rvalue_constant_expression (tree);
 extern tree cxx_constant_value			(tree, tree = NULL_TREE);
 extern tree cxx_constant_init			(tree, tree = NULL_TREE);
diff --git gcc/cp/semantics.c gcc/cp/semantics.c
index 35569d0cb0d..87c5c669a55 100644
--- gcc/cp/semantics.c
+++ gcc/cp/semantics.c
@@ -8671,7 +8671,7 @@ finish_static_assert (tree condition, tree message, location_t location,
       else if (condition && condition != error_mark_node)
 	{
 	  error ("non-constant condition for static assertion");
-	  if (require_potential_rvalue_constant_expression (condition))
+	  if (require_rvalue_constant_expression (condition))
 	    cxx_constant_value (condition);
 	}
       input_location = saved_loc;
diff --git gcc/testsuite/g++.dg/cpp0x/static_assert14.C gcc/testsuite/g++.dg/cpp0x/static_assert14.C
index e69de29bb2d..ee709f4200f 100644
--- gcc/testsuite/g++.dg/cpp0x/static_assert14.C
+++ gcc/testsuite/g++.dg/cpp0x/static_assert14.C
@@ -0,0 +1,7 @@
+// PR c++/84596
+// { dg-do compile { target c++11 } }
+
+template<int x>
+void b(int c) {
+  static_assert (c, "c"); // { dg-error "non-constant|not a constant" }
+}

	Marek