On Wed, Oct 24, 2018 at 05:16:33PM -0400, Jason Merrill wrote:
> On 10/11/18 8:56 PM, Marek Polacek wrote:
> > Here potential_constant_expression_1 rejects the testcase because the body
> > of
> > the for loop calls a non-constexpr function. But the range is empty so the
> > function would never get called.
> > The trick with evaluating the for-condition doesn't work here, because we're
> > dealing with a converted range-based for and can't evaluate
> >
> > __for_begin != __for_end
> >
> > because the constexpr cache doesn't have the values of these two VAR_DECLs.
>
>
> > So either we can use this ugly hack (more specialized), or just not check
> > the
> > body of the loop (more general), similarly to what we do (don't do) with
> > switch.
> >
> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
> >
> > 2018-10-11 Marek Polacek <pola...@redhat.com>
> >
> > PR c++/87594 - constexpr rejects-valid with range-based for.
> > * constexpr.c (potential_constant_expression_1) <case FOR_STMT>: Return
> > true for a converted ange-based for-statement.
> >
> > * g++.dg/cpp1y/constexpr-loop8.C: New test.
> >
> > diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
> > index 4fa8c965a9d..685ca743859 100644
> > --- gcc/cp/constexpr.c
> > +++ gcc/cp/constexpr.c
> > @@ -5827,6 +5827,17 @@ potential_constant_expression_1 (tree t, bool
> > want_rval, bool strict, bool now,
> > tmp = cxx_eval_outermost_constant_expr (tmp, true);
> > if (integer_zerop (tmp))
> > return true;
> > + /* See if this is
> > + __for_begin != __for_end
> > + cp_convert_range_for created for us. If so, this is a converted
> > + range-based for-statement, and we're not able to evaluate this
> > + condition, so we might end up skipping the body entirely. */
> > + else if (TREE_CODE (tmp) == NE_EXPR
> > + && VAR_P (TREE_OPERAND (tmp, 0))
> > + && DECL_NAME (TREE_OPERAND (tmp, 0)) == for_begin_identifier
> > + && VAR_P (TREE_OPERAND (tmp, 1))
> > + && DECL_NAME (TREE_OPERAND (tmp, 1)) == for_end_identifier)
> > + return true;
>
> This seems over-specific; any other condition we can't immediately evaluate
> also might not ever be true. So I think we want to return here unless the
> condition is always true. Likewise for WHILE_STMT.
Yeah, that makes a lot of sense. Thus:
Bootstrapped/regtested on x86_64-linux, ok for trunk?
2018-10-29 Marek Polacek <pola...@redhat.com>
PR c++/87594 - constexpr rejects-valid with range-based for.
* constexpr.c (potential_constant_expression_1): If the condition
can't be evaluated, return true.
* g++.dg/cpp1y/constexpr-loop8.C: New test.
diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 4fa8c965a9d..7692b1727da 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -5825,7 +5825,9 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
{
if (!processing_template_decl)
tmp = cxx_eval_outermost_constant_expr (tmp, true);
- if (integer_zerop (tmp))
+ /* If we couldn't evaluate the condition, it might not ever be
+ true. */
+ if (!integer_onep (tmp))
return true;
}
if (!RECUR (FOR_EXPR (t), any))
@@ -5853,7 +5855,8 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
return false;
if (!processing_template_decl)
tmp = cxx_eval_outermost_constant_expr (tmp, true);
- if (integer_zerop (tmp))
+ /* If we couldn't evaluate the condition, it might not ever be true. */
+ if (!integer_onep (tmp))
return true;
if (!RECUR (WHILE_BODY (t), any))
return false;
diff --git gcc/testsuite/g++.dg/cpp1y/constexpr-loop8.C
gcc/testsuite/g++.dg/cpp1y/constexpr-loop8.C
index e69de29bb2d..bf132f2484e 100644
--- gcc/testsuite/g++.dg/cpp1y/constexpr-loop8.C
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-loop8.C
@@ -0,0 +1,44 @@
+// PR c++/87594
+// { dg-do compile { target c++14 } }
+
+constexpr bool always_false() { return false; }
+int f() { return 1; }
+
+constexpr int
+fn1()
+{
+ struct empty_range {
+ constexpr int* begin() { return 0; }
+ constexpr int* end() { return 0; }
+ } e;
+ for (auto x : e)
+ f();
+ return 0;
+}
+
+constexpr int
+fn2 ()
+{
+ int a[] = { 1, 2, 3 };
+ for (auto x : a)
+ f(); // { dg-error "call to non-.constexpr. function" }
+ return 0;
+}
+
+constexpr int
+fn3 ()
+{
+ __extension__ int a[] = { };
+ for (auto x : a)
+ f();
+ return 0;
+}
+
+
+void
+bar ()
+{
+ constexpr int i1 = fn1 ();
+ constexpr int i2 = fn2 (); // { dg-message "in .constexpr. expansion of " }
+ constexpr int i3 = fn3 ();
+}
