Re: [C++ PATCH] [PR18354] Unary plus as template argument

["Giovanni Bajo" <giovannibajo at libero dot it>]

Roger Sayle <roger@eyesopen.com> wrote:

> Hi Giovanni,
> I suspect a patch something like this might work:
>
> --- 3752,3759 ----
>         {
>           if (!noconvert)
>             arg = default_conversion (arg);
> !         if (real_lvalue_p (arg))
> !           arg = build1 (NON_LVALUE_EXPR, TREE_TYPE (arg), arg);
>         }
>         break;
>
>
> I believe that the real source of the problem is that INTEGER_CSTs
> should never be wrapped in NON_LVALUE_EXPRs,

No, the frontend actually needs this for this testcase:

int n = 0;
+n = 3;

Otherwise, one of the build_modify_expr family function will succeed its test
for lvalueness of the LHS, thus failing to reject this code.

> Both of Joseph's examples will get the NON_LVALUE_EXPR they require,
> but your X<+1> template instantiation should be fixed.


I would like to get rid of NON_LVALUE_EXPR instead, if possible at all. In
build_unary_op, the unary expression comes with a wrapping CONVERT_EXPR of the
same underlying type. This is a nifty trick to make the expression a rvalue
without a specific node. All we have to do is to preserve the CONVERT_EXPR.

Tested on i686-pc-linux-gnu, no new regressions (including the new testcases I
committed this morning). OK for mainline?

Giovanni Bajo


cp/
        PR c++/18354
        * typeck.c (build_unary_op) <CONVERT_EXPR, NEGATE_EXPR>: Unify code.
        Make sure unary plus is a lvalue, but avoid NON_LVALUE_EXPR.

testsuite/
        PR c++/18354
        * g++.dg/template/nontype11.C: New test.

Attachment: patch-pr18354.txt
Description: Text document