Re: [PATCH] c++: lvalueness of non-dependent assignment [PR114994]

[Jason Merrill <jason@redhat.com>]

On 5/9/24 16:23, Patrick Palka wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> OK for trunk/14?  For trunk as a follow-up I can implement the
> mentionted representation change to use CALL_EXPR instead of
> MODOP_EXPR for a non-dependent simple assignment expression that
> resolved to an operator= overload.
> 
> -- >8 --
> 
> r14-4111 made us check non-dependent assignment expressions ahead of
> time, as well as give them a type.  Unlike for compound assignment
> expressions however, if a simple assignment resolves to an operator
> overload we still represent it as a (typed) MODOP_EXPR instead of a
> CALL_EXPR to the selected overload.  This, I reckoned, was just a
> pessimization (since we'll have to repeat overload resolution at
> instantiatiation time) but should be harmless.  (And it should be
> easily fixable by giving cp_build_modify_expr an 'overload' parameter).
> 
> But it breaks the below testcase ultimately because MODOP_EXPR (of
> non-reference type) is always treated as an lvalue according to
> lvalue_kind, which is incorrect for the MODOP_EXPR representing x=42.
> 
> We can fix this by representing such assignment expressions as CALL_EXPRs
> matching what that of compound assignments, but that turns out to
> require some tweaking of our -Wparentheses warning logic which seems
> unsuitable for backporting.
> 
> So this patch instead more conservatively fixes this by refining
> lvalue_kind to consider the type of a (simple) MODOP_EXPR as we
> already do for COND_EXPR.
> 
> 	PR c++/114994
> 
> gcc/cp/ChangeLog:
> 
> 	* tree.cc (lvalue_kind) <case MODOP_EXPR>: Consider the
> 	type of a simple assignment expression.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/template/non-dependent32.C: New test.
> ---
>   gcc/cp/tree.cc                                 |  7 +++++++
>   .../g++.dg/template/non-dependent32.C          | 18 ++++++++++++++++++
>   2 files changed, 25 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/template/non-dependent32.C
> 
> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
> index f1a23ffe817..0b97b789aab 100644
> --- a/gcc/cp/tree.cc
> +++ b/gcc/cp/tree.cc
> @@ -275,6 +275,13 @@ lvalue_kind (const_tree ref)
>         /* We expect to see unlowered MODOP_EXPRs only during
>   	 template processing.  */
>         gcc_assert (processing_template_decl);
> +      if (TREE_CODE (TREE_OPERAND (ref, 1)) == NOP_EXPR
> +	  && CLASS_TYPE_P (TREE_TYPE (TREE_OPERAND (ref, 0))))
> +	/* As in the COND_EXPR case, but for non-dependent assignment
> +	   expressions created by build_x_modify_expr.  */
> +	goto default_;

This seems overly specific, I'd think the same thing would apply to += 
and such?

Jason