Re: [C++ Patch/RFC] PR 82085 ("[6/7/8 Regression] ICE: Template variable reference used in nested template alias")

[Jason Merrill <jason at redhat dot com>]

OK.

On Mon, Oct 30, 2017 at 9:52 AM, Paolo Carlini <paolo.carlini@oracle.com> wrote:
> Hi,
>
> lately I spent quite a bit of time triaging and resolving ICEs and noticed
> this recently filed regression which, in a wicked way, I find somehow funny.
> In short, we are seeing an ICE in tsubst_copy_and_build, when, for an
> INDIRECT_REF, TREE_TYPE (r) is found null:
>
> case INDIRECT_REF:
>    {
>     tree r = RECUR (TREE_OPERAND (t, 0));
>
>     if (REFERENCE_REF_P (t))
>       {
>         /* A type conversion to reference type will be enclosed in
>            such an indirect ref, but the substitution of the cast
>            will have also added such an indirect ref.  */
>         if (TREE_CODE (TREE_TYPE (r)) == REFERENCE_TYPE)
>           r = convert_from_reference (r);
>
> Since Jason's r226642, which fixed a serious issue with multiple Bugzillas,
> we have lookup_template_variable doing:
>
>   /* The type of the expression is NULL_TREE since the template-id could
> refer
>      to an explicit or partial specialization. */
>   tree type = NULL_TREE;
>
> instead of setting type = unknown_type_node and that ultimately leads to the
> ICE for the testcase at issue. Now, I found in a way funny that the body of
> convert_from_reference has:
>
> tree
> convert_from_reference (tree val)
> {
>   if (TREE_TYPE (val)
>       && TREE_CODE (TREE_TYPE (val)) == REFERENCE_TYPE)
>
> thus, if only somebody had cleaned up with no functionality change the above
> bits of tsubst_copy_and_build to unconditionally call convert_from_reference
> we would not have PR82085. Or at least we would not have it in this form,
> maybe we are only uncovering a much deeper issue?!? Anyway, the trivial
> patchlet below certainly passes testing on x86_64-linux. What do you think?
>
> Thanks, Paolo.
>
> /////////////////////
>