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

[Paolo Carlini <paolo dot carlini at oracle dot com>]

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.

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