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c++, tree: walk TREE_VEC (and VECTOR_CST) in natural order [PR101886]
Unfortunately the extract_autos_r fix in r13-4799-ga7c8036b26082d is derailed by the fact that walk_tree_1 currently walks the elements of a TREE_VEC in reverse, which means for A<auto, auto> in the below testcase extract_autos_r ends up adjusting the TEMPLATE_TYPE_IDX of the first auto rather than the second one, and the first auto is the canonical auto of level 2 and index 0, so we rightfully trigger the sanity check added in that commit. It seems TREE_VEC and VECTOR_CST are the only trees that we walk in reverse, and this has been the case since r21884 whence the original version of walk_tree_1 was introduced. But that's arguably unnatural and inconsistent with how we walk other compound trees such as CONSTRUCTORs and EXPR_P trees. So this patch makes walk_tree_1 walk these trees in forward order, which fixes the below testcase. This in turn revealed that keep_template_parm builds up the list of found template parameters in reverse, which previously compensated for the TREE_VEC behavior, but now we should be building the list in the natural order as well for sake of pretty printing parameter mappings. Bootstrapped and regtested on x86_64-pc-linux-gnu with --enable-languages=all,ada,go does this look OK for trunk? PR c++/101886 gcc/cp/ChangeLog: * pt.cc (find_template_parameter_info::parm_list_tail): New data member. (keep_template_parm): Use parm_list_tail to append rather than prepend to parm_list. gcc/ChangeLog: * tree.cc (walk_tree_1) <case TREE_VEC>: Walk the elements in the natural order. <case VECTOR_CST>: Likewise. gcc/testsuite/ChangeLog: * g++.dg/concepts/diagnostic12.C: Adjust expected order of template parameters within parameter mapping. * g++.dg/concepts/auto6.C: New test.
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