[Bug c++/96960] [C++20] ICE in tsubst_copy_and_build, at cp/pt.c:20531 from lambda in return-type-requirement

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96960

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:e52f8ec25c0e58ebd083e8370e2fbc8af4120d87

commit r11-7454-ge52f8ec25c0e58ebd083e8370e2fbc8af4120d87
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Mar 2 07:49:29 2021 -0500

    c++: Fix satisfaction of placeholder type constraints [PR96443]

    This fixes the way we check satisfaction of constraints on placeholder
    types in various deduction contexts, and in particular when the
    constraint is dependent.

    Firstly, when evaluating the return type requirement of a compound
    requirement, we currently substitute the outer template arguments into
    the constraint before checking satisfaction. But we should instead be
    passing in the complete set of template arguments to satisfaction and
    not do a prior separate substitution.  Our current approach leads to us
    incorrectly rejecting the testcase concepts-return-req2.C below.

    Secondly, when checking the constraints on a placeholder variable or
    return type, we don't consider the template arguments of the enclosing
    context at all.  This leads to bogus errors during satisfaction when the
    constraint is dependent as in the testcase concepts-placeholder3.C
    below.

    In order to fix these two issues, we need to be able to normalize the
    constraints on a placeholder 'auto' on demand, which in turn requires us
    to know the template parameters that were in scope where the 'auto' was
    introduced.  This information currently doesn't seem to be easily available
    when we need it, so this patch turns PLACEHOLDER_TYPE_CONSTRAINTS into a
    TREE_LIST whose TREE_PURPOSE additionally holds the value of
    current_template_parms whence a constrained 'auto' was formed.

    This patch also removes some seemingly wrong handling of placeholder
    type arguments from tsubst_parameter_mapping.  The code doesn't trigger
    with the example used in the comments, because type_uses_auto doesn't
    look inside non-deduced contexts such as the operand of decltype.  And
    the call to do_auto_deduction seems confused because if 'arg' is a type,
    then so is 'parm', and therefore 'init' too is a type, but
    do_auto_deduction expects it to be an expression.  Before this patch,
    this code was dead (as far as our testsuite can tell), but now it breaks
    other parts of this patch, so let's remove it.

    gcc/cp/ChangeLog:

            PR c++/96443
            PR c++/96960
            * constraint.cc (type_deducible_p): Don't substitute into the
            constraints, and instead just pass 'args' to do_auto_deduction
            as the outer template arguments.
            (tsubst_parameter_mapping): Remove confused code for handling
            placeholder type arguments.
            (normalize_placeholder_type_constraint): Define.
            (satisfy_constraint_expression): Use it to handle placeholder
            'auto' types.
            * cp-tree.h (PLACEHOLDER_TYPE_CONSTRAINTS_INFO): Define.
            (PLACEHOLDER_TYPE_CONSTRAINTS): Redefine in terms of the above.
            * pt.c (tsubst) <case TEMPLATE_TYPE_PARM>: Use
            PLACEHOLDER_TYPE_CONSTRAINTS_INFO instead.
            (make_constrained_placeholder_type): Set
            PLACEHOLDER_TYPE_CONSTRAINTS_INFO instead.
            (do_auto_deduction): Clarify comments about the outer_targs
            parameter.  Rework satisfaction of a placeholder type constraint
            to pass in the complete set of template arguments directly to
            constraints_satisfied_p.
            (splice_late_return_type): Use PLACEHOLDER_TYPE_CONSTRAINTS_INFO
            instead.  Also rebuild the the constraint info on the new auto.

    gcc/testsuite/ChangeLog:

            PR c++/96443
            PR c++/96960
            * g++.dg/concepts/abbrev9.C: New test.
            * g++.dg/cpp2a/concepts-lambda15.C: New test.
            * g++.dg/cpp2a/concepts-placeholder3.C: New test.
            * g++.dg/cpp2a/concepts-return-req2.C: New test.
            * g++.dg/cpp2a/concepts-ts1.C: Add dg-bogus directive to the
            call to f15 that we expect to accept.