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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.
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