From d74fe10b13336b9de2e025ced4af00a25ff1d3e7 Mon Sep 17 00:00:00 2001 From: Patrick Palka Date: Fri, 12 Apr 2024 14:52:31 -0400 Subject: [PATCH 1/1] c++: templated substitution into lambda-expr, cont [PR114393] The original PR114393 testcase is unfortunately still not accepted after r14-9938-g081c1e93d56d35 due to return type deduction confusion when a lambda-expr is used as a default template argument. The below reduced testcase demonstrates the bug. Here when forming the dependent specialization b_v we substitute the default argument of F, a lambda-expr, with _Descriptor=U. (In this case in_template_context is true since we're in the context of the template c_v, so we don't defer.) This substitution in turn lowers the level of the lambda's auto return type from 2 to 1 and so later, when instantiating c_v we wrongly substitute this auto with the template argument at level=0,index=0, i.e. int, instead of going through do_auto_deduction which would yield char. One way to fix this would be to use a level-less auto to represent a deduced return type of a lambda, but that might be too invasive of a change at this stage, and it might be better to do this across the board for all deduced return types. Another way would be to pass tf_partial from coerce_template_parms during dependent substitution into a default template argument so that the substitution doesn't do any level-lowering, but that wouldn't do the right thing in this case due to the tf_partial early exit in the LAMBDA_EXPR case of tsubst_expr. Yet another way, and the approach that this patch takes, is to just defer all dependent substitution into a lambda-expr, building upon the logic added in r14-9938-g081c1e93d56d35. This also helps ensure LAMBDA_EXPR_REGEN_INFO consists only of the concrete template arguments that were ultimately substituted into the most general lambda. PR c++/114393 gcc/cp/ChangeLog: * pt.cc (tsubst_lambda_expr): Also defer all dependent substitution. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/lambda-targ2a.C: New test. Reviewed-by: Jason Merrill --- gcc/cp/pt.cc | 9 +++++++-- gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C | 14 ++++++++++++++ 2 files changed, 21 insertions(+), 2 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc index ec259ee0fbf..3b2106dd3f6 100644 --- a/gcc/cp/pt.cc +++ b/gcc/cp/pt.cc @@ -19622,11 +19622,16 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl) in_decl = oldfn; args = add_extra_args (LAMBDA_EXPR_EXTRA_ARGS (t), args, complain, in_decl); - if (processing_template_decl && !in_template_context) + if (processing_template_decl + && (!in_template_context || any_dependent_template_arguments_p (args))) { /* Defer templated substitution into a lambda-expr if we lost the necessary template context. This may happen for a lambda-expr - used as a default template argument. */ + used as a default template argument. + + Defer dependent substitution as well so that we don't prematurely + lower the level of a deduced return type or any other auto or + template parameter belonging to the lambda. */ t = copy_node (t); LAMBDA_EXPR_EXTRA_ARGS (t) = NULL_TREE; LAMBDA_EXPR_EXTRA_ARGS (t) = build_extra_args (t, args, complain); diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C b/gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C new file mode 100644 index 00000000000..7136ce79872 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-targ2a.C @@ -0,0 +1,14 @@ +// PR c++/114393 +// { dg-do compile { target c++20 } } + +template struct c1 {}; + +template +inline constexpr auto b_v = F; + +template +inline constexpr auto c_v = b_v; + +auto f = c_v; +using type = decltype(f()); +using type = char; -- 2.39.2