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[130.44.146.16]) by smtp.gmail.com with ESMTPSA id v14-20020a0cdd8e000000b0065b29403540sm19510qvk.127.2023.11.14.14.57.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Nov 2023 14:57:21 -0800 (PST) Message-ID: Date: Tue, 14 Nov 2023 17:57:20 -0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] c++: constantness of local var in constexpr fn [PR111703, PR112269] To: Patrick Palka Cc: gcc-patches@gcc.gnu.org References: <20231031181726.3944801-1-ppalka@redhat.com> <805a8e2e-7944-c8f3-164f-a47dfe4de9bc@idea> From: Jason Merrill In-Reply-To: <805a8e2e-7944-c8f3-164f-a47dfe4de9bc@idea> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-12.3 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 11/1/23 11:07, Patrick Palka wrote: > On Tue, 31 Oct 2023, Patrick Palka wrote: > >> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for >> trunk? Does it look OK for release branches as well for sake of PR111703? >> >> -- >8 -- >> >> potential_constant_expression was incorrectly treating most local >> variables from a constexpr function as (potentially) constant because it >> wasn't considering the 'now' parameter. This patch fixes this by >> relaxing some var_in_maybe_constexpr_fn checks accordingly, which turns >> out to partially fix two recently reported regressions: >> >> PR111703 is a regression caused by r11-550-gf65a3299a521a4 for >> restricting constexpr evaluation during warning-dependent folding. >> The mechanism is intended to restrict only constant evaluation of the >> instantiated non-dependent expression, but it also ends up restricting >> constant evaluation (as part of satisfaction) during instantiation of >> the expression, in particular when resolving the ck_rvalue conversion of >> the 'x' argument into a copy constructor call. > > Oops, this analysis is inaccurate for this specific testcase (although > the general idea is the same)... We don't call fold_for_warn on 'f(x)' > but rather on its 'x' argument that has been processed by > convert_arguments into an IMPLICIT_CONV_EXPR. And it's the > instantiation of this IMPLICIT_CONV_EXPR that turns it into a copy > constructor call. There is no ck_rvalue conversion at all here since > 'f' is a function pointer, not an actual function, and so ICSes don't > get computed (IIUC). If 'f' is changed to be an actual function then > there's no issue since build_over_call doesn't perform argument > conversions when in a template context and therefore doesn't call > check_function_arguments on the converted arguments (from which the > problematic fold_for_warn call occurs). Sounds like a we want to adjust a call to a function pointer to compute conversions without performing them in template context? In any case, the patch is OK. >> This seems like a bug in >> the mechanism[1], though I don't know if we want to refine the mechanism >> or get rid of it completely since the original testcases which motivated >> the mechanism are fixed more simply by r13-1225-gb00b95198e6720. In any >> case, this patch partially fixes this by making us correctly treat 'x' >> and therefore 'f(x)' in the below testcase as non-constant, which >> prevents the problematic warning-dependent folding from occurring at >> all. If this bug crops up again then I figure we could decide what to >> do with the mechanism then. >> >> PR112269 is caused by r14-4796-g3e3d73ed5e85e7 for merging tsubst_copy >> into tsubst_copy_and_build. tsubst_copy used to exit early when 'args' >> was empty, behavior which that commit deliberately didn't preserve. >> This early exit masked the fact that COMPLEX_EXPR wasn't handled by >> tsubst at all, and is a tree code that apparently we could see during >> warning-dependent folding on some targets. A complete fix is to add >> handling for this tree code in tsubst_expr, but this patch should fix >> the reported testsuite failures since the situations where COMPLEX_EXPR >> crops up in turn out to not be constant expressions in the >> first place after this patch. >> >> [1]: The mechanism incorrectly assumes that instantiation of the >> non-dependent expression shouldn't induce any template instantiation >> since ahead of time checking of the expression should've already induced >> whatever template instantiation was needed, but in this case although >> overload resolution was performed ahead of time, a ck_rvalue conversion >> gets resolved to a copy constructor call only at instantiation time. >> >> PR c++/111703 >> >> gcc/cp/ChangeLog: >> >> * constexpr.cc (potential_constant_expression_1) : >> Only consider var_in_maybe_constexpr_fn if 'now' is false. >> : Likewise. >> >> gcc/testsuite/ChangeLog: >> >> * g++.dg/cpp2a/concepts-fn8.C: New test. >> --- >> gcc/cp/constexpr.cc | 4 ++-- >> gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C | 24 +++++++++++++++++++++++ >> 2 files changed, 26 insertions(+), 2 deletions(-) >> create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C >> >> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc >> index c05760e6789..8a6b210144a 100644 >> --- a/gcc/cp/constexpr.cc >> +++ b/gcc/cp/constexpr.cc >> @@ -9623,7 +9623,7 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now, >> return RECUR (DECL_VALUE_EXPR (t), rval); >> } >> if (want_rval >> - && !var_in_maybe_constexpr_fn (t) >> + && (now || !var_in_maybe_constexpr_fn (t)) >> && !type_dependent_expression_p (t) >> && !decl_maybe_constant_var_p (t) >> && (strict >> @@ -9737,7 +9737,7 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now, >> STRIP_NOPS (x); >> if (is_this_parameter (x) && !is_capture_proxy (x)) >> { >> - if (!var_in_maybe_constexpr_fn (x)) >> + if (now || !var_in_maybe_constexpr_fn (x)) >> { >> if (flags & tf_error) >> constexpr_error (loc, fundef_p, "use of % in a " >> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C b/gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C >> new file mode 100644 >> index 00000000000..3f63a5b28d7 >> --- /dev/null >> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-fn8.C >> @@ -0,0 +1,24 @@ >> +// PR c++/111703 >> +// { dg-do compile { target c++20 } } >> + >> +template >> +constexpr bool always_true() { return true; } >> + >> +struct P { >> + P() = default; >> + >> + template >> + requires (always_true()) // { dg-bogus "used before its definition" } >> + constexpr P(const T&) { } >> + >> + int n, m; >> +}; >> + >> +void (*f)(P); >> + >> +template >> +constexpr bool g() { >> + P x; >> + f(x); // { dg-bogus "from here" } >> + return true; >> +} >> -- >> 2.42.0.526.g3130c155df >> >> >