On Mon, 28 Aug 2023, Jason Merrill wrote: > On 8/24/23 09:31, Patrick Palka wrote: > > On Wed, 23 Aug 2023, Jason Merrill wrote: > > > > > On 8/21/23 21:51, Patrick Palka wrote: > > > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look like > > > > a reasonable approach? I didn't observe any compile time/memory impact > > > > of this change. > > > > > > > > -- >8 -- > > > > > > > > As described in detail in the PR, CWG 2369 has the surprising > > > > consequence of introducing constraint recursion in seemingly valid and > > > > innocent code. > > > > > > > > This patch attempts to fix this surpising behavior for the majority > > > > of problematic use cases. Rather than checking satisfaction before > > > > _all_ non-dependent conversions, as specified by the CWG issue, > > > > this patch makes us first check "safe" non-dependent conversions, > > > > then satisfaction, then followed by "unsafe" non-dependent conversions. > > > > In this case, a conversion is "safe" if computing it is guaranteed > > > > to not induce template instantiation. This patch heuristically > > > > determines "safety" by checking for a constructor template or conversion > > > > function template in the (class) parm or arg types respectively. > > > > If neither type has such a member, then computing the conversion > > > > should not induce instantiation (modulo satisfaction checking of > > > > non-template constructor and conversion functions I suppose). > > > > > > > > + /* We're checking only non-instantiating conversions. > > > > + A conversion may instantiate only if it's to/from a > > > > + class type that has a constructor template/conversion > > > > + function template. */ > > > > + tree parm_nonref = non_reference (parm); > > > > + tree type_nonref = non_reference (type); > > > > + > > > > + if (CLASS_TYPE_P (parm_nonref)) > > > > + { > > > > + if (!COMPLETE_TYPE_P (parm_nonref) > > > > + && CLASSTYPE_TEMPLATE_INSTANTIATION (parm_nonref)) > > > > + return unify_success (explain_p); > > > > + > > > > + tree ctors = get_class_binding (parm_nonref, > > > > + complete_ctor_identifier); > > > > + for (tree ctor : lkp_range (ctors)) > > > > + if (TREE_CODE (ctor) == TEMPLATE_DECL) > > > > + return unify_success (explain_p); > > > > > > Today we discussed maybe checking CLASSTYPE_NON_AGGREGATE? > > > > Done; all dups of this PR seem to use tag types that are aggregates, so this > > seems like a good simplification. I also made us punt if the arg type has a > > constrained non-template conversion function. > > > > > > > > Also, instantiation can also happen when checking for conversion to a > > > pointer > > > or reference to base class. > > > > Oops, I suppose we just need to strip pointer types upfront as well. The > > !COMPLETE_TYPE_P && CLASSTYPE_TEMPLATE_INSTANTIATION tests will then make > > sure we deem a potential derived-to-base conversion unsafe if appropriate > > IIUC. > > > > How does the following look? > > > > -- >8 -- > > > > Subject: [PATCH] c++: refine CWG 2369 satisfaction vs non-dep convs > > [PR99599] > > > > PR c++/99599 > > > > gcc/cp/ChangeLog: > > > > * config-lang.in (gtfiles): Add search.cc. > > * pt.cc (check_non_deducible_conversions): Add bool parameter > > passed down to check_non_deducible_conversion. > > (fn_type_unification): Call check_non_deducible_conversions > > an extra time before satisfaction with noninst_only_p=true. > > (check_non_deducible_conversion): Add bool parameter controlling > > whether to compute only conversions that are guaranteed to > > not induce template instantiation. > > * search.cc (conversions_cache): Define. > > (lookup_conversions): Use it to cache the lookup. Improve cache > > rate by considering TYPE_MAIN_VARIANT of the type. > > > > gcc/testsuite/ChangeLog: > > > > * g++.dg/cpp2a/concepts-nondep4.C: New test. > > --- > > gcc/cp/config-lang.in | 1 + > > gcc/cp/pt.cc | 81 +++++++++++++++++-- > > gcc/cp/search.cc | 14 +++- > > gcc/testsuite/g++.dg/cpp2a/concepts-nondep4.C | 21 +++++ > > 4 files changed, 110 insertions(+), 7 deletions(-) > > create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-nondep4.C > > > > @@ -22921,6 +22933,65 @@ check_non_deducible_conversion (tree parm, tree > > arg, unification_kind_t strict, > > { > > bool ok = false; > > tree conv_arg = TYPE_P (arg) ? NULL_TREE : arg; > > + if (conv_p && *conv_p) > > + { > > + /* This conversion was already computed earlier (when > > + computing only non-instantiating conversions). */ > > + gcc_checking_assert (!noninst_only_p); > > + return unify_success (explain_p); > > + } > > + if (noninst_only_p) > > + { > > + /* We're checking only non-instantiating conversions. > > + Computing a conversion may induce template instantiation > > + only if ... */ > > Let's factor this whole block out into another function.
Sounds good. > > Incidentally, CWG1092 is a related problem with defaulted functions, which I > dealt with in a stricter way: when LOOKUP_DEFAULTED we ignore a conversion > from the parameter being copied to a non-reference-related type. As a > follow-on, it might make sense to use this test there as well? Interesting, I can look into that. > > > + tree parm_inner = non_reference (parm); > > + tree type_inner = non_reference (type); > > + bool ptr_conv_p = false; > > + if (TYPE_PTR_P (parm_inner) > > + && TYPE_PTR_P (type_inner)) > > + { > > + parm_inner = TREE_TYPE (parm_inner); > > + type_inner = TREE_TYPE (type_inner); > > + ptr_conv_p = true; > > + } > > I think we also want to set ptr_conv_p if the types are reference_related_p? Ah, because in that case we know the selected conversion will always be a derived-to-base conversion? That sounds like a nice refinement. > > > + /* ... conversion functions are considered and the arg's class > > + type has one that is a template or is constrained. */ > > Maybe just check TYPE_HAS_CONVERSION without digging into the actual > conversions, like with CLASSTYPE_NON_AGGREGATE? > Sounds good, I split out the conversion function caching into a separate patch. Like so? -- >8 -- Subject: [PATCH] c++: refine CWG 2369 satisfaction vs non-dep convs [PR99599] PR c++/99599 gcc/cp/ChangeLog: * pt.cc (check_non_deducible_conversions): Add bool parameter passed down to check_non_deducible_conversion. (fn_type_unification): Call check_non_deducible_conversions an extra time before satisfaction with noninst_only_p=true. (conversion_may_instantiate_p): Define. (check_non_deducible_conversion): Add bool parameter controlling whether to compute only conversions that are guaranteed to not induce template instantiation. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/concepts-nondep4.C: New test. --- gcc/cp/pt.cc | 86 +++++++++++++++++-- gcc/testsuite/g++.dg/cpp2a/concepts-nondep4.C | 21 +++++ 2 files changed, 102 insertions(+), 5 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-nondep4.C diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc index 0790bbf948f..b528ae9a0b8 100644 --- a/gcc/cp/pt.cc +++ b/gcc/cp/pt.cc @@ -151,7 +151,7 @@ static tree get_partial_spec_bindings (tree, tree, tree); static void tsubst_enum (tree, tree, tree); static bool check_instantiated_args (tree, tree, tsubst_flags_t); static int check_non_deducible_conversion (tree, tree, unification_kind_t, int, - struct conversion **, bool); + struct conversion **, bool, bool); static int maybe_adjust_types_for_deduction (tree, unification_kind_t, tree*, tree*, tree); static int type_unification_real (tree, tree, tree, const tree *, @@ -22321,7 +22321,8 @@ pack_deducible_p (tree parm, tree fn) static int check_non_deducible_conversions (tree parms, const tree *args, unsigned nargs, tree fn, unification_kind_t strict, int flags, - struct conversion **convs, bool explain_p) + struct conversion **convs, bool explain_p, + bool noninst_only_p) { /* Non-constructor methods need to leave a conversion for 'this', which isn't included in nargs here. */ @@ -22357,7 +22358,7 @@ check_non_deducible_conversions (tree parms, const tree *args, unsigned nargs, int lflags = conv_flags (ia, nargs, fn, arg, flags); if (check_non_deducible_conversion (parm, arg, strict, lflags, - conv_p, explain_p)) + conv_p, explain_p, noninst_only_p)) return 1; } @@ -22657,6 +22658,16 @@ fn_type_unification (tree fn, deduced: + /* As a refinement of CWG2369, check first and foremost non-dependent + conversions that we know are not going to induce template instantiation + (PR99599). */ + if (strict == DEDUCE_CALL + && incomplete + && check_non_deducible_conversions (parms, args, nargs, fn, strict, flags, + convs, explain_p, + /*noninst_only_p=*/true)) + goto fail; + /* CWG2369: Check satisfaction before non-deducible conversions. */ if (!constraints_satisfied_p (fn, targs)) { @@ -22670,7 +22681,8 @@ fn_type_unification (tree fn, as the standard says that we substitute explicit args immediately. */ if (incomplete && check_non_deducible_conversions (parms, args, nargs, fn, strict, flags, - convs, explain_p)) + convs, explain_p, + /*noninst_only_p=*/false)) goto fail; /* All is well so far. Now, check: @@ -22897,6 +22909,60 @@ maybe_adjust_types_for_deduction (tree tparms, return result; } +/* Return true if computing a conversion from FROM to TO might + induce template instantiation. Conversely, if this predicate + returns false then computing the conversion definitely won't + induce template instantiation. */ + +static bool +conversion_may_instantiate_p (tree to, tree from) +{ + to = non_reference (to); + from = non_reference (from); + + bool ptr_conv_p = false; + if (TYPE_PTR_P (to) + && TYPE_PTR_P (from)) + { + to = TREE_TYPE (to); + from = TREE_TYPE (from); + ptr_conv_p = true; + } + + /* If one of the types is a not-yet-instantiated class template + specialization, then computing a conversion will definitely + perform instantiation in order to consider bases, converting + constructors and/or conversion functions. */ + if ((CLASS_TYPE_P (to) + && !COMPLETE_TYPE_P (to) + && CLASSTYPE_TEMPLATE_INSTANTIATION (to)) + || (CLASS_TYPE_P (from) + && !COMPLETE_TYPE_P (from) + && CLASSTYPE_TEMPLATE_INSTANTIATION (from))) + return true; + + /* Converting from one pointer type to another, or between + reference-related types, always yields a standard conversion. */ + if (ptr_conv_p || reference_related_p (to, from)) + return false; + + /* Converting to a non-aggregate class type will consider its + user-declared constructors, one of which might be a template. */ + if (CLASS_TYPE_P (to) + && CLASSTYPE_NON_AGGREGATE (to)) + return true; + + /* Converting from a class type will consider its conversion functions, + one of which might be a template (or constrained). */ + if (CLASS_TYPE_P (from) + && TYPE_HAS_CONVERSION (from)) + return true; + + /* Otherwise, computing this conversion definitely won't induce + template instantiation. */ + return false; +} + /* Subroutine of fn_type_unification. PARM is a function parameter of a template which doesn't contain any deducible template parameters; check if ARG is a suitable match for it. STRICT, FLAGS and EXPLAIN_P are as in @@ -22905,7 +22971,7 @@ maybe_adjust_types_for_deduction (tree tparms, static int check_non_deducible_conversion (tree parm, tree arg, unification_kind_t strict, int flags, struct conversion **conv_p, - bool explain_p) + bool explain_p, bool noninst_only_p) { tree type; @@ -22927,6 +22993,16 @@ check_non_deducible_conversion (tree parm, tree arg, unification_kind_t strict, { bool ok = false; tree conv_arg = TYPE_P (arg) ? NULL_TREE : arg; + if (conv_p && *conv_p) + { + /* This conversion was already computed earlier (when + computing only non-instantiating conversions). */ + gcc_checking_assert (!noninst_only_p); + return unify_success (explain_p); + } + if (noninst_only_p + && conversion_may_instantiate_p (parm, type)) + return unify_success (explain_p); if (conv_p) /* Avoid recalculating this in add_function_candidate. */ ok = (*conv_p diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-nondep4.C b/gcc/testsuite/g++.dg/cpp2a/concepts-nondep4.C new file mode 100644 index 00000000000..d26783524f7 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-nondep4.C @@ -0,0 +1,21 @@ +// PR c++/99599 +// { dg-do compile { target c++20 } } + +struct foo_tag { }; +struct bar_tag { }; + +template <class T> +concept fooable = requires(T it) { + invoke_tag(foo_tag{}, it); +}; + +template<class T> +void invoke_tag(foo_tag, T in); + +template<fooable T> +void invoke_tag(bar_tag, T it); + +int main() { + invoke_tag(foo_tag{}, 2); + invoke_tag(bar_tag{}, 2); +} -- 2.42.0.122.g1fc548b2d6