On 4/10/21 3:57 PM, Patrick Palka wrote:
Here, in C++17 mode, we only pedwarn about the use of alias CTAD and then later ICE from alias_ctad_tweaks when attempting to add a constraint to one of the guides. Since the construction of the guides of an alias template effectively relies on concepts, we shouldn't be permissive about alias CTAD in C++17 mode, so this patch turns the pertinent pedwarn in do_class_deduction into an error.
Sounds good.
In order to get a consistent diagnostic for B() vs the other forms in the added testcase, I had to remove the special handling of CTAD with empty initializer in build_functional_cast_1 so that we always pass 'complain' to do_auto_deduction.
Did you compare the resulting diagnostics when deduction fails other than for trying to do alias deduction in C++17 mode?
Tested on x86_64-pc-linux-gnu, does this look OK for trunk? gcc/cp/ChangeLog: PR c++/99008 * pt.c (do_class_deduction): Reject alias CTAD in C++17 mode rather than issuing a pedwarn. * typeck2.c (build_functional_cast_1): Handle CTAD uniformly. gcc/testsuite/ChangeLog: PR c++/99008 * g++.dg/parse/template2.C: Adjust expected diagnostic. * g++.dg/template/error8.C: Likewise. * g++.dg/cpp1z/class-deduction84.C: New test. --- gcc/cp/pt.c | 8 ++++---- gcc/cp/typeck2.c | 17 +++-------------- gcc/testsuite/g++.dg/cpp1z/class-deduction84.C | 9 +++++++++ gcc/testsuite/g++.dg/parse/template2.C | 2 +- gcc/testsuite/g++.dg/template/error8.C | 2 +- 5 files changed, 18 insertions(+), 20 deletions(-) create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction84.C diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c index abd1ad4d1a6..40458102304 100644 --- a/gcc/cp/pt.c +++ b/gcc/cp/pt.c @@ -29290,10 +29290,10 @@ do_class_deduction (tree ptype, tree tmpl, tree init, } else if (cxx_dialect < cxx20 && DECL_ALIAS_TEMPLATE_P (tmpl)) { - /* This doesn't affect conforming C++17 code, so just pedwarn. */ - if (complain & tf_warning_or_error) - pedwarn (input_location, 0, "alias template deduction only available " - "with %<-std=c++20%> or %<-std=gnu++20%>"); + if (complain & tf_error) + error ("alias template deduction only available " + "with %<-std=c++20%> or %<-std=gnu++20%>"); + return error_mark_node; }tree type = TREE_TYPE (tmpl);diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c index a58d397ca3f..4e9632f6a7d 100644 --- a/gcc/cp/typeck2.c +++ b/gcc/cp/typeck2.c @@ -2197,24 +2197,13 @@ build_functional_cast_1 (location_t loc, tree exp, tree parms, error_at (loc, "invalid use of %qT", anode); return error_mark_node; } - else if (!parms) + else { - /* Even if there are no parameters, we might be able to deduce from - default template arguments. Pass TF_NONE so that we don't - generate redundant diagnostics. */ - type = do_auto_deduction (type, parms, anode, tf_none, + type = do_auto_deduction (type, parms, anode, complain, adc_variable_type); if (type == error_mark_node) - { - if (complain & tf_error) - error_at (loc, "cannot deduce template arguments " - "for %qT from %<()%>", anode); - return error_mark_node; - } + return error_mark_node; } - else - type = do_auto_deduction (type, parms, anode, complain, - adc_variable_type); }if (processing_template_decl)diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction84.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction84.C new file mode 100644 index 00000000000..29f25e50c9e --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction84.C @@ -0,0 +1,9 @@ +// PR c++/99008 +// { dg-do compile { target c++17 } } + +template <class> struct A { A(int = 0); }; +template <class = void> using B = A<int>; +auto x = B{}; // { dg-error "alias template deduction only available with" "" { target c++17_only } } +auto y = B(); // { dg-error "alias template deduction only available with" "" { target c++17_only } } +auto z = B{1}; // { dg-error "alias template deduction only available with" "" { target c++17_only } } +auto w = B(1); // { dg-error "alias template deduction only available with" "" { target c++17_only } } diff --git a/gcc/testsuite/g++.dg/parse/template2.C b/gcc/testsuite/g++.dg/parse/template2.C index 3cb27a85c00..3cafd9f1f01 100644 --- a/gcc/testsuite/g++.dg/parse/template2.C +++ b/gcc/testsuite/g++.dg/parse/template2.C @@ -3,6 +3,6 @@ namespace N { }int main() {- N::C(); // { dg-error "6:cannot deduce template arguments" "" { target c++17 } } + N::C(); // { dg-error "8:class template argument deduction failed|no match" "" { target c++17 } } // { dg-error "7:missing template arguments" "" { target c++14_down } .-1 } } diff --git a/gcc/testsuite/g++.dg/template/error8.C b/gcc/testsuite/g++.dg/template/error8.C index 6cae360aa16..de1534cc637 100644 --- a/gcc/testsuite/g++.dg/template/error8.C +++ b/gcc/testsuite/g++.dg/template/error8.C @@ -3,6 +3,6 @@ template <typename T> struct S {};void f() {- throw S (); // { dg-error "9:cannot deduce template arguments" "" { target c++17 } } + throw S (); // { dg-error "12:class template argument deduction failed|no match" "" { target c++17 } } // { dg-error "11:missing template arguments" "" { target c++14_down } .-1 } }
