From 8913b2c2bcded39427ff27e6dfc276ae8555f6b8 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Tue, 13 Apr 2021 12:35:33 -0400
Subject: [PATCH] c++: Reject alias CTAD in C++17 [PR99008]
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 constrain
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.
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.
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
for consistent diagnostics.
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 6b63edda60d2..106dfe556f7f 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -29305,10 +29305,10 @@ 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 a58d397ca3fd..4e9632f6a7d4 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 000000000000..29f25e50c9e8
--- /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 3cb27a85c007..3cafd9f1f01a 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 6cae360aa161..de1534cc6373 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 }
}
--
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