[PATCH v2] c++: Fix CTAD for aggregates in template [PR95568]

[Marek Polacek]

On Wed, Jun 24, 2020 at 03:52:12PM -0400, Jason Merrill via Gcc-patches wrote:
> On 6/23/20 6:58 PM, Marek Polacek wrote:
> > 95568 complains that CTAD for aggregates doesn't work within
> > requires-clause and it turned out that it doesn't work when we try
> > the deduction in a template.  The reason is that maybe_aggr_guide
> > creates a guide that can look like this
> > 
> >    template<class T> X(decltype (X<T>::x))-> X<T>
> 
> Then that's the bug; there's no reason the guide should be different just
> because we're trying to do the deduction in a template.
> 
> I'm not sure why I used finish_decltype_type there.

Aha.  Here's a v2 using just TREE_TYPE; in the patch I explain why I didn't
go with unlowered_expr_type.  Which is fully expected since
is_bitfield_expr_with_lowered_type doesn't do anything with FIELD_DECLs.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
95568 complains that CTAD for aggregates doesn't work within
requires-clause and it turned out that it doesn't work when we try
the deduction in a template.  The reason is that maybe_aggr_guide
creates a guide that can look like this

  template<class T> X(decltype (X<T>::x))-> X<T>

where the parameter is a decltype, which is a non-deduced context.  So
the subsequent build_new_function_call fails because unify_one_argument
can't deduce anything from it ([temp.deduct.type]: "If a template
parameter is used only in non-deduced contexts and is not explicitly
specified, template argument deduction fails.")

Those decltypes come from finish_decltype_type.  We can just use
TREE_TYPE instead.  I pondered using unlowered_expr_type, but that
didn't make any difference for the FIELD_DECLs I saw in
class-deduction-aggr6.C.

gcc/cp/ChangeLog:

	PR c++/95568
	* pt.c (collect_ctor_idx_types): Use TREE_TYPE.

gcc/testsuite/ChangeLog:

	PR c++/95568
	* g++.dg/cpp2a/class-deduction-aggr5.C: New test.
	* g++.dg/cpp2a/class-deduction-aggr6.C: New test.
---
 gcc/cp/pt.c                                   |  2 +-
 .../g++.dg/cpp2a/class-deduction-aggr5.C      | 20 ++++++++++++
 .../g++.dg/cpp2a/class-deduction-aggr6.C      | 32 +++++++++++++++++++
 3 files changed, 53 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr6.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 53a64c3a15e..618bf68b2d6 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28329,7 +28329,7 @@ collect_ctor_idx_types (tree ctor, tree list, tree elt = NULL_TREE)
   tree idx, val; unsigned i;
   FOR_EACH_CONSTRUCTOR_ELT (v, i, idx, val)
     {
-      tree ftype = elt ? elt : finish_decltype_type (idx, true, tf_none);
+      tree ftype = elt ? elt : TREE_TYPE (idx);
       if (BRACE_ENCLOSED_INITIALIZER_P (val)
 	  && CONSTRUCTOR_NELTS (val)
 	  /* As in reshape_init_r, a non-aggregate or array-of-dependent-bound
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C
new file mode 100644
index 00000000000..01253f42006
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C
@@ -0,0 +1,20 @@
+// PR c++/95568
+// { dg-do compile { target c++20 } }
+
+template<typename T> struct X { T x; };
+template<typename T, typename U> struct X2 { T x; U y; };
+template<typename T> concept Y = requires { X{0}; };
+
+template<typename T>
+void g()
+{
+  X{0};
+  X2{1, 2.2};
+  Y auto y = X{1};
+}
+
+void
+fn ()
+{
+  g<int>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr6.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr6.C
new file mode 100644
index 00000000000..5332eb93bbe
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr6.C
@@ -0,0 +1,32 @@
+// PR c++/95568
+// { dg-do compile { target c++20 } }
+// CTAD with aggregates containing bit-fields.
+
+template<class, class> struct same_type;
+template<class T> struct same_type<T, T> {};
+
+enum E { e };
+
+template<typename T>
+struct X {
+  T a : 5;
+};
+
+template<typename>
+void g()
+{
+  auto x = X{ 0 };
+  same_type<decltype(x.a), int>();
+  auto x2 = X{ E::e };
+  same_type<decltype(x2.a), E>();
+  auto x3 = X{ false };
+  same_type<decltype(x3.a), bool>();
+  auto x4 = X{ 0u };
+  same_type<decltype(x4.a), unsigned int>();
+}
+
+void
+fn ()
+{
+  g<int>();
+}

base-commit: 77d455ee81ec3a23f8b20259a31ab963716f8e82
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA