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

[Jason Merrill via Gcc-patches]

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 X(decltype (X::x))-> X


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.

Jason

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

[Marek Polacek via Gcc-patches]

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 X(decltype (X::x))-> X

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.")

In such a case we probably want to return PTYPE and try again later like
we do when trying to deduce from a type-dependent expression.  Instead
of playing with build_new_function_call with only tf_decltype or with
uses_deducible_template_parms I'm only checking processing_template_decl
because it's cheaper and because when we arein a template it seems
pretty much guaranteed that the guide will have a DECLTYPE_TYPE
parameter.

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

gcc/cp/ChangeLog:

PR c++/95568
* pt.c (do_class_deduction): When attempting CTAD for aggregates
in a template, return PTYPE.

gcc/testsuite/ChangeLog:

PR c++/95568
* g++.dg/cpp2a/class-deduction-aggr5.C: New test.
---
 gcc/cp/pt.c   |  9 -
 .../g++.dg/cpp2a/class-deduction-aggr5.C  | 20 +++
 2 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr5.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 53a64c3a15e..1f6ad8289bd 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28830,7 +28830,14 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
 }
 
   if (tree guide = maybe_aggr_guide (tmpl, init, args))
-cands = lookup_add (guide, cands);
+{
+  /* In a template, GUIDE's type can have DECLTYPE_TYPE parameters
+and those are non-deduced contexts.  Wait until they have been
+substituted.  */
+  if (processing_template_decl)
+   return ptype;
+  cands = lookup_add (guide, cands);
+}
 
   tree call = error_mark_node;
 
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 000..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 struct X { T x; };
+template struct X2 { T x; U y; };
+template concept Y = requires { X{0}; };
+
+template
+void g()
+{
+  X{0};
+  X2{1, 2.2};
+  Y auto y = X{1};
+}
+
+void
+fn ()
+{
+  g();
+}

base-commit: 6b161257f9f8c7a26b7d119ebc32cbbc54d2e508
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA