[PATCH] c++: Fix CTAD from single-element initializer list [PR99103]

[Patrick Palka via Gcc-patches]

When determining whether to rule out initializer-list constructors
during CTAD with a single-element initializer list, the element type's
cv-qualifiers should be irrelevant.  This patch fixes this by making
is_spec_or_derived strip cv-qualifiers from the supplied expression
type.

In passing, I noticed in maybe_aggr_guide we were calling
is_spec_or_derived with swapped arguments.  This led us to prefer the
aggregate deduction candidate over copying deduction in the second
testcase below with -std=c++20.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

gcc/cp/ChangeLog:

        PR c++/99103
        * pt.c (is_spec_or_derived): Drop cv-qualifiers from 'etype'.
        (maybe_aggr_guide): Fix order of arguments to
        is_spec_or_derived.

gcc/testsuite/ChangeLog:

        PR c++/99103
        * g++.dg/cpp1z/class-deduction79.C: New test.
        * g++.dg/cpp1z/class-deduction80.C: New test.
---
 gcc/cp/pt.c                                    |  3 ++-
 gcc/testsuite/g++.dg/cpp1z/class-deduction79.C | 10 ++++++++++
 gcc/testsuite/g++.dg/cpp1z/class-deduction80.C |  9 +++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction79.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction80.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 5102bf02d0f..1acb5b3a097 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28829,6 +28829,7 @@ is_spec_or_derived (tree etype, tree tmpl)
   if (!etype || !CLASS_TYPE_P (etype))
     return false;
 
+  etype = cv_unqualified (etype);
   tree type = TREE_TYPE (tmpl);
   tree tparms = (INNERMOST_TEMPLATE_PARMS
                 (DECL_TEMPLATE_PARMS (tmpl)));
@@ -28859,7 +28860,7 @@ maybe_aggr_guide (tree tmpl, tree init, vec<tree,va_gc> 
*args)
   if (args->length() == 1)
     {
       tree val = (*args)[0];
-      if (is_spec_or_derived (tmpl, TREE_TYPE (val)))
+      if (is_spec_or_derived (TREE_TYPE (val), tmpl))
        return NULL_TREE;
     }
 
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction79.C 
b/gcc/testsuite/g++.dg/cpp1z/class-deduction79.C
new file mode 100644
index 00000000000..ad0ba9bb8f5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction79.C
@@ -0,0 +1,10 @@
+// PR c++/99103
+// { dg-do compile { target c++17 } }
+#include <initializer_list>
+
+template <class T>
+struct S { S(std::initializer_list<T>); };
+
+extern const S<int> x;
+using T = decltype(S{x});
+using T = S<int>; // not S<S<int>>
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction80.C 
b/gcc/testsuite/g++.dg/cpp1z/class-deduction80.C
new file mode 100644
index 00000000000..3dd7cb5890b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction80.C
@@ -0,0 +1,9 @@
+// PR c++/99103
+// { dg-do compile { target c++17 } }
+
+template <class T> struct X { T a; };
+template <class T> struct Y : X<T> {};
+
+extern const Y<int> y;
+using T = decltype(X{y});
+using T = X<int>; // not X<Y<int>>
-- 
2.30.1.489.g328c109303