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

When determining whether to rule out initializer-list constructors
during CTAD with a single-element initializer list (as per P0702), 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.

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.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 3576e0e..0623e92 100644 (file)
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28833,6 +28833,7 @@ 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)));
@@ -28863,7 +28864,7 @@ 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 (file)
index 0000000..ebbe2b2
--- /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 type = decltype(S{x});
+using type = 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 (file)
index 0000000..5ccfc93
--- /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 type = decltype(X{y});
+using type = X<int>; // not X<Y<int>>