Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?
-- >8 --
Complex alias templates (and their dependent specializations) use
structural equality because we want to treat them as transparent in some
contexts but not others. Structural-ness however wasn't being preserved
during partial instantiation, which for the below testcase leads to the
checking ICE
same canonical type node for different types 'S<int>::P<U>' and 'pair<int, U>'
when comparing those two types with comparing_dependent_aliases set.
This patch makes us preserve structural-ness for partially instantiated
typedefs in general.
PR c++/119379
gcc/cp/ChangeLog:
* pt.cc (tsubst_decl) <case TYPE_DECL>: Preserve structural-ness
of a partially instantiated typedef.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/class-deduction-alias24.C: New test.
---
gcc/cp/pt.cc | 5 +++++
.../g++.dg/cpp2a/class-deduction-alias24.C | 16 ++++++++++++++++
2 files changed, 21 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias24.C
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 538ff220d74..39c0ee610bb 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -15920,6 +15920,11 @@ tsubst_decl (tree t, tree args, tsubst_flags_t
complain,
if (TYPE_USER_ALIGN (TREE_TYPE (t)))
TREE_TYPE (r) = build_aligned_type (TREE_TYPE (r),
TYPE_ALIGN (TREE_TYPE (t)));
+
+ /* Preserve structural-ness of a partially instantiated typedef. */
+ if (TYPE_STRUCTURAL_EQUALITY_P (TREE_TYPE (t))
+ && dependent_type_p (TREE_TYPE (r)))
+ SET_TYPE_STRUCTURAL_EQUALITY (TREE_TYPE (r));
}
layout_decl (r, 0);
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias24.C
b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias24.C
new file mode 100644
index 00000000000..cceddac0359
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias24.C
@@ -0,0 +1,16 @@
+// PR c++/119379
+// { dg-do compile { target c++20 } }
+
+template<class T, class U>
+struct pair {
+ pair(T, U);
+};
+
+template<class T>
+struct S {
+ template<class U> requires true
+ using P = pair<T, U>;
+};
+
+using type = decltype(S<int>::P(1, 2));
+using type = S<int>::P<int>;
--
2.49.0.rc1.37.ge969bc8759
