On 3/21/25 3:58 PM, Patrick Palka wrote:
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for trunk?
OK.
-- >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>;
