Here, alias_ctad_tweaks expect tsubst_decl of a FUNCTION_DECL to return a FUNCTION_DECL. A reasonable expectation, but in this case we were replacing the template args of the class-scope deduction guide with equivalent args, so looking in the hash table we found the partial instantiation stored when instantiating A<int>, which is a TEMPLATE_DECL. It's fine for that to be what is stored, but tsubst_function_decl should never return it.
Tested x86_64-pc-linux-gnu, applying to trunk. PR c++/105655 gcc/cp/ChangeLog: * pt.cc (build_template_decl): Add assert. (tsubst_function_decl): Don't return a template. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/class-deduction-alias13.C: New test. --- gcc/cp/pt.cc | 6 ++++- .../g++.dg/cpp2a/class-deduction-alias13.C | 24 +++++++++++++++++++ 2 files changed, 29 insertions(+), 1 deletion(-) create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias13.C diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc index 76913cb1409..021af019840 100644 --- a/gcc/cp/pt.cc +++ b/gcc/cp/pt.cc @@ -5021,6 +5021,8 @@ maybe_update_decl_type (tree orig_type, tree scope) static tree build_template_decl (tree decl, tree parms, bool member_template_p) { + gcc_checking_assert (TREE_CODE (decl) != TEMPLATE_DECL); + tree tmpl = build_lang_decl (TEMPLATE_DECL, DECL_NAME (decl), NULL_TREE); SET_DECL_LANGUAGE (tmpl, DECL_LANGUAGE (decl)); DECL_TEMPLATE_PARMS (tmpl) = parms; @@ -14074,7 +14076,9 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t complain, { hash = hash_tmpl_and_args (gen_tmpl, argvec); if (tree spec = retrieve_specialization (gen_tmpl, argvec, hash)) - return spec; + /* The spec for these args might be a partial instantiation of the + template, but here what we want is the FUNCTION_DECL. */ + return STRIP_TEMPLATE (spec); } } else diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias13.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias13.C new file mode 100644 index 00000000000..0a90a83081b --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias13.C @@ -0,0 +1,24 @@ +// PR c++/105655 +// { dg-do compile { target c++20 } } + +template <class T> +struct A +{ + template <class L, class R> + struct B + { + B(const L & left, const R & right) + {} + }; + + template <class L, class R> + B(const L &, const R &) -> B<L, R>; +}; + +template <class L, class R> +using C = A<int>::B<L, R>; + +int main() +{ + C x{0, 0}; +} base-commit: 850a9ce8bcca59c7efabcdeeca14c5bd905e8363 -- 2.27.0