[gcc r15-10736] c++: non-dep decltype folding of concept-id C<Ts...> [PR123676]

[Patrick Palka]

https://gcc.gnu.org/g:3f5a4b71b7bdc921e52d9ea65586119847834963

commit r15-10736-g3f5a4b71b7bdc921e52d9ea65586119847834963
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Jan 26 22:00:16 2026 -0500

    c++: non-dep decltype folding of concept-id C<Ts...> [PR123676]
    
    Here since the expression within the decltype C<Ts...> is not instantiation
    dependent (we know its type is bool, and don't care about its value)
    finish_decltype_type instantiates it immediately via the usual tsubst_expr
    with NULL_TREE args.  During which however tsubst_pack_expansion isn't
    prepared to handle such a substitution due to an overly strict assert.
    This patch relaxes the assert accordingly.
    
            PR c++/123676
    
    gcc/cp/ChangeLog:
    
            * pt.cc (tsubst_pack_expansion): Relax unsubsituted_packs
            assert to allow !processing_template_decl when args is
            NULL_TREE.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-decltype5.C: New test.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit 22f51c0f5e62a4e05bccbd0dfb23762ef2eeeff8)

Diff:
---
 gcc/cp/pt.cc                                    |  3 ++-
 gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 5823e9bf7cd5..05018a98dac8 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -13945,7 +13945,8 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 	  /* We can't substitute for this parameter pack.  We use a flag as
 	     well as the missing_level counter because function parameter
 	     packs don't have a level.  */
-	  gcc_assert (processing_template_decl || is_auto (parm_pack));
+	  gcc_assert (processing_template_decl || is_auto (parm_pack)
+		      || args == NULL_TREE);
 	  unsubstituted_packs = true;
 	}
     }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C
new file mode 100644
index 000000000000..1b6f7f78f2c3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype5.C
@@ -0,0 +1,10 @@
+// PR c++/123676
+// { dg-do compile { target c++20 } }
+
+template<class...>
+concept C = true;
+
+template<class... Ts>
+auto f() -> decltype(C<Ts...>) {
+  return true;
+}