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From: Jason Merrill <jason@gcc.gnu.org>
To: gcc-cvs@gcc.gnu.org
Subject: [gcc r12-6872] c++: alias template and typename [PR103057]
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https://gcc.gnu.org/g:f3e6ef7d873885ffaa0db7f4185364bbd7a70505

commit r12-6872-gf3e6ef7d873885ffaa0db7f4185364bbd7a70505
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Jan 25 15:12:16 2022 -0500

    c++: alias template and typename [PR103057]
    
    Usually we handle DR1558 substitution near the top of tsubst, but in this
    case while substituting TYPENAME_TYPE we were passing an alias
    specialization to tsubst_aggr_type, which ignored its aliasness.
    
            PR c++/103057
    
    gcc/cp/ChangeLog:
    
            * pt.cc (tsubst_aggr_type): Call tsubst for alias template
            specialization.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp0x/alias-decl-void1.C: New test.

Diff:
---
 gcc/cp/pt.cc                                  |  8 ++++++++
 gcc/testsuite/g++.dg/cpp0x/alias-decl-void1.C | 18 ++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 8f50b9c4d3c..6fbda676527 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -13585,6 +13585,14 @@ tsubst_aggr_type (tree t,
   if (t == NULL_TREE)
     return NULL_TREE;
 
+  /* If T is an alias template specialization, we want to substitute that
+     rather than strip it, especially if it's dependent_alias_template_spec_p.
+     It should be OK not to handle entering_scope in this case, since
+     DECL_CONTEXT will never be an alias template specialization.  We only get
+     here with an alias when tsubst calls us for TYPENAME_TYPE.  */
+  if (alias_template_specialization_p (t, nt_transparent))
+    return tsubst (t, args, complain, in_decl);
+
   switch (TREE_CODE (t))
     {
     case RECORD_TYPE:
diff --git a/gcc/testsuite/g++.dg/cpp0x/alias-decl-void1.C b/gcc/testsuite/g++.dg/cpp0x/alias-decl-void1.C
new file mode 100644
index 00000000000..accc1a45abc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alias-decl-void1.C
@@ -0,0 +1,18 @@
+// PR c++/103057
+// { dg-do compile { target c++11 } }
+
+template <class T> struct A { };
+template <class T> struct B { using type = A<T>; };
+template <class T> struct C {
+  using type = typename T::foo;	// { dg-error "int" }
+};
+template <class T> using L = B<void>;
+
+template <class T>
+typename L<typename C<T>::type>::type
+f(T) { };
+
+int main()
+{
+  f(42);			// { dg-error "no match" }
+}