c++: alias CTAD and aggregate deduction cand [PR98832]

During alias CTAD, we're accidentally ignoring the aggregate deduction
candidate for the underlying template because this guide is added
separately via maybe_aggr_guide (which doesn't yet handle alias
templates) instead of via deduction_guides_for (which does).  This patch
makes maybe_aggr_guide handle alias templates in a manner similar to
deduction_guides_for.

	PR c++/98832

gcc/cp/ChangeLog:

	* pt.c (maybe_aggr_guide): Handle alias templates appropriately.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias9.C: New test.

2 files changed, 17 insertions, 0 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 1af8120..f73c747 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28886,6 +28886,8 @@ is_spec_or_derived (tree etype, tree tmpl)
   return !err;
 }
 
+static tree alias_ctad_tweaks (tree, tree);
+
 /* Return a C++20 aggregate deduction candidate for TYPE initialized from
    INIT.  */
 
@@ -28898,6 +28900,15 @@ maybe_aggr_guide (tree tmpl, tree init, vec<tree,va_gc> *args)
   if (init == NULL_TREE)
     return NULL_TREE;
 
+  if (DECL_ALIAS_TEMPLATE_P (tmpl))
+    {
+      tree under = DECL_ORIGINAL_TYPE (DECL_TEMPLATE_RESULT (tmpl));
+      tree tinfo = get_template_info (under);
+      if (tree guide = maybe_aggr_guide (TI_TEMPLATE (tinfo), init, args))
+	return alias_ctad_tweaks (tmpl, guide);
+      return NULL_TREE;
+    }
+
   /* We might be creating a guide for a class member template, e.g.,
 
        template<typename U> struct A {
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias9.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias9.C
new file mode 100644
index 0000000..5cc7b7c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias9.C
@@ -0,0 +1,6 @@
+// PR c++/98832
+// { dg-do compile { target c++20 } }
+
+template<class T, class U> struct X { U u; };
+template<class T> using Y = X<int, T>;
+Y y{0};