[PATCH 1/2] c++: alias and non-type template parm [PR116223]

[Jason Merrill]

Tested x86_64-pc-linux-gnu, applying to trunk and 14.

-- 8< --

My r14-8291 for PR112632 introduced IMPLICIT_CONV_EXPR_FORCED to express
conversions to the type of an alias template parameter.  In this example,
that broke deduction of X in the call to foo, so let's teach deduction to
look through it.

        PR c++/116223
        PR c++/112632

gcc/cp/ChangeLog:

        * pt.cc (deducible_expression): Also look through
        IMPLICIT_CONV_EXPR_FORCED.
        (unify): Likewise.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp1z/nontype-auto25.C: New test.
---
 gcc/cp/pt.cc                                |  6 +++++-
 gcc/testsuite/g++.dg/cpp1z/nontype-auto25.C | 18 ++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/nontype-auto25.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 35a9c5619f9..cf65b347f6c 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -23031,6 +23031,8 @@ deducible_expression (tree expr)
   /* Strip implicit conversions and implicit INDIRECT_REFs.  */
   while (CONVERT_EXPR_P (expr)
         || TREE_CODE (expr) == VIEW_CONVERT_EXPR
+        || (TREE_CODE (expr) == IMPLICIT_CONV_EXPR
+            && IMPLICIT_CONV_EXPR_FORCED (expr))
         || REFERENCE_REF_P (expr))
     expr = TREE_OPERAND (expr, 0);
   return (TREE_CODE (expr) == TEMPLATE_PARM_INDEX);
@@ -24560,7 +24562,9 @@ unify (tree tparms, tree targs, tree parm, tree arg, 
int strict,
      signedness is the only information lost, and I think that will be
      okay.  VIEW_CONVERT_EXPR can appear with class NTTP, thanks to
      finish_id_expression_1, and are also OK.  */
-  while (CONVERT_EXPR_P (parm) || TREE_CODE (parm) == VIEW_CONVERT_EXPR)
+  while (CONVERT_EXPR_P (parm) || TREE_CODE (parm) == VIEW_CONVERT_EXPR
+        || (TREE_CODE (parm) == IMPLICIT_CONV_EXPR
+            && IMPLICIT_CONV_EXPR_FORCED (parm)))
     parm = TREE_OPERAND (parm, 0);
 
   if (arg == error_mark_node)
diff --git a/gcc/testsuite/g++.dg/cpp1z/nontype-auto25.C 
b/gcc/testsuite/g++.dg/cpp1z/nontype-auto25.C
new file mode 100644
index 00000000000..36b38b48ec2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nontype-auto25.C
@@ -0,0 +1,18 @@
+// PR c++/116223
+// { dg-do compile { target c++17 } }
+
+template <int T> struct A { int value = T; };
+
+template <unsigned char X> using B = A<X>;
+
+template <auto X>
+void foo(B<X>& mat) noexcept
+{
+  //    std::cout << mat.value << "\n";
+}
+
+int main()
+{
+    A<2> mat;
+    foo(mat);
+}

base-commit: 352c21c8a22a48d34cbd2fbfe398ee12c0a1d681
-- 
2.45.2