[gcc r12-4238] c++: NTTP with array/function type after substitution [PR61355]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:fba228e259dd5112851527f2dbb62c5601100985

commit r12-4238-gfba228e259dd5112851527f2dbb62c5601100985
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Oct 7 16:39:16 2021 -0400

    c++: NTTP with array/function type after substitution [PR61355]
    
    We're performing the [temp.param]/10 adjustment at parse time but not
    also at substitution time.
    
            PR c++/61355
    
    gcc/cp/ChangeLog:
    
            * pt.c (convert_template_argument): Perform array/function to
            pointer conversion on the substituted type of an NTTP.
    
    gcc/testsuite/ChangeLog:
    
            * g++.old-deja/g++.pt/nontype5.C: Adjust.
            * g++.dg/template/param6.C: New test.

Diff:
---
 gcc/cp/pt.c                                  |  4 ++++
 gcc/testsuite/g++.dg/template/param6.C       | 32 ++++++++++++++++++++++++++++
 gcc/testsuite/g++.old-deja/g++.pt/nontype5.C |  2 +-
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 1e52aa757e1..009fe6db573 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8530,6 +8530,10 @@ convert_template_argument (tree parm,
       else
 	t = tsubst (t, args, complain, in_decl);
 
+      /* Perform array-to-pointer and function-to-pointer conversion
+	 as per [temp.param]/10.  */
+      t = type_decays_to (t);
+
       if (invalid_nontype_parm_type_p (t, complain))
 	return error_mark_node;
 
diff --git a/gcc/testsuite/g++.dg/template/param6.C b/gcc/testsuite/g++.dg/template/param6.C
new file mode 100644
index 00000000000..8306e753d70
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/param6.C
@@ -0,0 +1,32 @@
+// PR c++/61355
+// Verify we perform array-to-pointer and function-to-pointer conversion
+// on the substituted/deduced type of an NTTP.
+
+int f();
+int p[5];
+
+namespace cpp98 {
+  template<class T, T> struct X;
+  typedef X<int(), f> ty1;
+  typedef X<int[5], p> ty2;
+}
+
+namespace cpp11 {
+#if __cpp_variadic_templates
+  template<class T, T...> struct X;
+  using ty1 = X<int(), f>;
+  using ty2 = X<int[5], p>;
+#endif
+}
+
+namespace cpp17 {
+#if __cpp_nontype_template_parameter_auto
+  template<decltype(auto)> struct X;
+  using ty1 = X<f>;
+  using ty2 = X<p>;
+
+  template<decltype(auto)...> struct Y;
+  using ty3 = Y<f>;
+  using ty4 = Y<p>;
+#endif
+}
diff --git a/gcc/testsuite/g++.old-deja/g++.pt/nontype5.C b/gcc/testsuite/g++.old-deja/g++.pt/nontype5.C
index 2678cf78a7d..e24dca43622 100644
--- a/gcc/testsuite/g++.old-deja/g++.pt/nontype5.C
+++ b/gcc/testsuite/g++.old-deja/g++.pt/nontype5.C
@@ -19,5 +19,5 @@ static int g() { return f(); }
 int f() { return 0; }
 
 int main() {
-return B<int,&f>::g();  // { dg-error "" } could not convert arg
+return B<int,&f>::g();
 }