Re: [PATCH] c++: NTTP with array/function type after substitution [PR61355]

[Jason Merrill via Gcc-patches]

On 10/7/21 09:31, Patrick Palka wrote:

We're implementing [temp.param]/10 at parse time but not also at
substitution time.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

OK.

        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:
        * g++.dg/template/param6.C: New test.
---
  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(-)
  create mode 100644 gcc/testsuite/g++.dg/template/param6.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 5af3a6472f8..170edbd6f1e 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8528,6 +8528,10 @@ 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();
  }