c++: converted lambda as template argument [PR83258, ...]

r8-1253-g3d2e25a240c711 removed the template argument linkage requirement
in convert_nontype_argument for C++17 (which r9-3836-g4be5c72cf3ea3e later
factored out into invalid_tparm_referent_p), but we need to also remove
the one in convert_nontype_argument_function for benefit of the first and
third testcase which we currently reject even in C++17/20 mode.

And in invalid_tparm_referent_p we're inadvertendly returning false for
the address of a lambda's static op() since it's DECL_ARTIFICIAL, which
currently causes us to reject the second (C++20) testcase.  But this
DECL_ARTIFICIAL check seems to be relevant only for VAR_DECL, and in fact
this code path was originally reachable only for VAR_DECL until recently
(r13-6970-gb5e38b1c166357).  So this patch restricts the check to VAR_DECL.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

	PR c++/83258
	PR c++/80488
	PR c++/97700

gcc/cp/ChangeLog:

	* pt.cc (convert_nontype_argument_function): Remove linkage
	requirement for C++17 and later.
	(invalid_tparm_referent_p) <case ADDR_EXPR>: Restrict
	DECL_ARTIFICIAL rejection test to VAR_DECL.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/visibility/anon8.C: Don't expect a "no linkage"
	error for the template argument &B2:fn in C++17 mode.
	* g++.dg/cpp0x/lambda/lambda-conv15.C: New test.
	* g++.dg/cpp2a/nontype-class56.C: New test.
	* g++.dg/template/function2.C: New test.

(cherry picked from commit c3afdb8ba8f1839544c414f57e41a58c8fda5349)

gcc/cp/pt.cc

@@ -6780,7 +6780,8 @@ convert_nontype_argument_function (tree type, tree expr,
     }
 
   linkage = decl_linkage (fn_no_ptr);
-  if (cxx_dialect >= cxx11 ? linkage == lk_none : linkage != lk_external)
+  if ((cxx_dialect < cxx11 && linkage != lk_external)
+      || (cxx_dialect < cxx17 && linkage == lk_none))
     {
       if (complain & tf_error)
 	{
@@ -7178,7 +7179,7 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
 	   * a string literal (5.13.5),
 	   * the result of a typeid expression (8.2.8), or
 	   * a predefined __func__ variable (11.4.1).  */
-	else if (DECL_ARTIFICIAL (decl))
+	else if (VAR_P (decl) && DECL_ARTIFICIAL (decl))
 	  {
 	    if (complain & tf_error)
 	      error ("the address of %qD is not a valid template argument",

gcc/testsuite/g++.dg/cpp0x/lambda/lambda-conv15.C

@@ -0,0 +1,11 @@
+// PR c++/83258
+// PR c++/80488
+// { dg-do compile { target c++11 } }
+
+template<void(*)()> struct A { };
+
+int main() {
+  constexpr auto fp = +[]{}; // { dg-error "non-'constexpr' function" "" { target c++14_down } }
+  A<fp> a1;    // { dg-error "not a valid template argument" "" { target c++14_down } }
+  A<[]{}> a2;  // { dg-error "lambda-expression in template-argument|invalid" "" { target c++17_down } }
+}

gcc/testsuite/g++.dg/cpp2a/nontype-class56.C

@@ -0,0 +1,8 @@
+// PR c++/97700
+// { dg-do compile { target c++20 } }
+
+struct S { void (*f)(); };
+
+template<S> struct X { };
+
+X<S{[]{}}> x;

gcc/testsuite/g++.dg/ext/visibility/anon8.C

@@ -2,7 +2,7 @@
 // { dg-do compile }
 
 template <void (*fn) ()>
-void call ()			// { dg-message "note" }
+void call ()			// { dg-message "note" "" { target c++14_down } }
 {
   fn ();
 }
@@ -26,7 +26,7 @@ int main ()
     static void fn2 () {}
   };
   call<&B1::fn1> ();
-  call<&B2::fn2> ();	// { dg-error "linkage|no matching" }
+  call<&B2::fn2> ();	// { dg-error "linkage|no matching" "" { target c++14_down } }
   call<&fn3> ();
   call<&B1::fn4> ();
   call<&fn5> ();	// { dg-error "linkage|no matching" "" { target { ! c++11 } } }

gcc/testsuite/g++.dg/template/function2.C

@@ -0,0 +1,8 @@
+// PR c++/83258
+
+template<void(*)()> struct A { };
+
+int main() {
+  struct B { static void f() { } };
+  A<B::f> a; // { dg-error "linkage" "" { target c++14_down } }
+}