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

[Patrick Palka]

r8-1253-g3d2e25a240c711 removed the template argument linkage requirement
in convert_nontype_argument for C++17, but we need to also remove the one
in convert_nontype_argument_function for sake of the first and third test
case which we incorrectly reject (in C++17/20 mode).

And in invalid_tparm_referent_p we're inadvertendly rejecting using the
address of a lambda's static op() due to the DECL_ARTIFICIAL check.
This patch relaxes this check for sake of the second test case which we
incorrectly reject (in C++20 mode).

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk and perhaps 13 (since it's a relatively easy/safe fix for a
popular non-regression bug).

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): Disable linkage
	requirement for C++17 and later.
	(invalid_tparm_referent_p): Relax DECL_ARTIFICIAL check for
	the artificial static op() of a lambda.

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.
---
 gcc/cp/pt.cc                                      |  7 +++++--
 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-conv15.C | 11 +++++++++++
 gcc/testsuite/g++.dg/cpp2a/nontype-class56.C      |  8 ++++++++
 gcc/testsuite/g++.dg/ext/visibility/anon8.C       |  4 ++--
 gcc/testsuite/g++.dg/template/function2.C         |  8 ++++++++
 5 files changed, 34 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-conv15.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class56.C
 create mode 100644 gcc/testsuite/g++.dg/template/function2.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 696df2bdd9f..c9b089f8fa7 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -6782,7 +6782,8 @@ 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)
 	{
@@ -7180,7 +7181,9 @@ 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 (DECL_ARTIFICIAL (decl)
+		 /* Accept the artificial static op() of a lambda.  */
+		 && !LAMBDA_TYPE_P (CP_DECL_CONTEXT (decl)))
 	  {
 	    if (complain & tf_error)
 	      error ("the address of %qD is not a valid template argument",
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-conv15.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-conv15.C
new file mode 100644
index 00000000000..cf45e06a33d
--- /dev/null
+++ b/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 } }
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class56.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class56.C
new file mode 100644
index 00000000000..0efd735c8a3
--- /dev/null
+++ b/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;
diff --git a/gcc/testsuite/g++.dg/ext/visibility/anon8.C b/gcc/testsuite/g++.dg/ext/visibility/anon8.C
index b8507497d32..bfcc2d06df6 100644
--- a/gcc/testsuite/g++.dg/ext/visibility/anon8.C
+++ b/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 } } }
diff --git a/gcc/testsuite/g++.dg/template/function2.C b/gcc/testsuite/g++.dg/template/function2.C
new file mode 100644
index 00000000000..ad19f24c1cd
--- /dev/null
+++ b/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 } }
+}
-- 
2.40.1.476.g69c786637d