c++: improve non-constant template arg diagnostic

A conversation today pointed out that the current diagnostic for this case
doesn't mention the constant evaluation failure, it just says e.g.

"'p' is not a valid template argument for 'int*' because it is not the
address of a variable"

This patch improves the diagnostic in C++17 and above (post-N4268) to
diagnose failed constant-evaluation.

gcc/cp/ChangeLog:

	* pt.cc (convert_nontype_argument_function): Check
	cxx_constant_value on failure.
	(invalid_tparm_referent_p): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/tc1/dr49.C: Adjust diagnostic.
	* g++.dg/template/func2.C: Likewise.
	* g++.dg/cpp1z/nontype8.C: New test.

4 files changed, 45 insertions, 26 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 71ae764..0b7a05c 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -6952,14 +6952,22 @@ convert_nontype_argument_function (tree type, tree expr,
 	{
 	  auto_diagnostic_group d;
 	  location_t loc = cp_expr_loc_or_input_loc (expr);
-	  error_at (loc, "%qE is not a valid template argument for type %qT",
-		    expr, type);
-	  if (TYPE_PTR_P (type))
-	    inform (loc, "it must be the address of a function "
-		    "with external linkage");
+	  tree c;
+	  if (cxx_dialect >= cxx17
+	      && (c = cxx_constant_value (fn),
+		  c == error_mark_node))
+	    ;
 	  else
-	    inform (loc, "it must be the name of a function with "
-		    "external linkage");
+	    {
+	      error_at (loc, "%qE is not a valid template argument for "
+			"type %qT", expr, type);
+	      if (TYPE_PTR_P (type))
+		inform (loc, "it must be the address of a function "
+			"with external linkage");
+	      else
+		inform (loc, "it must be the name of a function with "
+			"external linkage");
+	    }
 	}
       return NULL_TREE;
     }
@@ -7402,22 +7410,22 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
 	/* Null pointer values are OK in C++11.  */;
       else
 	{
-	  if (VAR_P (expr))
-	    {
-	      if (complain & tf_error)
-		error ("%qD is not a valid template argument "
-		       "because %qD is a variable, not the address of "
-		       "a variable", expr, expr);
-	      return true;
-	    }
+	  tree c;
+	  if (!(complain & tf_error))
+	    ;
+	  else if (cxx_dialect >= cxx17
+		   && (c = cxx_constant_value (expr),
+		       c == error_mark_node))
+	    ;
+	  else if (VAR_P (expr))
+	    error ("%qD is not a valid template argument "
+		   "because %qD is a variable, not the address of "
+		   "a variable", expr, expr);
 	  else
-	    {
-	      if (complain & tf_error)
-		error ("%qE is not a valid template argument for %qT "
-		       "because it is not the address of a variable",
-		       expr, type);
-	      return true;
-	    }
+	    error ("%qE is not a valid template argument for %qT "
+		   "because it is not the address of a variable",
+		   expr, type);
+	  return true;
 	}
     }
   return false;
diff --git a/gcc/testsuite/g++.dg/cpp1z/nontype8.C b/gcc/testsuite/g++.dg/cpp1z/nontype8.C
new file mode 100644
index 0000000..b81e85b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nontype8.C
@@ -0,0 +1,12 @@
+// Test that the diagnostic mentions lack of constexpr
+// { dg-do compile { target c++17 } }
+
+template <auto f> void g() {}
+void x()
+{
+  using fp = void (*)();
+  fp f = nullptr;		// { dg-message "constexpr" }
+  g<f>();			// { dg-error "" }
+  int *p = nullptr;		// { dg-message "constexpr" }
+  g<p>();			// { dg-error "" }
+}
diff --git a/gcc/testsuite/g++.dg/tc1/dr49.C b/gcc/testsuite/g++.dg/tc1/dr49.C
index 753d96b..6ddea6b 100644
--- a/gcc/testsuite/g++.dg/tc1/dr49.C
+++ b/gcc/testsuite/g++.dg/tc1/dr49.C
@@ -10,8 +10,8 @@ template struct R<&p>; // OK
 template struct S<&p>; // OK due to parameter adjustment
 
 int *ptr;
-template struct R<ptr>; // { dg-error "argument" }
-template struct S<ptr>; // { dg-error "argument" }
+template struct R<ptr>; // { dg-error "template argument|constant expression" }
+template struct S<ptr>; // { dg-error "template argument|constant expression" }
 
 int v[5];
 template struct R<v>; // OK due to implicit argument conversion
diff --git a/gcc/testsuite/g++.dg/template/func2.C b/gcc/testsuite/g++.dg/template/func2.C
index 0116f23..360f430 100644
--- a/gcc/testsuite/g++.dg/template/func2.C
+++ b/gcc/testsuite/g++.dg/template/func2.C
@@ -4,8 +4,7 @@ typedef void (*fptr)();
 fptr zeroptr = 0;
 template<typename T, fptr F> struct foo { };
 template<typename T> struct foo<T,zeroptr> { };
-// { dg-error "not a valid template argument" "not valid" { target *-*-* } .-1 } 
-// { dg-message "must be the address" "must be the address " { target *-*-* } .-2 }
+// { dg-error "template argument|constant expression" "not valid" { target *-*-* } .-1 } 
 
 // The rest is needed to trigger the ICE in 4.0 to 4.3:
 void f() { }