Fix handling of namespace-scope undeduced auto decls.

	* decl2.c (decl_dependent_p): New.
	(mark_used): Check it instead of just processing_template_decl.

From-SVN: r274894

gcc/cp/ChangeLog

 2019-08-22  Jason Merrill  <jason@redhat.com>
 
+	* decl2.c (decl_dependent_p): New.
+	(mark_used): Check it instead of just processing_template_decl.
+
+2019-08-22  Jason Merrill  <jason@redhat.com>
+
 	* parser.c (cp_parser_nested_name_specifier_opt): Avoid redundant
 	error.
 

gcc/cp/decl2.c

@@ -5425,6 +5425,25 @@ cp_warn_deprecated_use_scopes (tree scope)
     }
 }
 
+/* True if DECL or its enclosing scope have unbound template parameters.  */
+
+bool
+decl_dependent_p (tree decl)
+{
+  if (DECL_FUNCTION_SCOPE_P (decl)
+      || TREE_CODE (decl) == CONST_DECL
+      || TREE_CODE (decl) == USING_DECL
+      || TREE_CODE (decl) == FIELD_DECL)
+    decl = CP_DECL_CONTEXT (decl);
+  if (tree tinfo = get_template_info (decl))
+    if (any_dependent_template_arguments_p (TI_ARGS (tinfo)))
+      return true;
+  if (LAMBDA_FUNCTION_P (decl)
+      && dependent_type_p (DECL_CONTEXT (decl)))
+    return true;
+  return false;
+}
+
 /* Mark DECL (either a _DECL or a BASELINK) as "used" in the program.
    If DECL is a specialization or implicitly declared class member,
    generate the actual definition.  Return false if something goes
@@ -5451,6 +5470,9 @@ mark_used (tree decl, tsubst_flags_t complain)
       decl = OVL_FIRST (decl);
     }
 
+  if (!DECL_P (decl))
+    return true;
+
   /* Set TREE_USED for the benefit of -Wunused.  */
   TREE_USED (decl) = 1;
   /* And for structured bindings also the underlying decl.  */
@@ -5498,7 +5520,7 @@ mark_used (tree decl, tsubst_flags_t complain)
       || DECL_LANG_SPECIFIC (decl) == NULL
       || DECL_THUNK_P (decl))
     {
-      if (!processing_template_decl
+      if (!decl_dependent_p (decl)
 	  && !require_deduced_type (decl, complain))
 	return false;
       return true;

gcc/testsuite/g++.dg/cpp1z/constexpr-if30.C

+// { dg-do compile { target c++17 } }
+
+auto fn = [](auto i) {
+    if constexpr (sizeof(i) == 1)
+        return fn(123);		// { dg-error "auto" }
+};
+
+int main() {
+    fn('!');
+}