* @-patterns as in Haskell. For instance, in a function definition

f = args @ {x, y, z}: ...;

  `args' refers to the argument as a whole, which is further
  pattern-matched against the attribute set pattern {x, y, z}.

tests/lang/eval-okay-patterns.exp

@@ -0,0 +1 @@
+Str("abcxyzDDDDEFgh",[])

tests/lang/eval-okay-patterns.nix

@@ -0,0 +1,16 @@
+let
+
+  f = args@{x, y, z}: x + args.y + z;
+
+  g = {x, y, z}@args: f args;
+
+  h = {x ? "d", y ? x, z ? args.x}@args: x + y + z;
+
+  i = args@args2: args.x + args2.y;
+
+in
+  f {x = "a"; y = "b"; z = "c";} +
+  g {x = "x"; y = "y"; z = "z";} +
+  h {x = "D";} +
+  h {x = "D"; y = "E"; z = "F";} +
+  i {x = "g"; y = "h";}

tests/lang/eval-okay-xml.exp.xml

@@ -4,6 +4,18 @@
     <attr name="a">
       <string value="foo" />
     </attr>
+    <attr name="at">
+      <function>
+        <at>
+          <varpat name="args" />
+          <attrspat>
+            <attr name="x" />
+            <attr name="y" />
+            <attr name="z" />
+          </attrspat>
+        </at>
+      </function>
+    </attr>
     <attr name="b">
       <string value="bar" />
     </attr>

tests/lang/eval-okay-xml.nix

@@ -12,4 +12,6 @@ rec {
 
   id = x: x;
 
+  at = args@{x, y, z}: x;
+
 }

tests/lang/parse-fail-patterns-1.nix

@@ -0,0 +1 @@
+args@{args, x, y, z}: x