Enable use of unquote in defguard(p) (#13716)

lib/elixir/lib/kernel.ex

@@ -5765,8 +5765,11 @@ defmodule Kernel do
                 "only a single when clause is allowed"
 
       {call, impls} ->
-        case Macro.decompose_call(call) do
-          {_name, args} ->
+        case decompose_args(call) do
+          :error ->
+            raise ArgumentError, "invalid syntax in defguard #{Macro.to_string(call)}"
+
+          args ->
             validate_variable_only_args!(call, args)
 
             macro_definition =
@@ -5788,13 +5791,20 @@ defmodule Kernel do
               Elixir.Kernel.@(doc(guard: true))
               unquote(macro_definition)
             end
-
-          _invalid_definition ->
-            raise ArgumentError, "invalid syntax in defguard #{Macro.to_string(call)}"
         end
     end
   end
 
+  defp decompose_args({name, _, args}) when is_atom(name) and is_atom(args), do: []
+
+  defp decompose_args({name, _, args}) when is_atom(name) and is_list(args), do: args
+
+  defp decompose_args({{:unquote, _, _}, _, args}) when is_atom(args), do: []
+
+  defp decompose_args({{:unquote, _, _}, _, args}) when is_list(args), do: args
+
+  defp decompose_args(_), do: :error
+
   defp validate_variable_only_args!(call, args) do
     Enum.each(args, fn
       {ref, _meta, context} when is_atom(ref) and is_atom(context) ->

lib/elixir/test/elixir/kernel/guard_test.exs

@@ -60,6 +60,18 @@ defmodule Kernel.GuardTest do
       refute MacrosInGuards.is_foobar(:baz)
     end
 
+    defmodule UnquotedInGuardCall do
+      @value :foo
+
+      defguard unquote(String.to_atom("is_#{@value}"))(x) when x == unquote(@value)
+    end
+
+    test "guards names can be defined dynamically using unquote" do
+      require UnquotedInGuardCall
+      assert UnquotedInGuardCall.is_foo(:foo)
+      refute UnquotedInGuardCall.is_foo(:bar)
+    end
+
     defmodule GuardsInGuards do
       defguard is_foo(atom) when atom == :foo
       defguard is_foobar(atom) when is_foo(atom) or atom == :bar