Ensure %_{} is precise in guards, closes #15635

Author: josevalim

lib/elixir/lib/module/types/pattern.ex

@@ -777,7 +777,8 @@ defmodule Module.Types.Pattern do
 
     if compatible?(refined, atom()) do
       fields = [__struct__: {refined, false}]
-      of_open_map(args, singleton?(refined), fields, [], path, stack, context)
+      precise? = match?({:_, _, _}, var) or singleton?(refined)
+      of_open_map(args, precise?, fields, [], path, stack, context)
     else
       error = {:badstruct, refined, expr, context}
       {error_type(), false, error(__MODULE__, error, meta, stack, context)}

lib/elixir/test/elixir/module/types/pattern_test.exs

@@ -1414,7 +1414,11 @@ defmodule Module.Types.PatternTest do
     end
 
     test "maps in patterns" do
+      assert precise?([%{}])
       assert precise?([%{ok: _}])
+      assert precise?([%_{}])
+      assert precise?([%x{}])
+      assert precise?([%URI{}])
       assert precise?([%URI{path: _}])
 
       refute precise?([%{ok: 123}])