Better error message for when all patterns are exhausted

Author: josevalim

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

@@ -1362,10 +1362,26 @@ defmodule Module.Types.Pattern do
              #{to_quoted_string(type) |> indent(4)}
          """}
 
+      subtype?(type, previous_type) ->
+        {pattern,
+         """
+         the following clause cannot match because the previous clauses already matched all possible values:
+
+             #{expr_to_string(pattern) |> indent(4)} ->
+
+         it attempts to match on the result of:
+
+             #{expr_to_string(expr) |> indent(4)}
+
+         and the following types have already been matched:
+
+             #{to_quoted_string(previous_type) |> indent(4)}
+         """}
+
       true ->
         {pattern,
          """
-         the following clause cannot match because a previous clauses already matched this pattern:
+         the following clause cannot match because previous clauses already matched this pattern:
 
              #{expr_to_string(pattern) |> indent(4)} ->
 

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

@@ -1685,14 +1685,35 @@ defmodule Module.Types.ExprTest do
     end
 
     test "reports error from redundant clauses" do
+      assert typeerror!(
+               [x],
+               case System.get_env(x) do
+                 nil -> 1
+                 b when is_binary(b) -> 2
+                 other -> other
+               end
+             ) == ~l"""
+             the following clause cannot match because the previous clauses already matched all possible values:
+
+                 other ->
+
+             it attempts to match on the result of:
+
+                 System.get_env(x)
+
+             and the following types have already been matched:
+
+                 nil or binary()
+             """
+
       assert typeerror!(
                [x],
                case String.to_atom(x) do
                  :ok -> 1
                  :ok -> 2
                end
              ) == ~l"""
-             the following clause cannot match because a previous clauses already matched this pattern:
+             the following clause cannot match because previous clauses already matched this pattern:
 
                  :ok ->
 
@@ -1708,7 +1729,7 @@ defmodule Module.Types.ExprTest do
                  {x, y} when is_integer(x) and is_integer(y) -> 2
                end
              ) =~ ~l"""
-             the following clause cannot match because a previous clauses already matched this pattern:
+             the following clause cannot match because previous clauses already matched this pattern:
 
                  {x, y} when is_integer(x) and is_integer(y) ->