Improve error message for protocols with no implementation, closes #14364

Author: josevalim

lib/elixir/lib/module/types/apply.ex

@@ -1031,11 +1031,21 @@ defmodule Module.Types.Apply do
 
         Code.ensure_loaded?(mod) and
             Keyword.has_key?(mod.module_info(:attributes), :__protocol__) ->
-          # Protocol errors can be very verbose, so we collapse structs
-          """
-          but expected a type that implements the #{inspect(mod)} protocol, it must be one of:
-          #{clauses_args_to_quoted_string(clauses, converter, collapse_structs: true)}
-          """
+          if function_exported?(mod, :__protocol__, 1) and
+               mod.__protocol__(:impls) == {:consolidated, []} do
+            """
+            but the protocol was not yet implemented for any type and therefore will always fail. \
+            This error typically happens within libraries that define protocols and will disappear as \
+            soon as there is one implementation. If you expect the protocol to be implemented later on, \
+            you can define an implementation specific for development/test.
+            """
+          else
+            # Protocol errors can be very verbose, so we collapse structs
+            """
+            but expected a type that implements the #{inspect(mod)} protocol, it must be one of:
+            #{clauses_args_to_quoted_string(clauses, converter, collapse_structs: true)}
+            """
+          end
 
         true ->
           """

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

@@ -384,7 +384,7 @@ defmodule Module.Types.IntegrationTest do
       assert_no_warnings(files)
     end
 
-    test "mismatched impl" do
+    test "mismatched implementation" do
       files = %{
         "a.ex" => """
         defprotocol Itself do
@@ -420,6 +420,50 @@ defmodule Module.Types.IntegrationTest do
       assert_warnings(files, warnings)
     end
 
+    @tag :require_ast
+    test "no implementation" do
+      files = %{
+        "a.ex" => """
+        defprotocol NoImplProtocol do
+          def callback(data)
+        end
+        """,
+        "b.ex" => """
+        defmodule NoImplProtocol.Caller do
+          def run do
+            NoImplProtocol.callback(:hello)
+          end
+        end
+        """
+      }
+
+      warnings = [
+        """
+            warning: incompatible types given to NoImplProtocol.callback/1:
+
+                NoImplProtocol.callback(:hello)
+
+            given types:
+
+                -:hello-
+
+            but the protocol was not yet implemented for any type and therefore will always fail. \
+        This error typically happens within libraries that define protocols and will disappear as \
+        soon as there is one implementation. If you expect the protocol to be implemented later on, \
+        you can define an implementation specific for development/test.
+
+            typing violation found at:
+            │
+          3 │     NoImplProtocol.callback(:hello)
+            │                    ~
+            │
+            └─ b.ex:3:20: NoImplProtocol.Caller.run/0
+        """
+      ]
+
+      assert_warnings(files, warnings, consolidate_protocols: true)
+    end
+
     @tag :require_ast
     test "String.Chars protocol dispatch" do
       files = %{