Refactor variable tracking in patterns to prepare for guards (#15026)

Author: josevalim

lib/elixir/lib/module/types/descr.ex

@@ -2099,9 +2099,9 @@ defmodule Module.Types.Descr do
   defp list_hd_static(%{}), do: none()
 
   @doc """
-   Returns the tail of a list.
+  Returns the tail of a list.
 
-   For a `non_empty_list(t)`, the tail type is `list(t)`.
+  For a `non_empty_list(t)`, the tail type is `list(t)`.
   For an improper list `non_empty_list(t, s)`, the tail type is
   `list(t, s) or s` (either the rest of the list or the terminator)
   """

lib/elixir/lib/module/types/expr.ex

@@ -580,6 +580,7 @@ defmodule Module.Types.Expr do
         _ ->
           expected = if structs == [], do: @exception, else: Enum.reduce(structs, &union/2)
           expr = {:__block__, [type_check: info], [expr]}
+          context = Of.declare_var(var, context)
           {_ok?, _type, context} = Of.refine_head_var(var, expected, expr, stack, context)
           context
       end

lib/elixir/lib/module/types/helpers.ex

@@ -148,7 +148,7 @@ defmodule Module.Types.Helpers do
           version = meta[:version]
 
           case vars do
-            %{^version => %{off_traces: off_traces, name: name, context: context}} ->
+            %{^version => %{off_traces: [_ | _] = off_traces, name: name, context: context}} ->
               {:ok,
                Map.put(versions, version, %{
                  type: :variable,

lib/elixir/lib/module/types/of.ex

@@ -30,19 +30,40 @@ defmodule Module.Types.Of do
 
   @doc """
   Marks a variable with error.
+
+  This purposedly deletes all traces of the variable,
+  as it is often invoked when the cause for error is elsewhere.
+  """
+  def error_var({_var_name, meta, _var_context}, context) do
+    version = Keyword.fetch!(meta, :version)
+
+    update_in(context.vars[version], fn
+      %{errored: true} = data -> data
+      data -> Map.put(%{data | type: error_type(), off_traces: []}, :errored, true)
+    end)
+  end
+
+  @doc """
+  Declares a variable.
   """
-  def error_var(var, context) do
+  def declare_var(var, context) do
     {var_name, meta, var_context} = var
     version = Keyword.fetch!(meta, :version)
 
-    data = %{
-      type: error_type(),
-      name: var_name,
-      context: var_context,
-      off_traces: []
-    }
+    case context.vars do
+      %{^version => _} ->
+        context
+
+      vars ->
+        data = %{
+          type: term(),
+          name: var_name,
+          context: var_context,
+          off_traces: []
+        }
 
-    put_in(context.vars[version], data)
+        %{context | vars: Map.put(vars, version, data)}
+    end
   end
 
   @doc """
@@ -56,7 +77,7 @@ defmodule Module.Types.Of do
     version = Keyword.fetch!(meta, :version)
     %{vars: %{^version => %{type: old_type, off_traces: off_traces} = data} = vars} = context
 
-    if gradual?(old_type) and type not in [term(), dynamic()] do
+    if gradual?(old_type) and type not in [term(), dynamic()] and not is_map_key(data, :errored) do
       case compatible_intersection(old_type, type) do
         {:ok, new_type} when new_type != old_type ->
           data = %{
@@ -82,11 +103,13 @@ defmodule Module.Types.Of do
   because we want to refine types. Otherwise we should
   use compatibility.
   """
-  def refine_head_var(var, type, expr, stack, context) do
-    {var_name, meta, var_context} = var
+  def refine_head_var({_, meta, _}, type, expr, stack, context) do
     version = Keyword.fetch!(meta, :version)
 
     case context.vars do
+      %{^version => %{errored: true}} ->
+        {:ok, error_type(), context}
+
       %{^version => %{type: old_type, off_traces: off_traces} = data} = vars ->
         new_type = intersection(type, old_type)
 
@@ -96,26 +119,14 @@ defmodule Module.Types.Of do
             off_traces: new_trace(expr, type, stack, off_traces)
         }
 
-        context = %{context | vars: %{vars | version => data}}
-
-        # We need to return error otherwise it leads to cascading errors
         if empty?(new_type) do
-          {:error, error_type(),
-           error({:refine_head_var, old_type, type, var, context}, meta, stack, context)}
+          data = Map.put(%{data | type: error_type()}, :errored, true)
+          context = %{context | vars: %{vars | version => data}}
+          {:error, old_type, context}
         else
+          context = %{context | vars: %{vars | version => data}}
           {:ok, new_type, context}
         end
-
-      %{} = vars ->
-        data = %{
-          type: type,
-          name: var_name,
-          context: var_context,
-          off_traces: new_trace(expr, type, stack, [])
-        }
-
-        context = %{context | vars: Map.put(vars, version, data)}
-        {:ok, type, context}
     end
   end
 
@@ -546,23 +557,6 @@ defmodule Module.Types.Of do
     error(__MODULE__, warning, meta, stack, context)
   end
 
-  def format_diagnostic({:refine_head_var, old_type, new_type, var, context}) do
-    traces = collect_traces(var, context)
-
-    %{
-      details: %{typing_traces: traces},
-      message:
-        IO.iodata_to_binary([
-          """
-          incompatible types assigned to #{format_var(var)}:
-
-              #{to_quoted_string(old_type)} !~ #{to_quoted_string(new_type)}
-          """,
-          format_traces(traces)
-        ])
-    }
-  end
-
   def format_diagnostic({:badbinary, kind, meta, expr, expected_type, actual_type, context}) do
     type = if kind == :match, do: "matching", else: "construction"
     hints = if meta[:inferred_bitstring_spec], do: [:inferred_bitstring_spec], else: []