Introduce assert_valid_join which checks for unsupported join syntax

This improves the error messages for `JOIN VALUES (...) AS` clauses
which were being mistakenly tagged as a table prefixing issue. Now the
user gets a more accurate message.

It also puts all the join syntax checking into a single function which
makes it a bit eaiser to reason about (e.g. that hints, values, etc. are
not OK, but other join constructs are)

Author: aseigo

lib/ecto/adapters/sqlite3/connection.ex

@@ -1014,14 +1014,9 @@ defmodule Ecto.Adapters.SQLite3.Connection do
         on: %QueryExpr{expr: expression},
         qual: qual,
         ix: ix,
-        source: source,
-        hints: hints
-      } ->
-        if hints != [] do
-          raise Ecto.QueryError,
-            query: query,
-            message: "join hints are not supported by SQLite3"
-        end
+        source: source
+      } = join ->
+        assert_valid_join(join, query)
 
         {join, name} = get_source(query, sources, ix, source)
 
@@ -1035,6 +1030,20 @@ defmodule Ecto.Adapters.SQLite3.Connection do
     end)
   end
 
+  defp assert_valid_join(%JoinExpr{hints: hints}, query) when hints != [] do
+    raise Ecto.QueryError,
+      query: query,
+      message: "join hints are not supported by SQLite3"
+  end
+
+  defp assert_valid_join(%JoinExpr{source: {:values, _, _}}, query) do
+    raise Ecto.QueryError,
+      query: query,
+      message: "SQLite3 adapter does not support values lists"
+  end
+
+  defp assert_valid_join(_join_expr, _query), do: :ok
+
   defp join_on(:cross, true, _sources, _query), do: []
 
   defp join_on(_qual, expression, sources, query),

test/ecto/adapters/sqlite3/connection/join_test.exs

@@ -135,6 +135,24 @@ defmodule Ecto.Adapters.SQLite3.Connection.JoinTest do
     end
   end
 
+  test "join with values is not supported" do
+    assert_raise Ecto.QueryError, fn ->
+      rows = [%{x: 1, y: 1}, %{x: 2, y: 2}]
+      types = %{x: :integer, y: :integer}
+
+      Schema
+      |> join(
+        :inner,
+        [p],
+        q in values(rows, types),
+        on: [x: p.x(), y: p.y()]
+      )
+      |> select([p, q], {p.id, q.x})
+      |> plan()
+      |> all()
+    end
+  end
+
   test "join with fragment" do
     query =
       Schema