diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index f2043ef..1dd34d3 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -4,33 +4,57 @@ on: [push, pull_request]
 
 jobs:
   credo:
-    name: Credo
+    name: Static analysis
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - name: Install OTP and Elixir
         uses: actions/setup-elixir@v1
         with:
-          otp-version: 22.1
-          elixir-version: 1.9.4
+          otp-version: 23.1.1
+          elixir-version: 1.11.x
 
       - run: mix deps.get
-      - run: mix compile
-      - run: mix credo
+      - run: mix format --check-formatted
+      - run: mix compile --warnings-as-errors
+      - run: mix credo --all --strict
 
   test:
     name: Test
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - elixir: 1.7.x
+            otp: 19.3.6.13
+          - elixir: 1.8.x
+            otp: 20.3.8.26
+          - elixir: 1.9.x
+            otp: 20.3.8.26
+          - elixir: 1.10.x
+            otp: 21.3.8.18
+          - elixir: 1.11.x
+            otp: 21.3.8.18
+          - elixir: 1.11.x
+            otp: 23.1.1
+            warnings_as_errors: true
+    env:
+      MIX_ENV: test
+
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
 
       - name: Install OTP and Elixir
         uses: actions/setup-elixir@v1
         with:
-          otp-version: 22.1
-          elixir-version: 1.9.4
+          otp-version: ${{matrix.otp}}
+          elixir-version: ${{matrix.elixir}}
 
-      - run: mix deps.get
-      - run: MIX_ENV=test mix compile
-      - run: mix test
+      - run: mix deps.get --only test
+      - name: Run mix test
+        run: mix test --color 2>/tmp/tests.stderr
+      - name: Make sure tests do not have warnings
+        run: "! grep -v 'warning: ' /tmp/tests.stderr"
+        if: matrix.warnings_as_errors
diff --git a/lib/norm/core/all_of.ex b/lib/norm/core/all_of.ex
index cd5f75c..b3636f5 100644
--- a/lib/norm/core/all_of.ex
+++ b/lib/norm/core/all_of.ex
@@ -25,4 +25,3 @@ defmodule Norm.Core.AllOf do
     end
   end
 end
-
diff --git a/test/norm/contract_test.exs b/test/norm/contract_test.exs
index 4004a07..e7eb597 100644
--- a/test/norm/contract_test.exs
+++ b/test/norm/contract_test.exs
@@ -94,7 +94,7 @@ defmodule Norm.ContractTest do
     defmodule Reflection do
       use Norm
 
-      def int(), do: spec(is_integer())
+      def int, do: spec(is_integer())
 
       @contract foo(a :: int(), int()) :: int()
       def foo(a, b), do: a + b
diff --git a/test/norm/core/delegate_test.exs b/test/norm/core/delegate_test.exs
index ae2c5a8..bbf8553 100644
--- a/test/norm/core/delegate_test.exs
+++ b/test/norm/core/delegate_test.exs
@@ -2,7 +2,7 @@ defmodule Norm.Core.DelegateTest do
   use Norm.Case, async: true
 
   defmodule TreeTest do
-    def spec() do
+    def spec do
       schema(%{
         "value" => spec(is_integer()),
         "left" => delegate(&TreeTest.spec/0),
diff --git a/test/norm/core/schema_test.exs b/test/norm/core/schema_test.exs
index 729b60e..a2e5324 100644
--- a/test/norm/core/schema_test.exs
+++ b/test/norm/core/schema_test.exs
@@ -175,7 +175,7 @@ defmodule Norm.Core.SchemaTest do
 
     test "allows defaults" do
       spec = schema(%Movie{})
-      assert movie = conform(%Movie{}, spec)
+      assert conform(%Movie{}, spec)
     end
 
     property "can generate proper structs" do
diff --git a/test/norm_test.exs b/test/norm_test.exs
index eac68e6..2c8f5aa 100644
--- a/test/norm_test.exs
+++ b/test/norm_test.exs
@@ -263,7 +263,7 @@ defmodule NormTest do
       assert {:error, _errors} = conform([{:foo, :bar} | list], spec)
 
       assert list == conform!(list, coll_of(opts, [min_count: 2, distinct: true]))
-      assert {:error, errors} = conform([], coll_of(opts, [min_count: 2, distinct: true]))
+      assert {:error, _errors} = conform([], coll_of(opts, [min_count: 2, distinct: true]))
     end
 
     property "can be generated" do