diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index aca64f3509c..6a6a5888768 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -10,12 +10,16 @@ on:
   push:
     branches:
       - master
+      - diff-cover-mypy
+      - test-diff-cover-mypy
     tags:
       - "*"
 
   pull_request:
     branches:
       - master
+      - diff-cover-mypy
+      - test-diff-cover-mypy
 
 jobs:
   build:
@@ -25,24 +29,7 @@ jobs:
       fail-fast: false
       matrix:
         name: [
-          "windows-py35",
-          "windows-py36",
-          "windows-py37",
-          "windows-py37-pluggy",
-          "windows-py38",
-
-          "ubuntu-py35",
-          "ubuntu-py36",
-          "ubuntu-py37",
-          "ubuntu-py37-pluggy",
-          "ubuntu-py37-freeze",
-          "ubuntu-py38",
-          "ubuntu-pypy3",
-
-          "macos-py37",
-          "macos-py38",
-
-          "linting",
+          "mypy-diff",
         ]
 
         include:
@@ -189,3 +176,19 @@ jobs:
       run: |
         sudo apt-get install pandoc
         tox -e publish-gh-release-notes
+
+  mypy-diff:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up Python
+      uses: actions/setup-python@v1
+      with:
+        python-version: "3.8"
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install --upgrade wheel setuptools tox
+    - name: tox -e mypy-diff
+      run: "tox -e mypy-diff"
diff --git a/src/_pytest/debugging.py b/src/_pytest/debugging.py
index 9155d7e98e3..602b683d148 100644
--- a/src/_pytest/debugging.py
+++ b/src/_pytest/debugging.py
@@ -8,6 +8,10 @@
 from _pytest.config.exceptions import UsageError
 
 
+def untyped() -> None:
+    pass
+
+
 def _validate_usepdb_cls(value):
     """Validate syntax of --pdbcls option."""
     try:
@@ -19,6 +23,10 @@ def _validate_usepdb_cls(value):
     return (modname, classname)
 
 
+def not_untyped() -> None:
+    pass
+
+
 def pytest_addoption(parser):
     group = parser.getgroup("general")
     group._addoption(
diff --git a/tox.ini b/tox.ini
index cf50945fbd7..91c50ececf0 100644
--- a/tox.ini
+++ b/tox.ini
@@ -59,6 +59,15 @@ commands = pre-commit run --all-files --show-diff-on-failure {posargs:}
 extras = checkqa-mypy, testing
 commands = mypy {posargs:src testing}
 
+[testenv:mypy-diff]
+extras = checkqa-mypy, testing
+deps =
+    lxml
+    diff-cover
+commands =
+  -mypy --cobertura-xml-report {envtmpdir} {posargs:src testing}
+  diff-cover --fail-under=100 --compare-branch={env:DIFF_BRANCH:origin/{env:GITHUB_BASE_REF:master}} {envtmpdir}/cobertura.xml
+
 [testenv:docs]
 basepython = python3
 usedevelop = True