Better error message when view return type is not supported

Before, returning a `bool` from a route caused the error

```
[2019-05-31 10:08:42,216] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2070, in make_response
    rv = self.response_class.force_type(rv, request.environ)
  File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/wrappers/base_response.py", line 269, in force_type
    response = BaseResponse(*_run_wsgi_app(response, environ))
  File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/wrappers/base_response.py", line 26, in _run_wsgi_app
    return _run_wsgi_app(*args)
  File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/test.py", line 1119, in run_wsgi_app
    app_rv = app(environ, start_response)
TypeError: 'bool' object is not callable

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2393, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 1906, in full_dispatch_request
    return self.finalize_request(rv)
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 1921, in finalize_request
    response = self.make_response(rv)
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2078, in make_response
    reraise(TypeError, new_error, sys.exc_info()[2])
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/_compat.py", line 39, in reraise
    raise value.with_traceback(tb)
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2070, in make_response
    rv = self.response_class.force_type(rv, request.environ)
  File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/wrappers/base_response.py", line 269, in force_type
    response = BaseResponse(*_run_wsgi_app(response, environ))
  File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/wrappers/base_response.py", line 26, in _run_wsgi_app
    return _run_wsgi_app(*args)
  File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/test.py", line 1119, in run_wsgi_app
    app_rv = app(environ, start_response)
TypeError: 'bool' object is not callable
The view function did not return a valid response. The return type must be a string, tuple, Response instance, or WSGI callable, but it was a bool.
```

Now, it returns the more readable

```
[2019-05-31 10:36:19,500] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2400, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 1907, in full_dispatch_request
    return self.finalize_request(rv)
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 1922, in finalize_request
    response = self.make_response(rv)
  File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2085, in make_response
    " {rv.__class__.__name__}.".format(rv=rv))
TypeError: The view function did not return a valid response. The return type must be a string, dict, tuple, Response instance, or WSGI callable, but it was a bool.
```

Fixes #3214

CHANGES.rst

@@ -64,6 +64,8 @@ Unreleased
 -   When using the test client as a context manager (``with client:``),
     all preserved request contexts are popped when the block exits,
     ensuring nested contexts are cleaned up correctly. :pr:`3157`
+-   Show a better error message when the view return type is not
+    supported. :issue:`3214`
 
 .. _#2935: https://github.com/pallets/flask/issues/2935
 .. _#2957: https://github.com/pallets/flask/issues/2957

flask/app.py

@@ -27,6 +27,7 @@ from werkzeug.exceptions import (
     default_exceptions,
 )
 from werkzeug.routing import BuildError, Map, RequestRedirect, RoutingException, Rule
+from werkzeug.wrappers import BaseResponse
 
 from . import cli, json
 from ._compat import integer_types, reraise, string_types, text_type
@@ -2063,7 +2064,7 @@ class Flask(_PackageBoundObject):
                 status = headers = None
             elif isinstance(rv, dict):
                 rv = jsonify(rv)
-            else:
+            elif isinstance(rv, BaseResponse) or callable(rv):
                 # evaluate a WSGI callable, or coerce a different response
                 # class to the correct type
                 try:
@@ -2071,11 +2072,18 @@ class Flask(_PackageBoundObject):
                 except TypeError as e:
                     new_error = TypeError(
                         "{e}\nThe view function did not return a valid"
-                        " response. The return type must be a string, tuple,"
+                        " response. The return type must be a string, dict, tuple,"
                         " Response instance, or WSGI callable, but it was a"
                         " {rv.__class__.__name__}.".format(e=e, rv=rv)
                     )
                     reraise(TypeError, new_error, sys.exc_info()[2])
+            else:
+                raise TypeError(
+                    "The view function did not return a valid"
+                    " response. The return type must be a string, dict, tuple,"
+                    " Response instance, or WSGI callable, but it was a"
+                    " {rv.__class__.__name__}.".format(rv=rv)
+                )
 
         # prefer the status if it was provided
         if status is not None:

tests/test_basic.py

@@ -1218,17 +1218,21 @@ def test_response_type_errors():
 
     with pytest.raises(TypeError) as e:
         c.get("/none")
-        assert "returned None" in str(e)
+
+    assert "returned None" in str(e)
 
     with pytest.raises(TypeError) as e:
         c.get("/small_tuple")
-        assert "tuple must have the form" in str(e)
+
+    assert "tuple must have the form" in str(e)
 
     pytest.raises(TypeError, c.get, "/large_tuple")
 
     with pytest.raises(TypeError) as e:
         c.get("/bad_type")
-        assert "it was a bool" in str(e)
+
+    assert "object is not callable" not in str(e)
+    assert "it was a bool" in str(e)
 
     pytest.raises(TypeError, c.get, "/bad_wsgi")