Repository: mui/material-ui (TypeScript) — Bug Fix

Issue:
[Autocomplete] Adding the loading prop does nothing if freeSolo is true
<!-- Provide a general summary of the issue in the Title above -->
In the Asynchronous example of Autocomplete in the docs - https://material-ui.com/components/autocomplete/#asynchronous-requests, the loading prop triggers a "Loading..." disabled option to be displayed. When you take that same example and add `freeSolo`, as demonstrated here - https://stackblitz.com/edit/ca3vpr?file=demo.js, the loading option no longer shows up. 

I found a similar issue describing this problem on StackOverflow from 4 months ago - https://stackoverflow.com/questions/59266457/using-react-material-ui-autocomplete-with-loading-and-freesolo-props.

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- [x] The issue is present in the latest release.
- [x] I have searched the [issues](https://github.com/mui-org/material-ui/issues) of this repository and believe that this is not a duplicate.

## Current Behavior 😯

<!-- Describe what happens instead of the expected behavior. -->

When the `freeSolo` prop is added, the loading option no longer appears.

## Expected Behavior 🤔

<!-- Describe what should happen. -->

When the `freeSolo` prop is added, the loading option should still appear (if loading is true).

## Steps to Reproduce 🕹

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Steps:

1. Visit https://codesandbox.io/s/dazzling-shirley-yo1qp
2. Notice the addition of `freeSolo` prop.
3. Loading option does not appear when clicking into Autocomplete input.


## Context 🔦

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  What are you trying to accomplish? How has this issue affected you?
  Providing context helps us come up with a solution that is most useful in the real world.
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Would like to show this loading state while still using the freeSolo mode of Autocomplete.

## Your Environment 🌎

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  Include as many relevant details about the environment with which you experienced the bug.
  If you encounter issues with typescript please include version and tsconfig.
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| Tech        | Version |
| ----------- | ------- |
| Material-UI | v4.9.12  |
| Material-UI/lab | v4.0.0-alpha.51  |
| React       |   16.13.1  |
| Browser     |    Chrome Version 81.0.4044.122 (Official Build) (64-bit)     |



Discussion:
What do you think @oliviertassinari? 

As far as I can see the 'fix' would be:

```diff
@@ -260,7 +261,7 @@ export default function useAutocomplete(props) {
       )
     : [];

-  popupOpen = freeSolo && filteredOptions.length === 0 ? false : popupOpen;
+  popupOpen = freeSolo && filteredOptions.length === 0 && !loading ? false : popupOpen;

   if (process.env.NODE_ENV !== 'production') {
     if (value !== null && !freeSolo && options.length > 0) {

```

But I haven't decided if this is the right thing to do :/ 
@joshwooding looks good
@Bdthomson Would you like to work on it? 🙂

Resolving patch:
diff --git a/packages/material-ui-lab/src/useAutocomplete/useAutocomplete.js b/packages/material-ui-lab/src/useAutocomplete/useAutocomplete.js
--- a/packages/material-ui-lab/src/useAutocomplete/useAutocomplete.js
+++ b/packages/material-ui-lab/src/useAutocomplete/useAutocomplete.js
@@ -242,7 +242,7 @@ export default function useAutocomplete(props) {
   const inputValueIsSelectedValue =
     !multiple && value != null && inputValue === getOptionLabel(value);
 
-  let popupOpen = open;
+  const popupOpen = open;
 
   const filteredOptions = popupOpen
     ? filterOptions(
@@ -263,8 +263,6 @@ export default function useAutocomplete(props) {
       )
     : [];
 
-  popupOpen = freeSolo && filteredOptions.length === 0 ? false : popupOpen;
-
   if (process.env.NODE_ENV !== 'production') {
     if (value !== null && !freeSolo && options.length > 0) {
       const missingValue = (multiple ? value : [value]).filter(
@@ -405,7 +403,7 @@ export default function useAutocomplete(props) {
   });
 
   React.useEffect(() => {
-    if (!open) {
+    if (!popupOpen) {
       return;
     }
 
@@ -451,7 +449,7 @@ export default function useAutocomplete(props) {
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [
     value,
-    open,
+    popupOpen,
     filterSelectedOptions,
     changeHighlightedIndex,
     setHighlightedIndex,


Test patch:
diff --git a/packages/material-ui-lab/src/Autocomplete/Autocomplete.test.js b/packages/material-ui-lab/src/Autocomplete/Autocomplete.test.js
--- a/packages/material-ui-lab/src/Autocomplete/Autocomplete.test.js
+++ b/packages/material-ui-lab/src/Autocomplete/Autocomplete.test.js
@@ -54,6 +54,21 @@ describe('<Autocomplete />', () => {
     });
   });
 
+  describe('prop: loading', () => {
+    it('should show a loading message when open', () => {
+      render(
+        <Autocomplete
+          {...defaultProps}
+          freeSolo
+          loading
+          renderInput={(params) => <TextField autoFocus {...params} />}
+        />,
+      );
+      fireEvent.keyDown(document.activeElement, { key: 'ArrowDown' });
+      expect(document.querySelector(`.${classes.paper}`).textContent).to.equal('Loading…');
+    });
+  });
+
   describe('prop: autoHighlight', () => {
     it('should set the focus on the first item', () => {
       const options = ['one', 'two'];