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

Issue:
[Checkbox][Radio] Icon font size override
<!-- Provide a general summary of the issue in the Title above -->
When using the `icon` and `checkedIcon` props for the `Checkbox` component, passing a custom `fontSize` doesn't change size of the icons.

This issue is also present in the Checkbox demos, where the Custom Size example is the same size as all the other examples.

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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. -->
Passing a `fontSize` prop to the `icon` or `customIcon`  component does nothing.

```javascript
<Checkbox
  icon={<CheckBoxOutlineBlankIcon fontSize="small" />} // this doesn't do anything
  checkedIcon={<CheckBoxIcon fontSize="large" />} // neither does this
/>

// other props like color work as expected
<Checkbox
  icon={<CheckBoxOutlineBlankIcon color="secondary" />}
/>
```

## Expected Behavior 🤔

<!-- Describe what should happen. -->
Passing a valid `fontSize` prop to `icon` or `customIcon` should increase the size of the ` Icon`.

## Steps to Reproduce 🕹

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Steps:

1. Create a `Checkbox` component
2. Pass a custom component to `icon` or `checkedIcon` props
3. Add a custom `fontSize`
4. See nothing happen

### Links
Sandbox with error reproduced: https://codesandbox.io/s/checkbox-issue-demo-ksr7k

Documentation section with the same error: https://material-ui.com/components/checkboxes/#checkbox-with-formcontrollabel



## Context 🔦

<!--
  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.
-->
I want to be able to customize the size of my Checkboxes. Note that other props like `color` are not overridden in the same manner.

A somewhat similar issue is currently open regarding `Input` size props:
https://github.com/mui-org/material-ui/issues/19796

## Your Environment 🌎

<!--
  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.
-->

| Tech        | Version |
| ----------- | ------- |
| Material-UI | v4.10.1 |
| React       | 16.13.1 |
| Browser     | Chrome version 83.0.4103.61 (64 bit) |



Discussion:
@samuelcolburn Thanks for reporting the issue. It comes from the usage of cloneElement that forces a value. It affects the Radio component too. What do you think of this fix?

```diff
diff --git a/packages/material-ui/src/Checkbox/Checkbox.js b/packages/material-ui/src/Checkbox/Checkbox.js
index 7916091ab..6d28ca15c 100644
--- a/packages/material-ui/src/Checkbox/Checkbox.js
+++ b/packages/material-ui/src/Checkbox/Checkbox.js
@@ -64,14 +64,17 @@ const Checkbox = React.forwardRef(function Checkbox(props, ref) {
     checkedIcon = defaultCheckedIcon,
     classes,
     color = 'secondary',
-    icon = defaultIcon,
+    icon: iconProp = defaultIcon,
     indeterminate = false,
-    indeterminateIcon = defaultIndeterminateIcon,
+    indeterminateIcon: indeterminateIconProp = defaultIndeterminateIcon,
     inputProps,
     size = 'medium',
     ...other
   } = props;

+  const icon = indeterminate ? indeterminateIconProp : iconProp;
+  const indeterminateIcon = indeterminate ? indeterminateIconProp : checkedIcon;
+
   return (
     <SwitchBase
       type="checkbox"
@@ -87,11 +90,11 @@ const Checkbox = React.forwardRef(function Checkbox(props, ref) {
         'data-indeterminate': indeterminate,
         ...inputProps,
       }}
-      icon={React.cloneElement(indeterminate ? indeterminateIcon : icon, {
-        fontSize: size === 'small' ? 'small' : 'default',
+      icon={React.cloneElement(icon, {
+        fontSize: icon.props.fontSize === undefined && size !== 'medium' ? size : icon.props.fontSize,
       })}
-      checkedIcon={React.cloneElement(indeterminate ? indeterminateIcon : checkedIcon, {
-        fontSize: size === 'small' ? 'small' : 'default',
+      checkedIcon={React.cloneElement(indeterminateIcon, {
+        fontSize: indeterminateIcon.props.fontSize === undefined && size !== 'medium' ? size : indeterminateIcon.props.fontSize
       })}
       ref={ref}
       {...other}
@@ -171,7 +174,7 @@ Checkbox.propTypes = {
    * The size of the checkbox.
    * `small` is equivalent to the dense checkbox styling.
    */
-  size: PropTypes.oneOf(['medium', 'small']),
+  size: PropTypes.oneOf(['large', 'medium', 'small']),
   /**
    * The value of the component. The DOM API casts this to a string.
    * The browser uses "on" as the default value.
```

It's a start. Do you want to work on a pull request? :)
@oliviertassinari I wish to work on this as my first issue if it is not taken by someone else already, thanks.
@kn1ves Awesome, it's all yours. Note that the above fix isn't meant to be the final solution, but a proof of concept, they will be a couple of rough edges to polish :).

Resolving patch:
diff --git a/packages/material-ui/src/Checkbox/Checkbox.js b/packages/material-ui/src/Checkbox/Checkbox.js
--- a/packages/material-ui/src/Checkbox/Checkbox.js
+++ b/packages/material-ui/src/Checkbox/Checkbox.js
@@ -64,14 +64,17 @@ const Checkbox = React.forwardRef(function Checkbox(props, ref) {
     checkedIcon = defaultCheckedIcon,
     classes,
     color = 'secondary',
-    icon = defaultIcon,
+    icon: iconProp = defaultIcon,
     indeterminate = false,
-    indeterminateIcon = defaultIndeterminateIcon,
+    indeterminateIcon: indeterminateIconProp = defaultIndeterminateIcon,
     inputProps,
     size = 'medium',
     ...other
   } = props;
 
+  const icon = indeterminate ? indeterminateIconProp : iconProp;
+  const indeterminateIcon = indeterminate ? indeterminateIconProp : checkedIcon;
+
   return (
     <SwitchBase
       type="checkbox"
@@ -87,11 +90,15 @@ const Checkbox = React.forwardRef(function Checkbox(props, ref) {
         'data-indeterminate': indeterminate,
         ...inputProps,
       }}
-      icon={React.cloneElement(indeterminate ? indeterminateIcon : icon, {
-        fontSize: size === 'small' ? 'small' : 'default',
+      icon={React.cloneElement(icon, {
+        fontSize:
+          icon.props.fontSize === undefined && size !== 'medium' ? size : icon.props.fontSize,
       })}
-      checkedIcon={React.cloneElement(indeterminate ? indeterminateIcon : checkedIcon, {
-        fontSize: size === 'small' ? 'small' : 'default',
+      checkedIcon={React.cloneElement(indeterminateIcon, {
+        fontSize:
+          indeterminateIcon.props.fontSize === undefined && size !== 'medium'
+            ? size
+            : indeterminateIcon.props.fontSize,
       })}
       ref={ref}
       {...other}


Test patch:
diff --git a/packages/material-ui/src/Checkbox/Checkbox.test.js b/packages/material-ui/src/Checkbox/Checkbox.test.js
--- a/packages/material-ui/src/Checkbox/Checkbox.test.js
+++ b/packages/material-ui/src/Checkbox/Checkbox.test.js
@@ -115,4 +115,20 @@ describe('<Checkbox />', () => {
       });
     });
   });
+
+  it('should allow custom icon font sizes', () => {
+    const fontSizeSpy = spy();
+    const MyIcon = (props) => {
+      const { fontSize, ...other } = props;
+
+      React.useEffect(() => {
+        fontSizeSpy(fontSize);
+      });
+
+      return <div {...other} />;
+    };
+    render(<Checkbox icon={<MyIcon fontSize="foo" />} />);
+
+    expect(fontSizeSpy.args[0][0]).to.equal('foo');
+  });
 });