"@material-ui/core": "4.12.1" causes prop type error instead of the intended deprecation warning

[ggascoigne]

I was seeing this:

index.js:1 Warning: Failed prop type: (intermediate value).join is not a function

in my browser console after upgrading to 4.12.1

Looks like the cause is at: https://github.com/mui-org/material-ui/blob/master/packages/material-ui/src/Collapse/Collapse.js#L243

unrelated there's another one like this at https://github.com/mui-org/material-ui/blob/master/packages/material-ui/src/Accordion/Accordion.js#L177

throw new Error([
        'Material-UI: the classes.container key is deprecated.',
        'Use `classes.root` instead',
        'The name of the pseudo-class was changed for consistency.',
      ]).join('\n');

should be:

throw new Error([
        'Material-UI: the classes.container key is deprecated.',
        'Use `classes.root` instead',
        'The name of the pseudo-class was changed for consistency.',
      ].join('\n'));

The join should be on the array and not on the Error object.

[mnajdova]

Thanks for reporting @ggascoigne As pointed out this should fix it:

index f80142d72a..d17c20ef53 100644
--- a/packages/material-ui/src/Accordion/Accordion.js
+++ b/packages/material-ui/src/Accordion/Accordion.js
@@ -178,7 +178,7 @@ Accordion.propTypes = {
         'Material-UI: the classes.focused key is deprecated.',
         'Use `classes.focusedVisible` instead',
         'The name of the pseudo-class was changed for consistency.',
-      ]).join('\n');
+      ].join('\n'));
     }

     return null;
diff --git a/packages/material-ui/src/Collapse/Collapse.js b/packages/material-ui/src/Collapse/Collapse.js
index f0aa748bcf..1161cac70f 100644
--- a/packages/material-ui/src/Collapse/Collapse.js
+++ b/packages/material-ui/src/Collapse/Collapse.js
@@ -244,7 +244,7 @@ Collapse.propTypes = {
         'Material-UI: the classes.container key is deprecated.',
         'Use `classes.root` instead',
         'The name of the pseudo-class was changed for consistency.',
-      ]).join('\n');
+      ].join('\n'));
     }

     return null;

[serenalin121]

Can I work on this?

[mnajdova]

Yes @serenalin121 I am assigning this to you :)

[eps1lon]

Fixed in #27307