Previously: #2995 (#2995 (comment)), #3745 (#3745 (comment))

The InnerBlocks component, used for block nesting, should allow a block implementer to assign a predefined block arrangement, optionally locking or otherwise restricting the types of blocks which are available for insertion in the block.

This could potentially align with existing block templating

Pseudo-code examples:

<InnerBlocks
	allowedBlockNames={ [ 'core/image', 'core/paragraph' ] }
	template={ [
		createBlock( 'core/image' ),
		createBlock( 'core/paragraph' ),
	] }
/>

For the inverse relationship (blocks restricted to certain parent contexts), we could either...

• Rely on a combination of block's isPrivate API (already used to omit block from inserter) and treat allowedBlockNames as overriding this preference
• Add a property to the block API, allowedRootBlockNames: [ ... ]

Related Work:

Discussion by @jorgefilipecosta about defining inner blocks supported:

https://wordpress.slack.com/archives/C02QB2JS7/p1520252046000229

cc @jorgefilipecosta