feat(compiler-core): error if portal doesn't have target

[dsseng]

This will help people see their mistakes, erroring compilation of code failing in runtime. E.g. if user uses Vetur, they'll see mistakes in dev-time, not in runtime.

Examples of valid code:

<portal :target="a">
  <div>Portal content</div>
</portal>
<portal target="#a">
  <div>Portal content</div>
</portal>

Examples of erroring code:

<portal>
  I'm an error
</portal>
<portal a="b">
  I'm an error
</portal>

[LinusBorg]

Even though an old PR, this still seems like a good addition.

Before getting back to this and solving merge conflicts though, we should fix some of the existing bugs first (see teleport labels)

[skirtles-code]

I know this is an old PR, but given it's still open...

Should this maybe take account of v-bind="obj" too? The target could be passed in obj rather than as a separate prop.

[edison1105]

Agreed with @skirtles-code. We also need to consider v-bind="obj", which cannot be done at compile phase. So we do it at runtime. see

core/packages/runtime-core/src/components/Teleport.ts

Lines 56 to 63 in 928af5f

	const target = select(targetSelector)
	if (__DEV__ && !target && !isTeleportDisabled(props)) {
	warn(
	`Failed to locate Teleport target with selector "${targetSelector}". ` +
	`Note the target element must exist before the component is mounted - ` +
	`i.e. the target cannot be rendered by the component itself, and ` +
	`ideally should be outside of the entire Vue component tree.`,
	)

I think this PR can be closed. @skirtles-code WDYT?