Issue 4 - Single child composable problem

[aballano]

Android Studio Build:

Android Studio 4.0 Canary 8
Build #AI-193.5233.102.40.6107147, built on January 2, 2020
Runtime version: 1.8.0_212-release-1586-b4-5784211 x86_64
VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
macOS 10.14.6
GC: ParNew, ConcurrentMarkSweep
Memory: 4012M
Cores: 12
Registry: ide.new.welcome.screen.force=true, ide.balloon.shadow.size=0
Non-Bundled Plugins: com.chrisrm.idea.MaterialThemeUI, com.squareup.sqldelight

Version of Gradle Plugin:

4.0.0-alpha08

Version of Gradle:

gradle-6.1-rc-1-all

Version of Java:

1.8

OS:

MacOs Mojave 10.14.6

Steps to Reproduce:

• Create a simple button with more than one components inside:
• Build the app

Expected result:

• Any component that can only have 1 child inside should explicitly mark it this way, so adding multiple components should give a compile error

Actual result:

• No errors, only first component is rendered.

Proposed solution:

• The Composable annotation could contain a parameter to specify an arbitrary arity that would be enforced at compile time by the compiler plugin itself.

Sample branch:
https://github.com/arrow-kt/compose-samples/blob/gotchas/gist4/JetNews/app/src/main/java/com/example/jetnews/MainActivity.kt