different kotlin versions in android project

I can't figure out what problem I can get if I have different Kotlin versions in the project and in the dependencies of that project.

Option 1:

• the project include Kotlin 1.3.72
• some libraries (okhttp for example) include Kotlin 1.4.10. (We now use Kotlin 1.4.x functional interfaces for Authenticator, Interceptor, and others.)

Option 2:

• the project include Kotlin 1.4.0
• some libraries include Kotlin 1.3.71

In option 2 I get next warning:
w: Runtime JAR files in the classpath should have the same version. These files were found in the classpath:
/.gradle/caches/transforms-2/files-2.1/ab41544fa06f7b55dec847efe3b9899c/jetified-kotlin-stdlib-jdk7-1.3.71.jar (version 1.3)
.gradle/caches/transforms-2/files-2.1/a4c6c3b949eb40b555dea1468ae75f87/jetified-kotlin-stdlib-1.4.10.jar (version 1.4)
.gradle/caches/transforms-2/files-2.1/f98f62bf33e752598311020043618780/jetified-kotlin-stdlib-common-1.4.10.jar (version 1.4)
w: Some runtime JAR files in the classpath have an incompatible version. Consider removing them from the classpath

/.gradle/caches/transforms-2/files-2.1/ab41544fa06f7b55dec847efe3b9899c/jetified-kotlin-stdlib-jdk7-1.3.71.jar (version 1.3)

this can be resolved with including this in your app build.gradle dependencies:

implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

Also, I have personally learned to add the following dependencies to avoid these errors:

implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
implementation "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

Also, make sure you have ext.kotlin_version = "1.4.10" in your project build.gradle

What problems you can get? When the differences are not just in the minor release version (like 1.3.70 / 1.3.72) but are between a 1.3.x and a 1.4.x release, you risk e.g. that one library will use a function that has changed its signature in the new release, and so there will be a runtime error. This does not necessarily happen, but it can happen (unless you know for certain that your dependency libraries do no use any functions that have changed).

If your project target Java 8 so just include the dependency on that Java version

implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"

Follow the same rule if you target Java 6 or 7.

You clarify more about that in the post https://medium.com/@mbonnin/the-different-kotlin-stdlibs-explained-83d7c6bf293