Confusing warnings for inferred flexible types in public methods

[flomebul]

Compiler version

3.7.4

Minimized code

//> using scala 3.7.4
//> using options-Yexplicit-nulls -Werror

trait AwtComponentLogging extends java.awt.Component, com.typesafe.scalalogging.StrictLogging:

  override def getPreferredSize: java.awt.Dimension =
    logger.trace("enter")
    val dim: java.awt.Dimension = super.getPreferredSize
    logger.trace("exit")
    dim

Output

method AwtComponentLogging$$super$getPreferredSize exposes a flexible type in its inferred result type (java.awt.Dimension)?. Consider annotating the type explicitly
    val dim: java.awt.Dimension = super.getPreferredSize

Expectation

No warning like in scala 3.7.3

[tanishiking]

Starting from 3.7.4, when using -Yexplicit-nulls, the compiler creates a synthetic method that returns FlexibleTypes (like (java.awt.Dimension)?) for Java methods without a @NotNull annotation on their reference type return values.

You might wanna try -Yno-flexible-types to disable this behavior, in that case the return type will be java.awt.Dimension | Null though.
While this is not wrong, I agree that the error message isn't very helpful here, it'd be helpful if the message contains a pointer what is "FlexibleType" (maybe link to the doc?), why Java methods are inferred like that, and how to disable it? with -Yno-flexible-types.

Flexible types can be disabled by using -Yno-flexible-types flag. The ordinary union type | Null will be used instead.
https://docs.scala-lang.org/scala3/reference/experimental/explicit-nulls.html

#23682
#23880

---

Removed external com.typesafe.scalalogging.StrictLogging dependency version:

//> using scala 3.7.4
//> using options-Yexplicit-nulls -Werror

trait AwtComponentLogging extends java.awt.Component:

  override def getPreferredSize: java.awt.Dimension =
    val dim: java.awt.Dimension = super.getPreferredSize
    dim

---

I took a liberty of changing the title.

[flomebul]

Hello,

I try to use "-Yno-flexible-types", but it imply to modify a lot of lines in my project. So I follow the recommendation and read the seceion "Java Interoperability and Flexible Types". According to the documentation, it is possible to assign a Dimension? to a value of type Dimension or a value of type Dimension | Null
But unfortunately, both are rejected with the same warning and I do not understand how I can annotate the type explicitly !!!

[olhotak]

The warning for flexible types was added recently in #23880 . It's possible that there is a bug in the code that issues the warning and that the warning should not be issued for this method.

What is method AwtComponentLogging$$super$getPreferredSize? From the name, it looks like a synthetic method. The code that issues the warning explicitly excludes methods marked as synthetic. Is it possible that it is somehow synthetic but the synthetic marker is missing?

cc @noti0na1

[noti0na1]

Normally -Yno-flexible-types should never be used unless you want stict nullable types from Java library.