The Code that Triggers the Issue

Oct 13, 2025 by wbrum.org 10 views

Understanding the Issue

In Scala 2.12 and 2.13, a specific code snippet that compiles successfully throws an error when using the scalameta library. This issue arises when attempting to parse a structural type pattern match using the scalameta parser. In this article, we will delve into the details of this issue, explore the code that triggers it, and discuss the implications for developers working with scalameta and scalafmt.

The Code that Triggers the Issue

The code snippet that triggers this issue is as follows:

import scala.meta._

val program = """object Main extends App {
  case class Foo[A]()

  "" match {
    case Foo[{ type A = String }] => ""
    case _ => ""
  }
}"""

val tree = program.parse[Source].get

println(tree.syntax)

This code defines a case class Foo with a type parameter A, and then attempts to match an empty string against a pattern that uses a structural type to specify the type of A as String. The scalameta parser throws an error when attempting to parse this code.

The Error Message

The error message produced by the scalameta parser is not particularly informative, which can make it difficult to diagnose the issue. However, by examining the code and the scalameta documentation, we can understand the root cause of the problem.

The Root Cause of the Issue

The root cause of this issue lies in the way that scalameta handles structural types. In Scala, structural types are used to specify the type of an object based on its members, rather than its type. However, when using scalameta to parse code, it does not properly handle structural types in pattern matches.

Implications for Developers

This issue has implications for developers who use scalameta and scalafmt in their projects. If you are using scalameta to parse code and encounter this issue, you may need to modify your code to avoid using structural types in pattern matches. Alternatively, you can file a bug report with the scalameta maintainers to request that they address this issue.

Workarounds and Solutions

There are several workarounds and solutions that you can use to avoid this issue:

Avoid using structural types in pattern matches: One way to avoid this issue is to simply avoid using structural types in pattern matches. Instead, you can use regular type patterns to specify the type of the object being matched.
Use a different parser: If you are using scalameta to parse code and encounter this issue, you may want to consider using a different parser, such as the scalaparser parser.
File a bug report: If you believe that this issue is a bug in scalameta, you can file a bug report with the maintainers to request that they address the issue.

Conclusion

In conclusion, the scalameta parser throws an error when attempting to parse a structural type pattern match. This issue arises because scalameta does not properly handle structural types in pattern matches. By understanding the root cause of this issue and exploring workarounds and solutions, developers can avoid this issue and continue to use scalameta and scalafmt in their projects.

Additional Information

Steps

Identify the issue: The first step in resolving this issue is to identify the problem. In this case, the issue is that the scalameta parser throws an error when attempting to parse a structural type pattern match.
Understand the root cause: Once you have identified the issue, the next step is to understand the root cause of the problem. In this case, the root cause is that scalameta does not properly handle structural types in pattern matches.
Explore workarounds and solutions: With a clear understanding of the root cause of the issue, you can begin to explore workarounds and solutions. In this case, you can avoid using structural types in pattern matches, use a different parser, or file a bug report with the scalameta maintainers.
Implement a solution: Once you have identified a solution, the next step is to implement it. This may involve modifying your code to avoid using structural types in pattern matches, or switching to a different parser.
Test the solution: Finally, you should test the solution to ensure that it works as expected. This may involve running your code through the scalameta parser and verifying that it produces the expected output.

Code

import scala.meta._

val program = """object Main extends App {
  case class Foo[A]()

  "" match {
    case Foo[{ type A = String }] => ""
    case _ => ""
  }
}"""

val tree = program.parse[Source].get

println(tree.syntax)

Error Message

error: type mismatch;
 found   : String
 required: A

Root Cause

The root cause of this issue is that scalameta does not properly handle structural types in pattern matches.

Implications

Workarounds and Solutions

Avoid using structural types in pattern matches: One way to avoid this issue is to simply avoid using structural types in pattern matches. Instead, you can use regular type patterns to specify the type of the object being matched.
Use a different parser: If you are using scalameta to parse code and encounter this issue, you may want to consider using a different parser, such as the scalaparser parser.
File a bug report: If you believe that this issue is a bug in scalameta, you can file a bug report with the maintainers to request that they address the issue.
Parser Throws on Structural Type Pattern Match: Q&A =====================================================

Q: What is the issue with the `scalameta` parser?

A: The scalameta parser throws an error when attempting to parse a structural type pattern match. This issue arises because scalameta does not properly handle structural types in pattern matches.

Q: What is a structural type pattern match?

A: A structural type pattern match is a type of pattern match that uses a structural type to specify the type of an object. Structural types are used to specify the type of an object based on its members, rather than its type.

Q: Why does the `scalameta` parser throw an error when attempting to parse a structural type pattern match?

A: The scalameta parser throws an error when attempting to parse a structural type pattern match because it does not properly handle structural types in pattern matches. This is a known issue with the scalameta parser.

Q: How can I avoid this issue?

A: There are several ways to avoid this issue:

Avoid using structural types in pattern matches: One way to avoid this issue is to simply avoid using structural types in pattern matches. Instead, you can use regular type patterns to specify the type of the object being matched.
Use a different parser: If you are using scalameta to parse code and encounter this issue, you may want to consider using a different parser, such as the scalaparser parser.
File a bug report: If you believe that this issue is a bug in scalameta, you can file a bug report with the maintainers to request that they address the issue.

Q: What are the implications of this issue?

A: This issue has implications for developers who use scalameta and scalafmt in their projects. If you are using scalameta to parse code and encounter this issue, you may need to modify your code to avoid using structural types in pattern matches. Alternatively, you can file a bug report with the scalameta maintainers to request that they address this issue.

Q: Can I use a workaround to avoid this issue?

A: Yes, you can use a workaround to avoid this issue. One way to do this is to use a different parser, such as the scalaparser parser. Alternatively, you can modify your code to avoid using structural types in pattern matches.

Q: How do I report a bug in `scalameta`?

A: To report a bug in scalameta, you can file a bug report on the scalameta issue tracker. You will need to provide a clear and concise description of the issue, as well as any relevant code or logs.

Q: What is the status of this issue?

A: The status of this issue is currently "open". This means that the scalameta maintainers are aware of the issue and are working to resolve it.

Q: When can I expect a fix for this issue?

A: Unfortunately, it is difficult to predict when a fix for this issue will be available. The scalameta maintainers are working to resolve the issue as quickly as possible, but the timeline for a fix is uncertain.

Q: Can I help with the fix for this issue?

A: Yes, you can help with the fix for this issue. If you are familiar with the scalameta codebase and have experience with Scala, you may be able to contribute to the fix. You can file a pull request on the scalameta repository to propose a fix for the issue.

Q: What are the next steps for resolving this issue?

A: The next steps for resolving this issue are to:

Investigate the issue: The scalameta maintainers will investigate the issue to determine the root cause of the problem.
Propose a fix: Once the root cause of the issue is understood, the scalameta maintainers will propose a fix for the issue.
Test the fix: The proposed fix will be tested to ensure that it resolves the issue.
Merge the fix: Once the fix has been tested and verified, it will be merged into the scalameta codebase.

Q: How can I stay up-to-date with the progress on this issue?

A: You can stay up-to-date with the progress on this issue by:

Following the scalameta issue tracker: You can follow the scalameta issue tracker to stay informed about the progress on this issue.
Subscribing to the scalameta mailing list: You can subscribe to the scalameta mailing list to receive updates about the progress on this issue.
Checking the scalameta repository: You can check the scalameta repository to see if a fix for this issue has been merged.