Add support for mimaReportSignatureProblems (#52)

README.md

@@ -112,8 +112,24 @@ Signature:
 def mimaForwardIssueFilters: Target[Map[String, Seq[ProblemFilter]]]
 ```
 
+### IncompatibleSignatureProblem
+
+Most MiMa checks (`DirectMissingMethod`,`IncompatibleResultType`, `IncompatibleMethType`, etc) are against the "method descriptor", which is the "raw" type signature, without any information about generic parameters.
+
+The `IncompatibleSignature` check compares the `Signature`, which includes the full signature including generic parameters. This can catch real incompatibilities, but also sometimes triggers for a change in generics that would not in fact cause problems at run time. Notably, it will warn when updating your project to scala 2.12.9+ or 2.13.1+, see [this issue](https://github.com/lightbend/mima/issues/423) for details.
+
+You can opt-in to this check by setting:
+
+```scala
+def mimaReportSignatureProblems = true
+```
+
 ## Changelog
 
+### 0.0.11
+
+- Add support for `mimaReportSignatureProblems`
+
 ### 0.0.10
 
 - Run Mima in a separate classloader.

mill-mima/src/com/github/lolgab/mill/mima/MimaBase.scala

@@ -79,6 +79,12 @@ private[mima] trait MimaBase
     Seq.empty[String]
   }
 
+  /** If true, report `IncompatibleSignatureProblem`s.
+    */
+  def mimaReportSignatureProblems: Target[Boolean] = T {
+    false
+  }
+
   private def mimaWorkerClasspath: T[Agg[os.Path]] = T {
     Lib
       .resolveDependencies(
@@ -128,7 +134,13 @@ private[mima] trait MimaBase
     def toWorkerApi(p: ProblemFilter) =
       new worker.api.ProblemFilter(p.name, p.problem)
 
-    val binaryFilters = mimaBinaryIssueFilters().map(toWorkerApi).toArray
+    val incompatibleSignatureProblemFilters =
+      if (mimaReportSignatureProblems()) Seq.empty
+      else Seq(ProblemFilter.exclude[IncompatibleSignatureProblem]("*"))
+    val binaryFilters =
+      (mimaBinaryIssueFilters() ++ incompatibleSignatureProblemFilters)
+        .map(toWorkerApi)
+        .toArray
     val backwardFilters =
       mimaBackwardIssueFilters().view
         .mapValues(_.map(toWorkerApi).toArray)