Ignore when first constructor parameter is dropped from Signature

core/src/main/scala/com/typesafe/tools/mima/lib/analyze/method/MethodChecker.scala

@@ -5,6 +5,7 @@ import com.typesafe.tools.mima.lib.analyze.Checker
 
 private[analyze] abstract class BaseMethodChecker extends Checker[MethodInfo, ClassInfo] {
   import MethodRules._
+  import BaseMethodChecker._
 
   protected val rules = Seq(AccessModifier, FinalModifier, AbstractModifier, JavaStatic)
 
@@ -13,7 +14,7 @@ private[analyze] abstract class BaseMethodChecker extends Checker[MethodInfo, Cl
     if (meths.isEmpty)
       Some(DirectMissingMethodProblem(method))
     else {
-      meths.find(m => m.descriptor == method.descriptor && methSigCheck(method, m)) match {
+      meths.find(newMethod => hasMatchingDescriptorAndSignature(method, newMethod)) match {
         case Some(found) => checkRules(rules)(method, found)
         case None        =>
           val filtered = meths.filter(method.matchesType(_))
@@ -34,15 +35,21 @@ private[analyze] abstract class BaseMethodChecker extends Checker[MethodInfo, Cl
     }
   }
 
-  private def methSigCheck(oldmeth: MethodInfo, newMeth: MethodInfo): Boolean = {
-    oldmeth.signature == newMeth.signature || (
-      newMeth.bytecodeName == MemberInfo.ConstructorName && newMeth.signature.isEmpty
-    )
-  }
-
   private def uniques(methods: List[MethodInfo]): List[MethodInfo] =
     methods.groupBy(_.parametersDesc).values.map(_.head).toList
 }
+private[analyze] object BaseMethodChecker {
+  def hasMatchingDescriptorAndSignature(oldMethod: MethodInfo, newMethod: MethodInfo): Boolean =
+    oldMethod.descriptor == newMethod.descriptor && hasMatchingSignature(oldMethod.signature, newMethod.signature, newMethod.bytecodeName)
+
+  // Assumes it is already checked that the descriptors match
+  def hasMatchingSignature(oldSignature: String, newSignature: String, bytecodeName: String): Boolean =
+  oldSignature == newSignature ||
+    // Special case for https://github.com/scala/scala/pull/7975:
+    (bytecodeName == MemberInfo.ConstructorName &&
+      (newSignature.isEmpty || (oldSignature.endsWith(newSignature.drop(2))))
+    )
+}
 
 private[analyze] class ClassMethodChecker extends BaseMethodChecker {
   def check(method: MethodInfo, inclazz: ClassInfo): Option[Problem] = {

core/src/test/scala/com/typesafe/tools/mima/lib/analyze/method/MethodCheckerSpec.scala

@@ -0,0 +1,36 @@
+package com.typesafe.tools.mima.lib.analyze.method
+
+import com.typesafe.tools.mima.core.MemberInfo
+
+import org.scalatest.Matchers
+import org.scalatest.WordSpec
+
+final class MethodCheckerSpec extends WordSpec with Matchers {
+    "The method checker" should {
+        val `signatureOn2.12.8` =
+            "(Lakka/http/impl/engine/server/GracefulTerminatorStage;Lscala/concurrent/Promise<Lscala/Function1<Lscala/concurrent/duration/FiniteDuration;Lscala/concurrent/Future<Lakka/http/scaladsl/Http$HttpTerminated;>;>;>;)V"
+        val `signatureOn2.12.9` =
+            "(Lscala/concurrent/Promise<Lscala/Function1<Lscala/concurrent/duration/FiniteDuration;Lscala/concurrent/Future<Lakka/http/scaladsl/Http$HttpTerminated;>;>;>;)V"
+
+        "allow dropping the first parameter of the Signature attribute of a constructor" in {
+            // Assuming the descriptor is the same, dropping the first
+            // parameter of the Signature attribute can only be explained by
+            // going from a Scala version that does not have the fix in
+            // https://github.com/scala/scala/pull/7975 (2.12.8, 2.13.0) to
+            // one that does
+            BaseMethodChecker.hasMatchingSignature(
+                `signatureOn2.12.8`,
+                `signatureOn2.12.9`,
+                MemberInfo.ConstructorName
+            ) should be(true)
+        }
+
+        "reject adding the first parameter of the Signature attribute of a constructor back" in {
+            BaseMethodChecker.hasMatchingSignature(
+                `signatureOn2.12.9`,
+                `signatureOn2.12.8`,
+                MemberInfo.ConstructorName
+            ) should be(false)
+        }
+    }
+}