[inspection] re-enable comparing unrelated types inspection for scala 3

Adds support for '--language:strictEquality' compiler flag and 'CanEqual' when running inspection on scala 3 code

Author: disordered

scala/scala-impl/src/org/jetbrains/plugins/scala/codeInspection/typeChecking/ComparingUnrelatedTypesInspection.scala

@@ -1,16 +1,17 @@
 package org.jetbrains.plugins.scala.codeInspection.typeChecking
 
-import com.intellij.codeInspection.{LocalInspectionTool, ProblemHighlightType, ProblemsHolder}
+import com.intellij.codeInspection.{LocalInspectionTool, ProblemsHolder}
 import com.intellij.psi.PsiMethod
 import com.siyeh.ig.psiutils.MethodUtils
 import org.jetbrains.annotations.Nls
 import org.jetbrains.plugins.scala.codeInspection.collections.MethodRepr
 import org.jetbrains.plugins.scala.codeInspection.typeChecking.ComparingUnrelatedTypesInspection._
 import org.jetbrains.plugins.scala.codeInspection.{PsiElementVisitorSimple, ScalaInspectionBundle}
 import org.jetbrains.plugins.scala.extensions._
+import org.jetbrains.plugins.scala.lang.psi.api.base.types.ScParameterizedTypeElement
 import org.jetbrains.plugins.scala.lang.psi.api.expr.{ScExpression, ScReferenceExpression}
 import org.jetbrains.plugins.scala.lang.psi.api.statements.ScFunction
-import org.jetbrains.plugins.scala.lang.psi.api.toplevel.typedef.ScClass
+import org.jetbrains.plugins.scala.lang.psi.api.toplevel.typedef.{ScClass, ScGiven}
 import org.jetbrains.plugins.scala.lang.psi.impl.toplevel.synthetic.ScSyntheticFunction
 import org.jetbrains.plugins.scala.lang.psi.types._
 import org.jetbrains.plugins.scala.lang.psi.types.api._
@@ -127,12 +128,36 @@ object ComparingUnrelatedTypesInspection {
       }
     }
   }
+
+  private def hasCanEqual(expr: ScExpression, source: ScType, target: ScType): Boolean = {
+    lazy val expressionTypes: Seq[ScType] = List(source, target)
+    lazy val canEqualExists: Boolean = expr
+      .contexts
+      .flatMap(_.children)
+      .filterByType[ScGiven]
+      .filter(_.`type`().map(_.canonicalText.matches("_root_\\.scala\\.CanEqual\\[.+?, .+?]")).getOrElse(false))
+      .flatMap(_.children.filterByType[ScParameterizedTypeElement])
+      .map(_.typeArgList.typeArgs.flatMap(_.`type`().map(_.tryExtractDesignatorSingleton).toSeq))
+      .exists(_
+        .zip(expressionTypes)
+        .forall {
+          case (givenType, compType) =>
+            !checkComparability(givenType, compType, isBuiltinOperation = true).shouldNotBeCompared
+        }
+      )
+
+    val wideSource: ScType = source.widenIfLiteral
+    // Even though CanEqual[Primitive | String, _] can be defined and will satisfy compiler in strictEquals mode,
+    // it is not possible to override equals method on the primitives or Strings
+    !wideSource.isPrimitive &&
+      !wideSource.canonicalText.matches("_root_\\.java\\.lang\\.String") &&
+      (expr.isCompilerStrictEqualityMode || canEqualExists)
+  }
 }
 
 class ComparingUnrelatedTypesInspection extends LocalInspectionTool {
 
   override def buildVisitor(holder: ProblemsHolder, isOnTheFly: Boolean): PsiElementVisitorSimple = {
-    case e if e.isInScala3File => () // TODO Handle Scala 3 code (`CanEqual` instances, etc.), SCL-19722
     case MethodRepr(expr, Some(left), Some(oper), Seq(right)) if isComparingFunctions(oper.refName) =>
       // "blub" == 3
       val needHighlighting = oper.resolve() match {
@@ -145,7 +170,8 @@ class ComparingUnrelatedTypesInspection extends LocalInspectionTool {
           case Seq(Right(leftType), Right(rightType)) =>
             val isBuiltinOperation = isIdentityFunction(oper.refName) || !hasNonDefaultEquals(leftType)
             val comparability = checkComparability(leftType, rightType, isBuiltinOperation)
-            if (comparability.shouldNotBeCompared) {
+            if ((!expr.isInScala3File && comparability.shouldNotBeCompared) ||
+              (expr.isInScala3File && comparability.shouldNotBeCompared && !hasCanEqual(expr, leftType, rightType))) {
               val message = generateComparingUnrelatedTypesMsg(leftType, rightType)(expr)
               holder.registerProblem(expr, message)
             }
@@ -158,7 +184,8 @@ class ComparingUnrelatedTypesInspection extends LocalInspectionTool {
         ParameterizedType(_, Seq(elemType)) <- receiverType(baseExpr, ref).map(_.tryExtractDesignatorSingleton)
         argType <- arg.`type`().toOption
         comparability = checkComparability(elemType, argType, isBuiltinOperation = !hasNonDefaultEquals(elemType))
-        if comparability.shouldNotBeCompared
+        if (!baseExpr.isInScala3File && comparability.shouldNotBeCompared) ||
+          (baseExpr.isInScala3File && comparability.shouldNotBeCompared && !hasCanEqual(baseExpr, elemType, argType))
       } {
         val message = generateComparingUnrelatedTypesMsg(elemType, argType)(arg)
         holder.registerProblem(arg, message)

scala/scala-impl/src/org/jetbrains/plugins/scala/project/ScalaModuleSettings.scala

@@ -7,7 +7,7 @@ import com.intellij.openapi.roots.{OrderEnumerator, OrderRootType, libraries}
 import com.intellij.openapi.util.ModificationTracker
 import com.intellij.openapi.util.io.JarUtil.{containsEntry, getJarAttribute}
 import com.intellij.openapi.vfs.VirtualFile
-import com.intellij.util.CommonProcessors.{CollectProcessor, FindProcessor}
+import com.intellij.util.CommonProcessors.FindProcessor
 import org.jetbrains.plugins.scala.ScalaVersion
 import org.jetbrains.plugins.scala.caches.cached
 import org.jetbrains.plugins.scala.project.ScalaFeatures.SerializableScalaFeatures
@@ -18,7 +18,6 @@ import org.jetbrains.sbt.project.SbtVersionProvider
 
 import java.io.File
 import java.util.jar.Attributes
-import scala.jdk.CollectionConverters.IteratorHasAsScala
 
 private class ScalaModuleSettings private(
   module: Module,
@@ -142,6 +141,9 @@ private class ScalaModuleSettings private(
   val isCompilerStrictMode: Boolean =
     settingsForHighlighting.exists(_.strict)
 
+  val isCompilerStrictEqualityMode: Boolean =
+    settingsForHighlighting.exists(_.strictEquality)
+
   val customDefaultImports: Option[Seq[String]] =
     additionalCompilerOptions.collectFirst {
       case Yimports(imports) if scalaLanguageLevel >= Scala_2_13 => imports

scala/scala-impl/src/org/jetbrains/plugins/scala/project/package.scala

@@ -288,6 +288,9 @@ package object project {
     def isCompilerStrictMode: Boolean =
       scalaModuleSettings.exists(_.isCompilerStrictMode)
 
+    def isCompilerStrictEqualityMode: Boolean =
+      scalaModuleSettings.exists(_.isCompilerStrictEqualityMode)
+
     def scalaCompilerClasspath: Seq[File] = module.scalaSdk
       .fold(throw new ScalaSdkNotConfiguredException(module)) {
         _.properties.compilerClasspath
@@ -537,6 +540,8 @@ package object project {
 
     def isCompilerStrictMode: Boolean = module.exists(_.isCompilerStrictMode)
 
+    def isCompilerStrictEqualityMode: Boolean = isInScala3Module && module.exists(_.isCompilerStrictEqualityMode)
+
     def scalaLanguageLevel: Option[ScalaLanguageLevel] =
       fromFeaturesOrModule(_.languageLevel, _.scalaLanguageLevel)
 

scala/scala-impl/src/org/jetbrains/plugins/scala/project/settings/ScalaCompilerSettings.scala

@@ -50,6 +50,8 @@ case class ScalaCompilerSettings(compileOrder: CompileOrder,
   val languageWildcard: Boolean = additionalCompilerOptions.contains("-language:_") ||
     additionalCompilerOptions.contains("--language:_")
   val strict: Boolean = additionalCompilerOptions.contains("-strict")
+  val strictEquality: Boolean = additionalCompilerOptions.contains("-language:strictEquality") ||
+    additionalCompilerOptions.contains("--language:strictEquality")
 
   def getOptionsAsStrings(forScala3Compiler: Boolean): Seq[String] = {
     val state = toState