Scala compiler options I typically use

1 - scalac options 2.13.scala

val scalacOptions = Seq(
  "-encoding",
  "utf-8", // Specify character encoding used by source files.
  "-Ybackend-parallelism", //
  "8",
  "-explaintypes", // Explain type errors in more detail.
  "-feature", // Emit warning and location for usages of features that should be imported explicitly.
  "-unchecked", // Enable additional warnings where generated code depends on assumptions.
  "-Xcheckinit", // Wrap field accessors to throw an exception on uninitialized access.
  // "-Ymacro-annotations", // Enable support for macro annotations, formerly in macro paradise.

  // ********** -language: Enable or disable language features *****************
  // "-language:dynamics", //Allow direct or indirect subclasses of scala.Dynamic
  // "-language:existentials", // Existential types (besides wildcard types) can be written and inferred
  // "-language:higherKinds", // Allow higher-kinded types
  // "-language:implicitConversions", // Allow definition of implicit functions called views
  // "-language:postfixOps", // Allow postfix operator notation, such as 1 to 10 toList (not recommended)
  // "-language:reflectiveCalls", // Allow reflective access to members of structural types
  // "-language:experimental.macros", // Allow macro definition (besides implementation and application)

  // ********** Warning Settings ***********************************************
  "-Werror", // Fail the compilation if there are any warnings.
  "-Wdead-code", //  Warn when dead code is identified.
  "-Wextra-implicit", // Warn when more than one implicit parameter section is defined.
  // "-Wmacros:none", // Do not inspect expansions or their original trees when generating unused symbol warnings.
  // "-Wmacros:before", // Only inspect unexpanded user-written code for unused symbols. (Default)
  "-Wmacros:after", // Only inspect expanded trees when generating unused symbol warnings.
  // "-Wmacros:both", // Inspect both user-written code and expanded trees when generating unused symbol warnings.
  "-Wnumeric-widen", // Warn when numerics are widened.
  "-Woctal-literal", // Warn on obsolete octal syntax.
  // "-Wself-implicit", // Warn when an implicit resolves to an enclosing self-definition.
  "-Wunused:imports", //Warn if an import selector is not referenced.
  "-Wunused:patvars", // Warn if a variable bound in a pattern is unused.
  "-Wunused:privates", // Warn if a private member is unused.
  "-Wunused:locals", // Warn if a local definition is unused.
  "-Wunused:explicits", // Warn if an explicit parameter is unused.
  "-Wunused:implicits", // Warn if an implicit parameter is unused.
  "-Wunused:params",  // Enable -Wunused:explicits,implicits.
  "-Wvalue-discard", // Warn when non-Unit expression results are unused.

  // ********** -Xlint: Enable recommended warnings ****************************
  "-Xlint:adapted-args", // Warn if an argument list is modified to match the receiver.
  "-Xlint:nullary-unit", // Warn when nullary methods return Unit.
  "-Xlint:inaccessible", // Warn about inaccessible types in method signatures.
  "-Xlint:infer-any", // Warn when a type argument is inferred to be Any.
  // "-Xlint:missing-interpolator", // A string literal appears to be missing an interpolator id.
  "-Xlint:doc-detached", // A Scaladoc comment appears to be detached from its element.
  "-Xlint:private-shadow", // A private field (or class parameter) shadows a superclass field.
  "-Xlint:type-parameter-shadow", // A local type parameter shadows a type already in scope.
  "-Xlint:poly-implicit-overload", // Parameterized overloaded implicit methods are not visible as view bounds.
  "-Xlint:option-implicit", // Option.apply used implicit view.
  "-Xlint:delayedinit-select", // Selecting member of DelayedInit.
  "-Xlint:package-object-classes", // Class or object defined in package object.
  "-Xlint:stars-align", // Pattern sequence wildcard must align with sequence component.
  "-Xlint:constant", // Evaluation of a constant arithmetic expression results in an error.
  "-Xlint:nonlocal-return", // A return statement used an exception for flow control.
  "-Xlint:implicit-not-found", // Check @implicitNotFound and @implicitAmbiguous messages.
  "-Xlint:serial", // @SerialVersionUID on traits and non-serializable classes.
  "-Xlint:valpattern", // Enable pattern checks in val definitions.
  "-Xlint:eta-zero", // Warn on eta-expansion (rather than auto-application) of zero-ary method.
  "-Xlint:eta-sam", // Warn on eta-expansion to meet a Java-defined functional interface that is not explicitly annotated with @FunctionalInterface.
  "-Xlint:deprecation", // Enable linted deprecations.
)

2 - scalac options 2.12.scala

val scalacOptions = Seq(
  "-encoding",
  "utf-8", // Specify character encoding used by source files.
  "-Ybackend-parallelism", // maximum worker threads for backend
  "8",
  "-explaintypes", // Explain type errors in more detail.
  "-feature", // Emit warning and location for usages of features that should be imported explicitly.
  "-unchecked", // Enable additional warnings where generated code depends on assumptions.
  "-Xcheckinit", // Wrap field accessors to throw an exception on uninitialized access.
  "-Ypartial-unification", // Enable partial unification in type constructor inference.
  "-Yno-adapted-args", // Do not adapt an argument list (either by inserting () or creating a tuple) to match the receiver.
  "-deprecation", // Emit warning and location for usages of deprecated APIs.
  // ********** -language: Enable or disable language features *****************
  // "-language:dynamics", //Allow direct or indirect subclasses of scala.Dynamic
  // "-language:existentials", // Existential types (besides wildcard types) can be written and inferred
  // "-language:higherKinds", // Allow higher-kinded types
  // "-language:implicitConversions", // Allow definition of implicit functions called views
  // "-language:postfixOps", // Allow postfix operator notation, such as 1 to 10 toList (not recommended)
  // "-language:reflectiveCalls", // Allow reflective access to members of structural types
  // "-language:experimental.macros", // Allow macro definition (besides implementation and application)
  // ********** Warning Settings ***********************************************
  "-Xfatal-warnings", // Fail the compilation if there are any warnings.
  "-Ywarn-dead-code", // Warn when dead code is identified.
  "-Ywarn-extra-implicit", // Warn when more than one implicit parameter section is defined.
  // "-Ywarn-macros:none", // Do not inspect expansions or their original trees when generating unused symbol warnings.
  // "-Ywarn-macros:before", // Only inspect unexpanded user-written code for unused symbols. (Default)
  "-Ywarn-macros:after", // Only inspect expanded trees when generating unused symbol warnings.
  // "-Ywarn-macros:both", // Inspect both user-written code and expanded trees when generating unused symbol warnings.
  "-Ywarn-numeric-widen", // Warn when numerics are widened.
  "-Ywarn-unused:implicits", // Warn if an implicit parameter is unused.
  "-Ywarn-unused:imports", // Warn if an import selector is not referenced.
  "-Ywarn-unused:locals", // Warn if a local definition is unused.
  "-Ywarn-unused:params", // Warn if a value parameter is unused.
  "-Ywarn-unused:patvars", // Warn if a variable bound in a pattern is unused.
  "-Ywarn-unused:privates", // Warn if a private member is unused.
  "-Ywarn-value-discard", // Warn when non-Unit expression results are unused.
  // ********** -Xlint: Enable recommended warnings ****************************
  "-Xlint:adapted-args", // Warn if an argument list is modified to match the receiver.
  "-Xlint:nullary-unit", // Warn when nullary methods return Unit.
  "-Xlint:inaccessible", // Warn about inaccessible types in method signatures.
  "-Xlint:nullary-override", // Warn when non-nullary `def f()' overrides nullary `def f'.
  "-Xlint:infer-any", // Warn when a type argument is inferred to be `Any`.
  // "-Xlint:missing-interpolator", // A string literal appears to be missing an interpolator id.
  "-Xlint:doc-detached", // A Scaladoc comment appears to be detached from its element.
  "-Xlint:private-shadow", // A private field (or class parameter) shadows a superclass field.
  "-Xlint:type-parameter-shadow", // A local type parameter shadows a type already in scope.
  "-Xlint:poly-implicit-overload", // Parameterized overloaded implicit methods are not visible as view bounds.
  "-Xlint:option-implicit", // Option.apply used implicit view.
  "-Xlint:delayedinit-select", // Selecting member of DelayedInit.
  "-Xlint:package-object-classes", // Class or object defined in package object.
  "-Xlint:stars-align", // Pattern sequence wildcard must align with sequence component.
  "-Xlint:constant", // Evaluation of a constant arithmetic expression results in an error.
  "-Xlint:by-name-right-associative", // By-name parameter of right associative operator.
  "-Xlint:unsound-match", // Pattern match may not be typesafe.
)

3 - links.md

• Scala Compiler Options | Scala Documentation
• Recommended Scalac Flags for 2.12 - tpolecat
• Recommended Scalac Flags For 2.13 · Nathan Kleyn