Support inline unapplySeq and with leading given parameters (#16358)

Based on #15191 by @gorilskij  and enables support for #12991.

Fixes #8577
Fixes #12991
Fixes #15188

Author: nicolasstucki

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -181,37 +181,28 @@ object Inlines:
     // as its right hand side. The call to the wrapper unapply serves as the signpost for pattern matching.
     // After pattern matching, the anonymous class is removed in phase InlinePatterns with a beta reduction step.
     //
-    // An inline unapply `P.unapply` in a plattern `P(x1,x2,...)` is transformed into
-    // `{ class $anon { def unapply(t0: T0)(using t1: T1, t2: T2, ...): R = P.unapply(t0)(using t1, t2, ...) }; new $anon }.unapply`
-    // and the call `P.unapply(x1, x2, ...)` is inlined.
+    // An inline unapply `P.unapply` in a pattern `P[...](using ...)(x1,x2,...)(using t1: T1, t2: T2, ...)` is transformed into
+    // `{ class $anon { def unapply(s: S)(using t1: T1, t2: T2, ...): R = P.unapply[...](using ...)(s)(using t1, t2, ...) }; new $anon }.unapply(using y1,y2,...)`
+    // and the call `P.unapply[...](using ...)(x1, x2, ...)(using t1, t2, ...)` is inlined.
     // This serves as a placeholder for the inlined body until the `patternMatcher` phase. After pattern matcher
     // transforms the patterns into terms, the `inlinePatterns` phase removes this anonymous class by β-reducing
     // the call to the `unapply`.
 
-    object SplitFunAndGivenArgs:
-      def unapply(tree: Tree): (Tree, List[List[Tree]]) = tree match
-        case Apply(SplitFunAndGivenArgs(fn, argss), args) => (fn, argss :+ args)
-        case _ => (tree, Nil)
-    val UnApply(SplitFunAndGivenArgs(fun, leadingImplicits), trailingImplicits, patterns) = unapp
-    if leadingImplicits.flatten.nonEmpty then
-      // To support them see https://github.com/lampepfl/dotty/pull/13158
-      report.error("inline unapply methods with given parameters before the scrutinee are not supported", fun)
+    val UnApply(fun, trailingImplicits, patterns) = unapp
 
     val sym = unapp.symbol
 
     var unapplySym1: Symbol = NoSymbol // created from within AnonClass() and used afterwards
 
     val newUnapply = AnonClass(ctx.owner, List(defn.ObjectType), sym.coord) { cls =>
-      val targs = fun match
-        case TypeApply(_, targs) => targs
-        case _ => Nil
-      val unapplyInfo = sym.info match
-        case info: PolyType => info.instantiate(targs.map(_.tpe))
-        case info => info
-
-      val unapplySym = newSymbol(cls, sym.name.toTermName, Synthetic | Method, unapplyInfo, coord = sym.coord).entered
+      // `fun` is a partially applied method that contains all type applications of the method.
+      // The methodic type `fun.tpe.widen` is the type of the function starting from the scrutinee argument
+      // and its type parameters are instantiated.
+      val unapplySym = newSymbol(cls, sym.name.toTermName, Synthetic | Method, fun.tpe.widen, coord = sym.coord).entered
       val unapply = DefDef(unapplySym.asTerm, argss =>
-        inlineCall(fun.appliedToArgss(argss).withSpan(unapp.span))(using ctx.withOwner(unapplySym))
+        val body = fun.appliedToArgss(argss).withSpan(unapp.span)
+        if body.symbol.is(Transparent) then inlineCall(body)(using ctx.withOwner(unapplySym))
+        else body
       )
       unapplySym1 = unapplySym
       List(unapply)

tests/neg/i12991.scala

@@ -0,0 +1,11 @@
+package i8577
+
+import scala.quoted._
+
+object Macro:
+  opaque type StrCtx = StringContext
+  def apply(ctx: StringContext): StrCtx = ctx
+  def unapply(ctx: StrCtx): Option[StringContext] = Some(ctx)
+
+def implUnapply(sc: Expr[Macro.StrCtx], input: Expr[Int])(using Quotes): Expr[Option[Seq[Int]]] =
+  '{ Some(Seq(${input})) }

tests/pos-macros/i8577a/Macro_1.scala

@@ -0,0 +1,9 @@
+package i8577
+
+def main: Unit =
+  extension (ctx: StringContext) def mac: Macro.StrCtx = Macro(ctx)
+  extension (inline ctx: Macro.StrCtx) inline def unapplySeq(inline input: Int): Option[Seq[Int]] =
+    ${ implUnapply('ctx, 'input) }
+
+  val mac"$x" = 1
+  assert(x == 1)

tests/pos-macros/i8577a/Main_2.scala

@@ -0,0 +1,11 @@
+package i8577
+
+import scala.quoted._
+
+object Macro:
+  opaque type StrCtx = StringContext
+  def apply(ctx: StringContext): StrCtx = ctx
+  def unapply(ctx: StrCtx): Option[StringContext] = Some(ctx)
+
+def implUnapply[U](sc: Expr[Macro.StrCtx], input: Expr[U])(using Type[U])(using Quotes): Expr[Option[Seq[U]]] =
+  '{ Some(Seq(${input})) }

tests/pos-macros/i8577b/Macro_1.scala

@@ -0,0 +1,9 @@
+package i8577
+
+def main: Unit =
+  extension (ctx: StringContext) def mac: Macro.StrCtx = Macro(ctx)
+  extension (inline ctx: Macro.StrCtx) inline def unapplySeq[U](inline input: U): Option[Seq[U]] =
+    ${ implUnapply('ctx, 'input) }
+
+  val mac"$x" = 1
+  assert(x == 1)

tests/pos-macros/i8577b/Main_2.scala

@@ -0,0 +1,11 @@
+package i8577
+
+import scala.quoted._
+
+object Macro:
+  opaque type StrCtx = StringContext
+  def apply(ctx: StringContext): StrCtx = ctx
+  def unapply(ctx: StrCtx): Option[StringContext] = Some(ctx)
+
+def implUnapply[T](sc: Expr[Macro.StrCtx], input: Expr[T])(using Type[T])(using Quotes): Expr[Option[Seq[T]]] =
+  '{ Some(Seq(${input})) }

tests/pos-macros/i8577c/Macro_1.scala

@@ -0,0 +1,9 @@
+package i8577
+
+def main: Unit =
+  extension (ctx: StringContext) def mac: Macro.StrCtx = Macro(ctx)
+  extension [T] (inline ctx: Macro.StrCtx) inline def unapplySeq(inline input: T): Option[Seq[T]] =
+    ${ implUnapply('ctx, 'input) }
+
+  val mac"$x" = 1
+  assert(x == 1)

tests/pos-macros/i8577c/Main_2.scala

@@ -0,0 +1,11 @@
+package i8577
+
+import scala.quoted._
+
+object Macro:
+  opaque type StrCtx = StringContext
+  def apply(ctx: StringContext): StrCtx = ctx
+  def unapply(ctx: StrCtx): Option[StringContext] = Some(ctx)
+
+def implUnapply[T](sc: Expr[Macro.StrCtx], input: Expr[T])(using Type[T])(using Quotes): Expr[Option[Seq[T]]] =
+  '{ Some(Seq(${input})) }

tests/pos-macros/i8577d/Macro_1.scala

@@ -0,0 +1,9 @@
+package i8577
+
+def main: Unit =
+  extension (ctx: StringContext) def mac: Macro.StrCtx = Macro(ctx)
+  extension [T] (inline ctx: Macro.StrCtx) inline def unapplySeq[U](inline input: T): Option[Seq[T]] =
+    ${ implUnapply('ctx, 'input) }
+
+  val mac"$x" = 1
+  assert(x == 1)