Handle leading given parameters in inline unapplies

Fixes #12991

Author: nicolasstucki

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

@@ -181,9 +181,9 @@ 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 y1,y2,...)(x1,x2,...)` is transformed into
+    // `{ class $anon { def unapply(using l1: L1, l2: L2, ...)(s: S)(using t1: T1, t2: T2, ...): R = P.unapply(using l1, l2, ...)(s)(using t1, t2, ...) }; new $anon }.unapply(using y1,y2,...)`
+    // and the call `P.unapply(using l1, l2,...)(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`.
@@ -193,9 +193,6 @@ object Inlines:
         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 sym = unapp.symbol
 
@@ -217,7 +214,7 @@ object Inlines:
       List(unapply)
     }
 
-    val newFun = newUnapply.select(unapplySym1).withSpan(unapp.span)
+    val newFun = newUnapply.select(unapplySym1).appliedToArgss(leadingImplicits).withSpan(unapp.span)
     cpy.UnApply(unapp)(newFun, trailingImplicits, patterns)
   end inlinedUnapply
 

tests/pos/i12991.scala

@@ -0,0 +1,15 @@
+object Foo:
+  inline def unapply(using String)(i: Int): Some[Int] = Some(i)
+
+object Bar:
+  inline def unapply(using String)(using String)(i: Int): Some[Int] = Some(i)
+
+object Baz:
+  inline def unapply[T](using String)(i: T): Some[T] = Some(i)
+
+given String = ""
+
+val i = 10 match
+  case Foo(x) => x
+  case Bar(x) => x
+  case Baz(x) => x