Fix #4203: Don't expand implicit functions in patterns

Author: smarter

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1821,6 +1821,7 @@ class Typer extends Namer
             xtree.isTerm &&
             !untpd.isImplicitClosure(xtree) &&
             !ctx.mode.is(Mode.ImplicitShadowing) &&
+            !ctx.mode.is(Mode.Pattern) &&
             !ctx.isAfterTyper)
           makeImplicitFunction(xtree, ifpt)
         else xtree match {
@@ -2317,6 +2318,7 @@ class Typer extends Namer
       if (defn.isImplicitFunctionClass(wtp.underlyingClassRef(refinementOK = false).classSymbol) &&
           !untpd.isImplicitClosure(tree) &&
           !isApplyProto(pt) &&
+          !ctx.mode.is(Mode.Pattern) &&
           !ctx.isAfterTyper) {
         typr.println(i"insert apply on implicit $tree")
         typed(untpd.Select(untpd.TypedSplice(tree), nme.apply), pt, locked)

tests/pos/i4203.scala

@@ -0,0 +1,9 @@
+case class Box[Z](unbox: Z)
+
+object Test {
+  def foo(b: Box[implicit Int => Int]): Int = b match {
+    case Box(f) =>
+      implicit val i: Int = 1
+      f
+  }
+}