Fix #5574

Re-typing the pattern in GADT context is unnecessary - reducing
the pattern will on its own perform the same subtyping checks.

Previously if a pattern binding a HK type was inlined, a warning
saying that its kind is different from the kind of its parameter
was emitted _after_ preparing for reduction. This replaced the type
of the Bind node with an error, preventing the bind from being
located and narrowed when reducing the inline match.

Author: abgruszecki

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

@@ -855,10 +855,9 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(implicit ctx: Context) {
         }
         if (!isImplicit) caseBindingsBuf += scrutineeBinding
         val gadtCtx = typer.gadtContext(gadtSyms).addMode(Mode.GADTflexible)
-        val pat1 = typer.typedPattern(cdef.pat, scrutType)(gadtCtx)
         val fromBuf = mutable.ListBuffer.empty[TypeSymbol]
         val toBuf = mutable.ListBuffer.empty[TypeSymbol]
-        if (reducePattern(caseBindingsBuf, fromBuf, toBuf, scrutineeSym.termRef, pat1)(gadtCtx) && guardOK) {
+        if (reducePattern(caseBindingsBuf, fromBuf, toBuf, scrutineeSym.termRef, cdef.pat)(gadtCtx) && guardOK) {
           val caseBindings = caseBindingsBuf.toList
           val from = fromBuf.toList
           val to = toBuf.toList

tests/pos/i5574.scala

@@ -0,0 +1,14 @@
+import scala.typelevel._
+
+object i5574 {
+  class Box[F[_]]
+
+  inline def foo[T] <: Any =
+    inline erasedValue[T] match {
+      case _: Box[f] =>
+        type t = f
+        23
+    }
+
+  foo[Box[List]]
+}