Look at the most precise type for inline matches

Fixes #11291
Fixes #6781

Author: nicolasstucki

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

@@ -1291,7 +1291,13 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
       if (!tree.isInline || ctx.owner.isInlineMethod) // don't reduce match of nested inline method yet
         super.typedMatchFinish(tree, sel, wideSelType, cases, pt)
       else {
-        val selType = if (sel.isEmpty) wideSelType else sel.tpe
+        def selTyped(sel: Tree): Type = sel match {
+          case Typed(sel2, _) => selTyped(sel2)
+          case Block(Nil, sel2) => selTyped(sel2)
+          case Inlined(_, Nil, sel2) => selTyped(sel2)
+          case _ => sel.tpe
+        }
+        val selType = if (sel.isEmpty) wideSelType else selTyped(sel)
         reduceInlineMatch(sel, selType, cases.asInstanceOf[List[CaseDef]], this) match {
           case Some((caseBindings, rhs0)) =>
             // drop type ascriptions/casts hiding pattern-bound types (which are now aliases after reducing the match)
@@ -1509,4 +1515,5 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
             case _ => None
   }
 
+
 }

tests/pos/i11291.scala

@@ -0,0 +1,11 @@
+inline def meth =
+  val x1 = inline ("a": Any) match
+    case _: String => "ok"
+  val x2 = inline { "a": Any } match
+    case _: String => "ok"
+  inline s match
+    case _: String => "ok"
+
+inline def s = "a": Any
+
+def test = meth

tests/pos/i6781.scala

@@ -0,0 +1,13 @@
+enum Nat {
+  case Zero
+  case Succ[N <: Nat](n: N)
+}
+import Nat._
+
+inline def toInt(inline n: Nat): Int = inline n match {
+  case Zero => 0
+  case Succ(n1) => toInt(n1) + 1
+}
+
+val natTwoA = toInt(Succ[Succ[Zero.type]](Succ(Zero)))
+val natTwoB = toInt(Succ(Succ(Zero)): Succ[Succ[Zero.type]])