Handle inlining of matches with sub cases

A sub match is inline iff the outer match is

Author: EugeneFlesselle

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

@@ -394,9 +394,13 @@ class InlineReducer(inliner: Inliner)(using Context):
             case ConstantValue(v: Boolean) => (v, true)
             case _ => (false, false)
           }
-        if guardOK then Some((caseBindings.map(_.subst(from, to)), cdef.body.subst(from, to), canReduceGuard))
-        else if canReduceGuard then None
-        else Some((caseBindings.map(_.subst(from, to)), cdef.body.subst(from, to), canReduceGuard))
+        if !canReduceGuard then Some((List.empty, EmptyTree, false))
+        else if !guardOK then None
+        else cdef.body.subst(from, to) match
+          case t: SubMatch => // a sub match of an inline match is also inlined
+            reduceInlineMatch(t.selector, t.selector.tpe, t.cases, typer).map:
+              (subCaseBindings, rhs) => (caseBindings.map(_.subst(from, to)) ++ subCaseBindings, rhs, true)
+          case b => Some((caseBindings.map(_.subst(from, to)), b, true))
       }
       else None
     }

tests/pos/inline-sub-match.scala

@@ -0,0 +1,30 @@
+import scala.language.experimental.matchWithSubCases
+
+object Test:
+
+  // using transparent to test whether test whether reduced as expected
+  transparent inline def foo(i: Int, j: Int): String =
+    inline i match
+      case 0 with j match
+        case 1 => "01"
+        case 2 => "02"
+      case 1 with j match
+        case 1 => "11"
+        case 2 => "12"
+      case _ => "3"
+
+  val r01: "01" = foo(0, 1)
+  val r02: "02" = foo(0, 2)
+  val r11: "11" = foo(1, 1)
+  val r31: "3" = foo(3, 1)
+
+  transparent inline def bar(x: Option[Any]): String =
+    inline x match
+      case Some(y: Int) with y match
+        case 1 => "a"
+        case 2 => "b"
+      case Some(z: String) => "c"
+      case _ => "d"
+
+  val x = bar(Some(2)) // FIX this reduces to "c", but this appears to be a more general issue than sub-matches
+  val y = bar(Some("hello"))