Backport "Fix #20372: Check pattern match exhaustivity in inlined cod…

…e" to LTS (#21115)

Backports #20403 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -56,6 +56,10 @@ class PatternMatcher extends MiniPhase {
       if !inInlinedCode then
         // check exhaustivity and unreachability
         SpaceEngine.checkMatch(tree)
+      else
+        // only check exhaustivity, as inlining may generate unreachable code 
+        // like in i19157.scala
+        SpaceEngine.checkMatchExhaustivityOnly(tree)
 
       translated.ensureConforms(matchType)
     }

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -903,6 +903,9 @@ object SpaceEngine {
   }
 
   def checkMatch(m: Match)(using Context): Unit =
-    if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
+    checkMatchExhaustivityOnly(m)
     if reachabilityCheckable(m.selector) then checkReachability(m)
+
+  def checkMatchExhaustivityOnly(m: Match)(using Context): Unit =
+    if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
 }

tests/warn/i20372.check

@@ -0,0 +1,8 @@
+-- [E029] Pattern Match Exhaustivity Warning: tests/warn/i20372.scala:8:5 ----------------------------------------------
+8 |  id(foo match { // warn
+  |     ^^^
+  |     match may not be exhaustive.
+  |
+  |     It would fail on pattern case: Baz
+  |
+  | longer explanation available when compiling with `-explain`

tests/warn/i20372.scala

@@ -0,0 +1,10 @@
+sealed trait Foo
+case object Bar extends Foo
+case object Baz extends Foo
+
+inline def id[A](a: A): A = a
+
+def shouldThrowAWarning(foo: Foo) =
+  id(foo match { // warn
+    case Bar => "Bar"
+  })