From 171607afa79bf53dd2857d5a5af78c47f3415b54 Mon Sep 17 00:00:00 2001
From: Erik Eckstein <eeckstein@apple.com>
Date: Thu, 2 Oct 2025 19:20:05 +0200
Subject: [PATCH] DeadStoreElimination: fix a corner case of storing a trivial
 case of a non-trivial enum

Storing a trivial enum case in a non-trivial enum must be treated like a non-trivial init or assign, e.g.
```
  %1 = enum $Optional<String>, #Optional.none!enumelt
  store %1 to [trivial] %0  // <- cannot delete this store!
  store %2 to [assign] %0
```
---
 .../FunctionPasses/DeadStoreElimination.swift         |  9 ++++++++-
 test/SILOptimizer/dead_store_elim.sil                 | 11 +++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/DeadStoreElimination.swift b/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/DeadStoreElimination.swift
index c7c4a1fa44986..ef7eb618e10f9 100644
--- a/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/DeadStoreElimination.swift
+++ b/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/DeadStoreElimination.swift
@@ -167,8 +167,15 @@ private extension StoreInst {
 
   var hasValidOwnershipForDeadStoreElimination: Bool {
     switch storeOwnership {
-    case .unqualified, .trivial:
+    case .unqualified:
       return true
+    case .trivial:
+      // Storing a trivial enum case in a non-trivial enum must be treated like a non-trivial
+      // init or assign, e.g.
+      //   %1 = enum $Optional<String>, #Optional.none!enumelt
+      //   store %1 to [trivial] %0  // <- cannot delete this store!
+      //   store %2 to [assign] %0
+      return source.type.isTrivial(in: parentFunction)
     case .initialize, .assign:
       // In OSSA, non-trivial values cannot be dead-store eliminated because that could shrink
       // the lifetime of the original stored value (because it's not kept in memory anymore).
diff --git a/test/SILOptimizer/dead_store_elim.sil b/test/SILOptimizer/dead_store_elim.sil
index 4cd51cceba8f7..c55bc4992cc9e 100644
--- a/test/SILOptimizer/dead_store_elim.sil
+++ b/test/SILOptimizer/dead_store_elim.sil
@@ -1615,6 +1615,17 @@ bb0(%0 : $Int, %1 : @owned $IntAndFoo, %2 : $*IntAndFoo):
   return %r : $()
 }
 
+// We don't support this, yet.
+// Just check that we don't crash.
+sil [ossa] @test_trivial_store_of_non_trivial_enum : $@convention(thin) (@owned Optional<String>) -> @out Optional<String> {
+bb0(%0 : $*Optional<String>, %1 : @owned $Optional<String>):
+  %2 = enum $Optional<String>, #Optional.none!enumelt
+  store %2 to [trivial] %0
+  store %1 to [assign] %0
+  %r = tuple ()
+  return %r
+}
+
 // CHECK-LABEL: sil @test_bind_memory  :
 // CHECK:         store %0
 // CHECK:       end sil function 'test_bind_memory'