Fix KeyPath with 16-byte-aligned subscript traps on 32-bit targets

stdlib/public/core/KeyPath.swift

@@ -1198,7 +1198,7 @@ internal struct ComputedArgumentSize {
 #if _pointerBitWidth(_64)
     0x8000_0000_0000_0000
 #elseif _pointerBitWidth(_32)
-    0x6000_0000
+    0xC000_0000
 #else
 #warning("Unsupported platform")
     fatalError()

test/stdlib/KeyPath32Bit.swift

@@ -0,0 +1,52 @@
+// RUN: %target-run-simple-swift
+// RUN: %target-run-simple-swift(-O)
+// REQUIRES: executable_test
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+// Regression test for a 32-bit-only mask-overlap bug in `ComputedArgumentSize`
+// (`stdlib/public/core/KeyPath.swift`).
+
+import StdlibUnittest
+
+let tests = TestSuite("KeyPath32Bit")
+
+struct Inner {
+  let base: Int
+  subscript(idx: SIMD4<Int32>) -> Int {
+    return base + Int(idx[0]) + Int(idx[1]) + Int(idx[2]) + Int(idx[3])
+  }
+}
+
+struct Outer {
+  var inner: Inner
+}
+
+tests.test("appended keypath with 16-byte-aligned subscript argument") {
+  // Precondition: SIMD4<Int32> must have 16-byte alignment on this target.
+  // If a future stdlib change demotes the alignment, this test would stop
+  // exercising the bug's codepath — assert loudly instead of silently.
+  expectEqual(16, MemoryLayout<SIMD4<Int32>>.alignment)
+  expectEqual(16, MemoryLayout<SIMD4<Int32>>.size)
+
+  let arg = SIMD4<Int32>(1, 2, 3, 4)
+
+  // The leaf keypath captures a 16-aligned argument.
+  let leaf: KeyPath<Inner, Int> = \Inner.[arg]
+
+  // `appending(path:)` forces the buggy `calculateAppendedKeyPathSize` /
+  // `_storeInto` paths that read `.alignment` back from the leaf component's
+  // header. Under the buggy mask, the read returns 0; the appended buffer is
+  // under-allocated; projection traps inside the stdlib.
+  let composed: KeyPath<Outer, Int> = (\Outer.inner).appending(path: leaf)
+
+  // Normal keypath projection. On a correct stdlib this returns the expected
+  // value. On a buggy stdlib it traps before returning, which lit records as
+  // a non-zero exit code (test FAIL).
+  let observed = Outer(inner: Inner(base: 100))[keyPath: composed]
+
+  // 100 (base) + 1 + 2 + 3 + 4 = 110.
+  expectEqual(110, observed)
+}
+
+runAllTests()