fix: emit replace patches for indices dropped by shrinking length

__tests__/patch.js

@@ -790,6 +790,74 @@ runPatchTests(
 	]
 )
 
+// #1355
+runPatchTests(
+	"arrays - truncate with length, then grow",
+	{x: [1]},
+	d => {
+		d.x.length = 0
+		d.x[1] = 2
+	},
+	[
+		{op: "replace", path: ["x", 0], value: undefined},
+		{op: "add", path: ["x", 1], value: 2}
+	],
+	[
+		{op: "replace", path: ["x", 0], value: 1},
+		{op: "remove", path: ["x", 1]}
+	],
+	{x: [undefined, 2]}
+)
+
+runPatchTests(
+	"arrays - truncate with length, then restore the length",
+	{x: [1, 2]},
+	d => {
+		d.x.length = 0
+		d.x.length = 2
+	},
+	[
+		{op: "replace", path: ["x", 0], value: undefined},
+		{op: "replace", path: ["x", 1], value: undefined}
+	],
+	[
+		{op: "replace", path: ["x", 0], value: 1},
+		{op: "replace", path: ["x", 1], value: 2}
+	],
+	{x: [undefined, undefined]}
+)
+
+runPatchTests(
+	"arrays - truncate with length, then push",
+	{x: [1, 2, 3]},
+	d => {
+		d.x.length = 1
+		d.x.push(4, 5)
+	},
+	[
+		{op: "replace", path: ["x", 1], value: 4},
+		{op: "replace", path: ["x", 2], value: 5}
+	],
+	[
+		{op: "replace", path: ["x", 1], value: 2},
+		{op: "replace", path: ["x", 2], value: 3}
+	],
+	{x: [1, 4, 5]}
+)
+
+runPatchTests(
+	"arrays - truncate with length, then grow back to the same values",
+	{x: [1, 2]},
+	d => {
+		d.x.length = 0
+		d.x[0] = 1
+		d.x[1] = 2
+	},
+	[],
+	[],
+	{x: [1, 2]}
+)
+
 runPatchTests(
 	"arrays - pop twice",
 	{x: [1, 2, 3]},

src/core/proxy.ts

@@ -284,7 +284,21 @@ arrayTraps.set = function (state, prop, value) {
 		isNaN(parseInt(prop as any))
 	)
 		die(14)
-	return objectTraps.set!.call(this, state[0], prop, value, state[0])
+	const arrayState = state[0]
+	const oldLength = prop === "length" ? latest(arrayState).length : 0
+	const result = objectTraps.set!.call(
+		this,
+		arrayState,
+		prop,
+		value,
+		arrayState
+	)
+	// Shrinking `length` drops the tail without going through `deleteProperty`,
+	// so mark the dropped base indices as assigned. Otherwise, if the array grows
+	// back, patch generation sees no assignment for them and emits no `replace`.
+	const end = Math.min(oldLength, arrayState.base_.length)
+	for (let i = +value; i < end; i++) arrayState.assigned_!.set("" + i, true)
+	return result
 }
 
 // Access a property without creating an Immer draft.