fix: default values in object destructuring (#14554)

* fix: enhance string/expression length check and fix closing character issues

* docs: add documentation for unterminated_string_constant error

* test: add tests for object destructuring with default values in "each" blocks

* Update .changeset/clean-planets-rush.md

* refactor: clean up unnecessary comments and whitespace

* fix: resolve formatting issues

* simplify

* tweak

* regenerate

---------

Co-authored-by: Rich Harris <[email protected]>

.changeset/clean-planets-rush.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle default values in object destructuring within "each" blocks when using characters like "}" and "]"

documentation/docs/98-reference/.generated/compile-errors.md

@@ -1012,6 +1012,12 @@ Unexpected end of input
 '%word%' is a reserved word in JavaScript and cannot be used here
 ```
 
+### unterminated_string_constant
+
+```
+Unterminated string constant
+```
+
 ### void_element_invalid_content
 
 ```

packages/svelte/messages/compile-errors/template.md

@@ -418,6 +418,10 @@ See https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-ele
 
 > '%word%' is a reserved word in JavaScript and cannot be used here
 
+## unterminated_string_constant
+
+> Unterminated string constant
+
 ## void_element_invalid_content
 
 > Void elements cannot have children or closing tags

packages/svelte/src/compiler/errors.js

@@ -1574,6 +1574,15 @@ export function unexpected_reserved_word(node, word) {
 	e(node, "unexpected_reserved_word", `'${word}' is a reserved word in JavaScript and cannot be used here\nhttps://svelte.dev/e/unexpected_reserved_word`);
 }
 
+/**
+ * Unterminated string constant
+ * @param {null | number | NodeLike} node
+ * @returns {never}
+ */
+export function unterminated_string_constant(node) {
+	e(node, "unterminated_string_constant", `Unterminated string constant\nhttps://svelte.dev/e/unterminated_string_constant`);
+}
+
 /**
  * Void elements cannot have children or closing tags
  * @param {null | number | NodeLike} node

packages/svelte/src/compiler/phases/1-parse/read/context.js

@@ -37,33 +37,11 @@ export default function read_pattern(parser) {
 		e.expected_pattern(i);
 	}
 
-	/** @type {string[]} */
-	const bracket_stack = [];
-
-	while (i < parser.template.length) {
-		const char = parser.template[i];
-
-		if (is_bracket_open(char)) {
-			bracket_stack.push(char);
-		} else if (is_bracket_close(char)) {
-			const popped = /** @type {string} */ (bracket_stack.pop());
-			const expected = /** @type {string} */ (get_bracket_close(popped));
-
-			if (char !== expected) {
-				e.expected_token(i, expected);
-			}
-
-			if (bracket_stack.length === 0) {
-				i += 1;
-				break;
-			}
-		}
-		i += 1;
-	}
-
+	i = match_bracket(parser, start);
 	parser.index = i;
 
 	const pattern_string = parser.template.slice(start, i);
+
 	try {
 		// the length of the `space_with_newline` has to be start - 1
 		// because we added a `(` in front of the pattern_string,
@@ -93,6 +71,75 @@ export default function read_pattern(parser) {
 	}
 }
 
+/**
+ * @param {Parser} parser
+ * @param {number} start
+ */
+function match_bracket(parser, start) {
+	const bracket_stack = [];
+
+	let i = start;
+
+	while (i < parser.template.length) {
+		let char = parser.template[i++];
+
+		if (char === "'" || char === '"' || char === '`') {
+			i = match_quote(parser, i, char);
+			continue;
+		}
+
+		if (is_bracket_open(char)) {
+			bracket_stack.push(char);
+		} else if (is_bracket_close(char)) {
+			const popped = /** @type {string} */ (bracket_stack.pop());
+			const expected = /** @type {string} */ (get_bracket_close(popped));
+
+			if (char !== expected) {
+				e.expected_token(i - 1, expected);
+			}
+
+			if (bracket_stack.length === 0) {
+				return i;
+			}
+		}
+	}
+
+	e.unexpected_eof(parser.template.length);
+}
+
+/**
+ * @param {Parser} parser
+ * @param {number} start
+ * @param {string} quote
+ */
+function match_quote(parser, start, quote) {
+	let is_escaped = false;
+	let i = start;
+
+	while (i < parser.template.length) {
+		const char = parser.template[i++];
+
+		if (is_escaped) {
+			is_escaped = false;
+			continue;
+		}
+
+		if (char === quote) {
+			return i;
+		}
+
+		if (char === '\\') {
+			is_escaped = true;
+		}
+
+		if (quote === '`' && char === '$' && parser.template[i] === '{') {
+			i = match_bracket(parser, i);
+		}
+	}
+
+	e.unterminated_string_constant(start);
+}
+
 /**
  * @param {Parser} parser
  * @returns {any}

packages/svelte/tests/parser-modern/samples/each-block-object-pattern-special-characters/input.svelte

@@ -0,0 +1,9 @@
+{#each x as { y = 'z' }}{/each}
+
+{#each x as { y = '{' }}{/each}
+
+{#each x as { y = ']' }}{/each}
+
+{#each x as { y = `${`"`}` }}{/each}
+
+{#each x as { y = `${`John`}` }}{/each}