fix: don't error on `{type}` in declaration tags (#18321)

The `\b` in the regex matched on `}`, too, so you would get a very
confusing "invalid declaration tag" error on `{type}`. Match on
whitespace instead (because that's what has to come afterwards)

---------

Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
Co-authored-by: Rich-Harris <hello@rich-harris.dev>

packages/svelte/src/compiler/phases/1-parse/state/tag.js

@@ -12,7 +12,10 @@ import { find_matching_bracket, match_bracket } from '../utils/bracket.js';
 
 const regex_whitespace_with_closing_curly_brace = /\s*}/y;
 const regex_supported_declaration = /(?:let|const)\b/y;
-const regex_unsupported_declaration = /(?:var|function|class|type|interface|enum)\b/y;
+// All except `type` are reserved keywords and cannot be used as variable names.
+// For type we check if it's not something like `type .x` / `type ()` / `type % 2` / ...
+const regex_unsupported_declaration =
+	/(?:(?:var|function|class|interface|enum)\b)|(?:type\s+[^?.(`<[&|%^}])/y;
 
 const pointy_bois = { '<': '>' };
 

packages/svelte/tests/validator/samples/declaration-tag-invalid-type-2/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "declaration_tag_invalid_type",
+		"message": "Declaration tags must be `let` or `const` declarations",
+		"start": {
+			"line": 14,
+			"column": 2
+		},
+		"end": {
+			"line": 14,
+			"column": 8
+		}
+	}
+]

packages/svelte/tests/validator/samples/declaration-tag-invalid-type-2/input.svelte

@@ -0,0 +1,15 @@
+{#if true}
+	<!-- check that these doesn't trigger already -->
+	{type}
+	{type }
+	{type && foo}
+	{type || bar}
+	{type % 2}
+	{type .x}
+	{type ?.x}
+	{type ()}
+	{type [1]}
+	{type `tag`}
+	<!-- ... this one should trigger though -->
+	{type foo = boolean}
+{/if}