fix: only match rest params with matchers when the matcher matches

.changeset/spotty-bears-doubt.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: only match rest params with matchers when the matcher matches

packages/kit/src/utils/routing.js

@@ -162,8 +162,12 @@ export function exec(match, params, matchers) {
 
 		// if `value` is undefined, it means this is an optional or rest parameter
 		if (value === undefined) {
-			if (param.rest) result[param.name] = '';
-			continue;
+			if (param.rest) {
+				// We need to allow the matcher to run so that it can decide if this optional rest param should be allowed to match
+				value = '';
+			} else {
+				continue;
+			}
 		}
 
 		if (!param.matcher || matchers[param.matcher](value)) {

packages/kit/src/utils/routing.spec.js

@@ -245,6 +245,26 @@ describe('exec', () => {
 			path: '/a/b/c',
 			expected: undefined
 		},
+		{
+			route: '/[...a=doesntmatch]/[b]',
+			path: '/foo',
+			expected: undefined
+		},
+		{
+			route: '/[...a=matches]/[b]',
+			path: '/foo',
+			expected: { a: '', b: 'foo' }
+		},
+		{
+			route: '/[...a=doesntmatch]/[b]/[c]',
+			path: '/foo/bar',
+			expected: undefined
+		},
+		{
+			route: '/[...a=matches]/[b]/[c]',
+			path: '/foo/bar',
+			expected: { a: '', b: 'foo', c: 'bar' }
+		},
 		{
 			route: '/[...catchall]',
 			path: '/\n',