fix(parser): report a src: range that points below the first slide (#2734)

Co-authored-by: Mahiro Hirakawa <mahirohirakawa@glovrex.com>

Author: mahirhir

packages/parser/src/fs.ts

@@ -98,6 +98,16 @@ export async function load(
     const directImporter = importers?.at(-1)
     for (const index of parseRangeString(md.slides.length, range)) {
       const subSlide = md.slides[index - 1]
+      if (!subSlide) {
+        // `parseRangeString` only drops indexes above the total, so a range
+        // such as `#0` or `#-3` still reaches here and has no slide.
+        md.errors ??= []
+        md.errors.push({
+          row: 0,
+          message: `Slide ${index} does not exist in "${path}", which has ${md.slides.length} slides`,
+        })
+        continue
+      }
       try {
         await loadSlide(md, subSlide, frontmatterOverride, importers)
       }

test/fixtures/markdown/out-of-range/entry.md

@@ -0,0 +1,5 @@
+# Entry
+
+---
+src: ./sub.md#0
+---

test/fixtures/markdown/out-of-range/sub.md

@@ -0,0 +1,5 @@
+# One
+
+---
+
+# Two

test/parser.test.ts

@@ -594,6 +594,15 @@ Some content
     expect(errors.some(e => /circular/i.test(e.message))).toBe(true)
   })
 
+  it('records an error when a src: import selects a slide below the first one', async () => {
+    const root = resolve(__dirname, 'fixtures/markdown/out-of-range')
+    const data = await load({ userRoot: root, roots: [root] }, resolve(root, 'entry.md'))
+    const errors = Object.values(data.markdownFiles).flatMap(md => md.errors ?? [])
+    expect(errors.map(e => e.message)).toEqual([
+      expect.stringContaining('Slide 0 does not exist'),
+    ])
+  })
+
   it('records an error when a src: import escapes the allowed roots', async () => {
     const root = resolve(__dirname, 'fixtures/markdown/escaping/root')
     const data = await load({ userRoot: root, roots: [root], allowedRoots: [root] }, resolve(root, 'entry.md'))