functionalscript · sergey-shandar · Aug 14, 2026 · Aug 14, 2026 · Aug 14, 2026 · Aug 14, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
diff --git a/changelog/unreleased/1553.md b/changelog/unreleased/1553.md
@@ -0,0 +1,5 @@
+- `djs/tokenizer`: `parseDjsMinusState` drops its `case '-'` — `js/tokenizer`
+  always merges two adjacent `-` characters into one `'--'` token, so
+  minus-state can never see another `-` — and `metadataAfterTag` drops an
+  `indexOf` miss check its one call site can never trigger
+  [#1553](https://github.com/functionalscript/functionalscript/pull/1553)
diff --git a/fjs/djs/tokenizer/module.f.mjs b/fjs/djs/tokenizer/module.f.mjs
@@ -511,10 +511,13 @@ export const tokenizeString = s => {
 // Finds `tag` in flatTokens and returns the metadata of the next code point after it.
 // numError/unterminated tags are followed by the poisoning char / EOF-hitting position
 // in the flattened AST walk order, so this pinpoints roughly where tokenization failed.
+//
+// The single call site only passes a `tag` it already confirmed via
+// `flatTokens.includes(tag)`, so `indexOf` here is never -1.
 /** @type {(tag: string, flatTokens: readonly _FlatToken[], fallback: TokenMetadata) => TokenMetadata} */
 const metadataAfterTag = (tag, flatTokens, fallback) => {
     const idx = flatTokens.indexOf(tag)
-    const found = idx < 0 ? undefined : flatTokens.slice(idx + 1).find((/** @type {_FlatToken} */ t) => t instanceof Array)
+    const found = flatTokens.slice(idx + 1).find((/** @type {_FlatToken} */ t) => t instanceof Array)
     return found === undefined ? fallback : found[1]
 }
 
@@ -592,11 +595,16 @@ const parseDjsDefaultState = input => {
 
 // Folds a leading '-' into the following number/bigint token, mirroring the old
 // fjs/djs/tokenizer's minus-state exactly.
+//
+// No `case '-'` here: the underlying `js/tokenizer` always merges two adjacent
+// `-` characters into a single `'--'` token (the decrement operator), so this
+// state — entered only after a single, unmerged `-` — can never itself see
+// another `'-'`-kind input. Such an input falls through to `default`, which
+// handles it exactly like any other non-number/bigint/eof token.
 /** @type {(input: JsToken) => readonly [List<DjsToken>, _DjsScanState]} */
 const parseDjsMinusState = input => {
     switch (input.kind) {
         case 'eof': return [[{ kind: 'error', message: 'invalid token' }, { kind: 'eof' }], { kind: 'def' }]
-        case '-': return [[{ kind: 'error', message: 'invalid token' }], { kind: '-' }]
         case 'bigint': return [[{ kind: 'bigint', value: -1n * input.value }], { kind: 'def' }]
         case 'number': return [[{ kind: 'number', bf: multiply(input.bf)(-1n), value: `-${input.value}` }], { kind: 'def' }]
         default: return [{ first: { kind: 'error', message: 'invalid token' }, tail: mapDjsToken(input) }, { kind: 'def' }]

diff --git a/fjs/djs/tokenizer/proof.f.mjs b/fjs/djs/tokenizer/proof.f.mjs
@@ -901,7 +901,10 @@ export const proof = {
             assertEq(stringify(/** @type {Unknown} */ (result)), '[{"metadata":{"column":2,"line":1,"path":""},"token":{"kind":"bigint","value":-1234567890n}},{"metadata":{"column":13,"line":1,"path":""},"token":{"kind":"eof"}}]')
         },
         () => {
-            // '-' followed by '-': one error, stays in minus-state waiting for what follows
+            // `js/tokenizer` merges '--' into one decrement-operator token, so
+            // this never enters (or re-enters) minus-state via a second '-';
+            // it's one error straight from the default state's unknown-token
+            // fallback, mapping the whole '--' token to a single error.
             const result = toArray(tokenize(stringToList('--'))(''))
             assertEq(stringify(/** @type {Unknown} */ (result)), '[{"metadata":{"column":1,"line":1,"path":""},"token":{"kind":"error","message":"invalid token"}},{"metadata":{"column":3,"line":1,"path":""},"token":{"kind":"eof"}}]')
         },
@@ -914,6 +917,12 @@ export const proof = {
             const result = toArray(tokenize(stringToList('-'))(''))
             assertEq(stringify(/** @type {Unknown} */ (result)), '[{"metadata":{"column":2,"line":1,"path":""},"token":{"kind":"error","message":"invalid token"}},{"metadata":{"column":2,"line":1,"path":""},"token":{"kind":"eof"}}]')
         },
+        () => {
+            // '-' followed by neither a number/bigint nor eof: one error for
+            // the dangling '-', then the following token maps through normally.
+            const result = toArray(tokenize(stringToList('-{'))(''))
+            assertEq(stringify(/** @type {Unknown} */ (result)), '[{"metadata":{"column":2,"line":1,"path":""},"token":{"kind":"error","message":"invalid token"}},{"metadata":{"column":2,"line":1,"path":""},"token":{"kind":"{"}},{"metadata":{"column":3,"line":1,"path":""},"token":{"kind":"eof"}}]')
+        },
         () => {
             const result = toArray(tokenize(stringToList('[-1234567890n]'))(''))
             assertEq(stringify(/** @type {Unknown} */ (result)), '[{"metadata":{"column":1,"line":1,"path":""},"token":{"kind":"["}},{"metadata":{"column":3,"line":1,"path":""},"token":{"kind":"bigint","value":-1234567890n}},{"metadata":{"column":14,"line":1,"path":""},"token":{"kind":"]"}},{"metadata":{"column":15,"line":1,"path":""},"token":{"kind":"eof"}}]')