Don't track syntax with unknown source locations

Avoids recording original syntaxes with a source or position
of #f, since these are usually intermediate syntax pairs that
don't correspond to actual source syntax.

For example, in a (#%module-begin . rst) in a submodule, the
underlying pair may contain an identifier and a cdr that is a
syntax object. That cdr will have #f source and position and also
does not correspond to any source syntax.

Leaving such syntax objects in the table means there are spurious
hits later when a syntax object with an unknown source location
is looked up.

Closes PR 14389

pkgs/typed-racket-pkgs/source-syntax/source-syntax.rkt

@@ -22,7 +22,12 @@
 
   ;; build `syntax-locs`
   (let loop ([stx orig])
-    (when (syntax? stx) (hash-set! syntax-locs (syntax-loc stx) stx))
+    (when (and (syntax? stx)
+               ;; avoid spurious hits in the table from syntaxes
+               ;; that have no useful source information
+               (and (syntax-source stx)
+                    (syntax-position stx)))
+      (hash-set! syntax-locs (syntax-loc stx) stx))
     (let ([stx (if (syntax? stx) (syntax-e stx) stx)])
       (when (pair? stx) (loop (car stx)) (loop (cdr stx)))))
 

pkgs/typed-racket-pkgs/typed-racket-test/tests/typed-racket/fail/pr14389.rkt

@@ -0,0 +1,10 @@
+#;
+(exn-pred (regexp-quote "in: (for/flvector"))
+#lang typed/racket/base
+
+(require racket/flonum)
+(module+ test
+  (require typed/rackunit)
+  (define (from upto)
+    (for/flvector ([i (in-range from upto)])
+      0.0)))