[math] better expt: when only exponent is known

test/math-fail.rkt

@@ -15,6 +15,7 @@
   (ann (let ([n 4]) (/: n n)) One)
   (ann (let ([n 2]) (expt: 3 (-: n n))) One)
   (ann (expt: 3 2) Zero)
+  (ann ((lambda ([x : Natural]) (expt x 3)) 2) Index)
   ;; -- lambda => back to racket/base
   (ann ((lambda ([f : (-> Natural Natural Natural)]) (f 0 0)) +:) Zero)
   (ann ((lambda ([f : (-> Natural Natural Integer)]) (f 0 0)) -:) Zero)

test/math-pass.rkt

@@ -151,5 +151,8 @@
       (define-num: n2 2)
       (ann (-: (expt: n1 n2) 64) Zero))
     0)
-
+  (check-true
+    (and (ann (lambda ([n : Natural]) (expt: n 0)) (-> Natural One)) #t))
+  (check-true
+    (and (ann (lambda ([n : Index]) (expt: n 1)) (-> Index Index)) #t))
 )

trivial/private/math.rkt

@@ -119,6 +119,11 @@
    [(_ n1 n2)
     (let ([v1 (stx->num #'n1)]
           [v2 (stx->num #'n2)])
-      (and v1 v2 ;; Should never fail
-        (quasisyntax/loc stx #,(expt v1 v2))))]
+      (cond
+       [(and v1 v2)
+        (quasisyntax/loc stx #,(expt v1 v2))]
+       [(and v2 (<= 0 v2 10))
+        (quasisyntax/loc stx (* #,@(for/list ([_i (in-range v2)]) (quasisyntax/loc stx n1))))]
+       [else
+        #f]))]
    [_ #f]))))