add typeasserts for captured variables with declared types

This provides an escape hatch for difficult-to-analyze bindings by
declaring their type. We assert that the type is as declared on
access.

fixes #16431

src/julia-syntax.scm

@@ -2715,11 +2715,17 @@ f(x) = yt(x)
                                    `($ (call (core QuoteNode) ,e))
                                    `(call (core getfield) ,fname (inert ,e)))))
                    (if (and (vinfo:asgn cv) (vinfo:capt cv))
-                       `(call (core getfield) ,access (inert contents))
+                       (let ((val `(call (core getfield) ,access (inert contents))))
+                         (if (eq? (vinfo:type cv) 'Any)
+                             val
+                             `(call (core typeassert) ,val ,(vinfo:type cv))))
                        access)))
                 (vi
                  (if (and (vinfo:asgn vi) (vinfo:capt vi))
-                     `(call (core getfield) ,e (inert contents))
+                     (let ((val `(call (core getfield) ,e (inert contents))))
+                       (if (eq? (vinfo:type vi) 'Any)
+                           val
+                           `(call (core typeassert) ,val ,(vinfo:type vi))))
                      e))
                 (else e))))
        ((atom? e) e)
@@ -2741,9 +2747,9 @@ f(x) = yt(x)
            (let ((vi (assq (cadr e) (car (lam:vinfo lam)))))
              (if (and vi (vinfo:asgn vi) (vinfo:capt vi)
                       ;; avoid redundant box for vars with newvar nodes
-                      (not (any (lambda (x) (and (length= x 2)
-                                                 (eq? (car x) 'newvar) (eq? (cadr x) (cadr e))))
-                                (lam:body lam))))
+                      (not (expr-contains-p (lambda (x) (and (length= x 2)
+                                                             (eq? (car x) 'newvar) (eq? (cadr x) (cadr e))))
+                                            (lam:body lam))))
                  `(= ,(cadr e) (call (core Box)))
                  `(newvar ,(cadr e)))))
           ((const)

test/core.jl

@@ -4149,3 +4149,11 @@ end
 @noinline MaybeFunc(T) = Union{T, Void}
 fMaybeFunc() = MaybeFunc(Int64)
 @test fMaybeFunc() == Union{Int64, Void}
+
+# issue #16431
+function f16431(x)
+    z::Int = x * 2
+    g(y) = begin z = z + y; y + x end
+    z * g(x)
+end
+@test @inferred(f16431(1)) == 4