Brute-force propagate let types upward to resolve specialization issue (#1433)

* debugging

* propagate let type to subexpr

* cleanup

* add note

---------

Co-authored-by: garlic0x1 <[email protected]>

Author: stylewarning

Diff for: src/codegen/optimizer.lisp

@@ -193,6 +193,16 @@
       (action (:before node-let) #'add-local-funs)
       (action (:before node-bind) #'add-bind-fun)))))
 
+;;; XXX HACK: This is a temporary hack (famous last words) to solve
+;;; #1390. This issue should be handled earlier.
+(defun %reset-let-node-type-by-subexpression! (node)
+  ;; This function overwrites a LET-NODE's type with its SUBEXPR's
+  ;; type.
+  (when (node-let-p node)
+    (setf (coalton-impl/codegen/ast::%node-type (node-let-subexpr node))
+          (node-type node)))
+  node)
+
 (defun optimize-bindings-initial (bindings package env)
   (declare (type binding-list bindings)
            (type package package)
@@ -201,7 +211,10 @@
   (let ((hoister (make-hoister)))
     (append
      (loop :for (name . node) :in bindings
-           :collect (cons name (static-dict-lift (optimize-node node env) name hoister package env)))
+           :collect (cons name (%reset-let-node-type-by-subexpression!
+                                (static-dict-lift
+                                 (optimize-node node env)
+                                 name hoister package env))))
      (pop-final-hoist-point hoister))))
 
 (defun optimize-node (node env)