DEFCONSTANT must also define the constant at compile time.

src/CHANGELOG

@@ -38,6 +38,9 @@ ECL 9.6.3:
  - Compiler messages no show the offending form and file position. Before this
    information was not properly stored in the compiled code tree.
 
+ - DEFCONSTANT only had the expected compile-time side effects in the bytecodes
+   compiler, not in the lisp->C translator.
+
 * Visible changes:
 
  - The toplevel only traps SERIOUS-CONDITIONS.

src/lsp/evalmacros.lsp

@@ -119,6 +119,7 @@ VARIABLE doc and can be retrieved by (DOCUMENTATION 'SYMBOL 'VARIABLE)."
     ,(ext:register-with-pde whole)
     ,(unless *bytecodes-compiler*
        `(eval-when (:compile-toplevel)
+          (sys:*make-constant ',var ,form)
           (si::register-global ',var)))
     ',var))