compiler: better checking whether a variable may be introduced

Previously c1make-var checked whether the symbol NAME is CONSTANTP, but
ECL expands symbol macros in CONSTANTP so this returned false positives.
A similar concern applied to the CMP-ENV-REGISTER-SYMBOL-MACRO-FUNCTION.

C1EXPR-INNER when encountered a symbol tried to yield C1CONSTANT-VALUE
for if it iwas CONSTANTP - this was correct except for that we didn't
pass the environment to the predicate and symbols weren't shadowed.

In this commit one function is added to the core - si:constp (with
similar purpose to si:specialp) and one function to the compiler -
constant-variable-p (similar to special-variable-p) and they are
appropriately used when necessary. A regression test is added.

Fixes #662.

src/cmp/cmpenv-api.lsp

@@ -152,7 +152,7 @@ the closure in let/flet forms for variables/functions it closes over."
                                           env))
 
 (defun cmp-env-register-symbol-macro-function (name function &optional (env *cmp-env*))
-  (when (or (constantp name) (special-variable-p name))
+  (when (or (constant-variable-p name) (special-variable-p name))
     (cmperr "Cannot bind the special or constant variable ~A with symbol-macrolet." name))
   (push (list name 'si::symbol-macro function)
         (cmp-env-variables env))

src/cmp/cmppass1-eval.lsp

@@ -25,7 +25,7 @@
                ((keywordp form)
                 (make-c1form* 'LOCATION :type (object-type form)
                               :args (add-symbol form)))
-               ((constantp form)
+               ((constantp form *cmp-env*)
                 (or (c1constant-value (symbol-value form) :only-small-values t)
                     (c1var form)))
                (t (c1var form))))

src/cmp/cmppass1-var.lsp

@@ -221,7 +221,7 @@
 
 (defun c1make-var (name specials ignores types)
   (cmpck (not (symbolp name)) "The variable ~s is not a symbol." name)
-  (cmpck (constantp name) "The constant ~s is being bound." name)
+  (cmpck (constant-variable-p name) "The constant ~s is being bound." name)
   (let ((ignorable (cdr (assoc name ignores)))
         (kind 'LEXICAL)                 ; we rely on check-vref to fix it
         (type (assoc name types)))

src/cmp/cmpvar.lsp

@@ -222,6 +222,9 @@
         ;; we also have to consider 'GLOBAL here.
         (and v (eq (var-kind v) 'SPECIAL)))))
 
+(defun constant-variable-p (name)
+  (si::constp name))
+
 (defun local-variable-p (name &optional (env *cmp-env*))
   (let ((record (cmp-env-search-var name env)))
     (and record (var-p record))))