compiler: bugfix: constant test in 2 or 3 arg if.

* share/txr/stdlib/compiler.tl (compiler comp-if): The two and
three argument cases assume that if the test is a constant
expression, the consequent "then" should be unconditionally
taken. The correct behavior is to evaluate the constant,
which could yield nil. I checked which library code changes
after this fix, and found that a number of (defset ...)
forms are generating different, shorter code.
This is due to (if ',restpar (if (consp ,restpar) ...))
in defset-expander. The intent there was to eliminate the
inner if code entirely if respar is nil (there is no rest
parameter); due to this bug, the code elimination didn't
happen.  The behavior is nevertheless correct because the
code does nothing if restpar is nil.

1 files changed, 2 insertions, 2 deletions

diff --git a/share/txr/stdlib/compiler.tl b/share/txr/stdlib/compiler.tl
index f510d892..5c397a4a 100644
--- a/share/txr/stdlib/compiler.tl
+++ b/share/txr/stdlib/compiler.tl
@@ -429,7 +429,7 @@
        ((null test)
         me.(compile oreg env else))
        ((constantp test)
-        me.(compile oreg env then))
+        me.(compile oreg env (if (eval test) then else)))
        ((and (consp test) (member (car test) %test-funs%))
         me.(compile oreg env ^(ift ,(car test) ,(cadr test) ,(caddr test)
                                    ,then ,else)))
@@ -457,7 +457,7 @@
      (cond
        ((null test) me.(compile oreg env nil))
        ((constantp test)
-        me.(compile oreg env then))
+        me.(compile oreg env (if (eval test) then)))
        ((and (consp test) (member (car test) %test-funs%))
         me.(compile oreg env ^(ift ,(car test) ,(cadr test) ,(caddr test)
                                    ,then)))