From e6f1010ef967123bcc1046383d228dbae78bcde4 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Mon, 7 Jun 2021 06:29:07 -0700
Subject: expander: expand must only ignores unbound warnings.

The expand function must not muffle all deferred warnings.
That causes the problem that a form like (inc var a.bar) fails
to produce a warning due to bar not being the slot of any
structure. The expand function must only muffle warnings about
undefined functions and variables.

* eval.c (muffle_unbound_warning): New static function.
(no_warn_expand): Use muffle_unbound_warning as handler,
rather than uw_muffle_warning.

* tests/012/struct.tl: Fix two test cases here which test the
expand function using a form that references a nonexistent
slot. These now generate a warning, so we use the slot name b
rather than d, which is defined.

* txr.1: Documented change to expand.
---
 eval.c              | 21 ++++++++++++++++++++-
 tests/012/struct.tl |  8 ++++----
 txr.1               | 15 ++++++++++++++-
 3 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/eval.c b/eval.c
index bf0dc246..2d840160 100644
--- a/eval.c
+++ b/eval.c
@@ -5088,12 +5088,31 @@ val expand(val form, val menv)
   return ret;
 }
 
+static val muffle_unbound_warning(val exc, struct args *args)
+{
+  (void) exc;
+
+  args_normalize_least(args, 2);
+
+  if (args->fill >= 2) {
+    val tag = args->arg[1];
+
+    if (consp(tag)) {
+      val type = car(tag);
+      if (type == var_s || type == fun_s || type == sym_s)
+        uw_rthrow(continue_s, nil);
+    }
+  }
+
+  return nil;
+}
+
 static val no_warn_expand(val form, val menv)
 {
   val ret;
   uw_frame_t uw_handler;
   uw_push_handler(&uw_handler, cons(defr_warning_s, nil),
-                  func_n1v(uw_muffle_warning));
+                  func_n1v(muffle_unbound_warning));
   ret = expand(form, menv);
   uw_pop_frame(&uw_handler);
   return ret;
diff --git a/tests/012/struct.tl b/tests/012/struct.tl
index 57234913..3f03552d 100644
--- a/tests/012/struct.tl
+++ b/tests/012/struct.tl
@@ -43,10 +43,10 @@
 (stest (ignwarn (expand 's.(a b c)))
        "(call (slot s 'a)\n                     \
        \      s b c)")
-(test (expand 's.[a].d)
-      (slot [(slot s 'a) s] 'd))
-(test (expand 's.[a b c].d)
-      (slot [(slot s 'a) s b c] 'd))
+(test (expand 's.[a].b)
+      (slot [(slot s 'a) s] 'b))
+(test (expand 's.[a b c].b)
+      (slot [(slot s 'a) s b c] 'b))
 (set *gensym-counter* 0)
 (stest (ignwarn (expand 's.(a).d))
        "(slot (call (slot s 'a)\n               \
diff --git a/txr.1 b/txr.1
index 88bfebfe..9cb01efb 100644
--- a/txr.1
+++ b/txr.1
@@ -36903,7 +36903,20 @@ and
 .code expand*
 is that
 .code expand
-suppresses any warning exceptions that are issued during expansion.
+suppresses expansion-time deferred warnings (exceptions of type
+.codn defr-warning ),
+issued for unbound variables or functions.
+To suppress a warning means to intercept the warning exception with a handler
+which throws a
+.code continue
+exception to resume processing.
+What this requirement means is that if unbound functions or variables
+occur in the
+.meta form
+being expanded by expand, the warning is effectively squelched. Rationale:
+.code expand
+is may be used by macros for expanding fragments which contain references to
+variables or functions which are not defined in those fragments.
 
 .coNP Function @ expand-with-free-refs
 .synb
-- 
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