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Fix bug preventing complex exprs on 'for in' loops

    'for in' loops had a bug where we would try to evaluate the
    sequence for the loop every time, instead of doing it once. This
    also caused the side effect of expecting lvalues instead of rvalues
    for the index expressions.

    This change fixes it.

--- a/6/simp.c

+++ b/6/simp.c

@@ -505,7 +505,7 @@

 static void simpiter(Simp *s, Node *n)

 {

     Node *lbody, *lstep, *lcond, *lmatch, *lend;

-    Node *idx, *len, *dcl, *val, *done;

+    Node *idx, *len, *dcl, *seq, *val, *done;

     Node *zero;

     lbody = genlbl();

@@ -517,6 +517,7 @@

     zero = mkintlit(n->line, 0);

     zero->expr.type = tyintptr;

+    seq = rval(s, n->iterstmt.seq, NULL);

     idx = gentemp(s, n, tyintptr, &dcl);

     declarelocal(s, dcl);

@@ -531,11 +532,11 @@

     simp(s, assign(s, idx, addk(idx, 1)));

     /* condition */

     simp(s, lcond);

-    len = seqlen(s, n->iterstmt.seq, tyintptr);

+    len = seqlen(s, seq, tyintptr);

     done = mkexpr(n->line, Olt, idx, len, NULL);

     cjmp(s, done, lmatch, lend);

     simp(s, lmatch);

-    val = load(idxaddr(s, n->iterstmt.seq, idx));

+    val = load(idxaddr(s, seq, idx));

     umatch(s, n->iterstmt.elt, val, val->expr.type, lbody, lstep);

     simp(s, lend);

 }

--- /dev/null

+++ b/test/subrangefor.myr

@@ -1,0 +1,8 @@

+use std

+

+const main = {

+	for i in [1,2,3,4][:2]

+		std.put("%i", i)

+	;;

+	std.put("\n")

+}

--- a/test/tests

+++ b/test/tests

@@ -56,6 +56,7 @@

 B nestfn	E	42

 # B closure	E	55      ## BUGGERED

 B loop		E	45

+B subrangefor	P       12

 B patiter	P	23512

 B condiftrue	E	7

 B condiffalse	E	9