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Don't put in a bogus offset when casting slices.

    How did this EVER work?

--- a/6/simp.c

+++ b/6/simp.c

@@ -626,11 +626,13 @@

 static Node *slicebase(Simp *s, Node *n, Node *off)

 {

     Node *t, *u, *v;

+    Type *ty;

     int sz;

     t = rval(s, n, NULL);

     u = NULL;

-    switch (exprtype(n)->type) {

+    ty = tybase(exprtype(n));

+    switch (ty->type) {

         case Typtr:     u = n; break;

         case Tyarray:   u = addr(t, base(exprtype(n))); break;

         case Tyslice:   u = load(addr(t, mktyptr(n->line, base(exprtype(n))))); break;

@@ -637,10 +639,14 @@

         default: die("Unslicable type %s", tystr(n->expr.type));

     }

     /* safe: all types we allow here have a sub[0] that we want to grab */

-    off = ptrsized(s, off);

-    sz = tysize(n->expr.type->sub[0]);

-    v = mul(off, disp(n->line, sz));

-    return add(u, v);

+    if (off) {

+      off = ptrsized(s, off);

+      sz = tysize(n->expr.type->sub[0]);

+      v = mul(off, disp(n->line, sz));

+      return add(u, v);

+    } else {

+      return u;

+    }

 }

 static Node *slicelen(Simp *s, Node *sl)

@@ -713,7 +719,6 @@

 static Node *simpcast(Simp *s, Node *val, Type *to)

 {

-    Node *sz;

     Node *r;

     Type *t;

     int issigned;

@@ -733,9 +738,7 @@

                     if (t->type == Typtr)

                         fatal(val->line, "Bad cast from %s to %s",

                               tystr(exprtype(val)), tystr(to));

-                    sz = mkintlit(val->line, Ptrsz);

-                    sz->expr.type = exprtype(val);

-                    r = slicebase(s, val, sz);

+                    r = slicebase(s, val, NULL);

                     break;

                 case Tyint8: case Tyint16: case Tyint32: case Tyint64:

                 case Tyint: case Tylong: