Make VCC not fail if you try to pass STRANDS to STRING_LIST.

Strenghen the case against STRING_LIST in the docs.

See also: #2873

Conflicts:
	lib/libvmod_debug/vmod.vcc

bin/varnishtest/tests/m00000.vtc

@@ -120,3 +120,10 @@ varnish v1 -errvcl {Expression has type STRING, expected REAL} {
 		set resp.http.who = std.random("foo", "bar");
 	}
 }
+
+varnish v1 -errvcl {Cannot convert type STRING(STRANDS) to STRING(STRING_LIST)} {
+	import debug;
+	sub vcl_deliver {
+		set resp.http.who = debug.return_strands(req.url + "bar");
+	}
+}

doc/sphinx/reference/vmod.rst

@@ -390,9 +390,8 @@ STEVEDORE
 STRING_LIST
 	C-type: ``const char *, ...``
 
-	`Notice: New vmod developments for 6.1 and higher should
-	consider STRANDS as a better alternative to STRING_LIST, which
-	will eventually be replaced entirely.`
+	`Notice: New vmod developments for 6.1 and later must
+	use STRANDS instead of STRING_LIST, which is going away.`
 
 	A multi-component text-string.  We try very hard to avoid
 	doing text-processing in Varnish, and this is one way we

lib/libvcc/vcc_expr.c

@@ -1312,7 +1312,14 @@ vcc_expr0(struct vcc *tl, struct expr **e, vcc_type_t fmt)
 	else
 		vcc_expr_cor(tl, e, fmt);
 	ERRCHK(tl);
-	assert(!(*e)->fmt->stringform);
+
+	if ((*e)->fmt->stringform) {
+		VSB_printf(tl->sb, "Cannot convert type %s(%s) to %s(%s)\n",
+		    vcc_utype((*e)->fmt), (*e)->fmt->name,
+		    vcc_utype(fmt), fmt->name);
+		vcc_ErrWhere2(tl, t1, tl->t);
+		return;
+	}
 
 	if ((*e)->fmt != STRINGS && fmt->stringform)
 		vcc_expr_tostring(tl, e, STRINGS);

lib/libvmod_debug/vmod.vcc

@@ -217,6 +217,8 @@ $Method STRING .meth_opt(PRIV_CALL, PRIV_VCL, PRIV_TASK,
 
 Test object method with all the fancy stuff.
 
+$Function STRANDS return_strands(STRANDS strand)
+
 $Function VOID store_ip(PRIV_TASK, IP)
 
 Store an IP address to be later found by ``debug.get_ip()`` in the same

lib/libvmod_debug/vmod_debug.c

@@ -548,3 +548,11 @@ xyzzy_get_ip(VRT_CTX, struct vmod_priv *priv)
 
 	return (ip);
 }
+
+VCL_STRANDS
+xyzzy_return_strands(VRT_CTX, VCL_STRANDS strand)
+{
+
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	return (strand);
+}