checker: check untyped nil in assignment

vlib/v/checker/assign.v

@@ -220,6 +220,9 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 							c.error('invalid use of reserved type `$left.name` as a variable name',
 								left.pos)
 						}
+						if right is ast.Nil {
+							c.error('use of untyped nil in assignment', right.pos())
+						}
 					}
 					mut ident_var_info := left.info as ast.IdentVar
 					if ident_var_info.share == .shared_t {

vlib/v/checker/tests/nil.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/nil.vv:3:18: error: use of untyped nil in assignment
+    1 | fn main() {
+    2 |     unsafe {
+    3 |         value := nil
+      |                  ~~~
+    4 |         println(value)
+    5 |     }

vlib/v/checker/tests/nil.vv

@@ -0,0 +1,6 @@
+fn main() {
+    unsafe {
+        value := nil
+        println(value)
+    }
+}