checker: disallow taking address of optional fields for now (#16487)

vlib/v/checker/checker.v

@@ -3544,9 +3544,13 @@ pub fn (mut c Checker) prefix_expr(mut node ast.PrefixExpr) ast.Type {
 				&& (node.right.typ == ast.bool_type_idx || (right_sym.kind == .enum_
 				&& !(right_sym.info as ast.Enum).is_flag
 				&& !(right_sym.info as ast.Enum).uses_exprs)) {
-				c.error('cannot take address of field in struct `${c.table.type_to_str(node.right.expr_type)}`, which is tagged as `[minify]`',
+				c.error('cannot take the address of field in struct `${c.table.type_to_str(node.right.expr_type)}`, which is tagged as `[minify]`',
 					node.pos.extend(node.right.pos))
 			}
+
+			if node.right.typ.has_flag(.optional) {
+				c.error('cannot take the address of an optional field', node.pos.extend(node.right.pos))
+			}
 		}
 	}
 	// TODO: testing ref/deref strategy
@@ -3562,7 +3566,7 @@ pub fn (mut c Checker) prefix_expr(mut node ast.PrefixExpr) ast.Type {
 		}
 		if mut node.right is ast.Ident {
 			if node.right.kind == .constant && !c.inside_unsafe && c.pref.experimental {
-				c.warn('cannot take an address of const outside `unsafe`', node.right.pos)
+				c.warn('cannot take the address of const outside `unsafe`', node.right.pos)
 			}
 		}
 		if node.right is ast.SelectorExpr {

vlib/v/checker/tests/optional_fields_addr_err.out

@@ -0,0 +1,13 @@
+vlib/v/checker/tests/optional_fields_addr_err.vv:9:10: error: cannot take the address of an optional field
+    7 | fn main() {
+    8 |     x := Wrapper{}
+    9 |     if _ := &x.value {
+      |             ~~~~~~~~
+   10 |     }
+   11 |
+vlib/v/checker/tests/optional_fields_addr_err.vv:12:7: error: cannot take the address of an optional field
+   10 |     }
+   11 | 
+   12 |     _ := &x.value
+      |          ~~~~~~~~
+   13 | }

vlib/v/checker/tests/optional_fields_addr_err.vv

@@ -0,0 +1,13 @@
+module main
+
+struct Wrapper {
+	value ?u8
+}
+
+fn main() {
+	x := Wrapper{}
+	if _ := &x.value {
+	}
+
+	_ := &x.value
+}