checker: fix sumtype cast of comptime var (#17287)

vlib/v/checker/checker.v

@@ -2688,12 +2688,26 @@ pub fn (mut c Checker) expr(node_ ast.Expr) ast.Type {
 // 	return ast.void_type
 // }
 
+fn (mut c Checker) get_comptime_selector_type(node ast.ComptimeSelector, default_type ast.Type) ast.Type {
+	if node.field_expr is ast.SelectorExpr
+		&& c.check_comptime_is_field_selector(node.field_expr as ast.SelectorExpr)
+		&& (node.field_expr as ast.SelectorExpr).field_name == 'name' {
+		return c.comptime_fields_default_type
+	}
+	return default_type
+}
+
 fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 	// Given: `Outside( Inside(xyz) )`,
 	//        node.expr_type: `Inside`
 	//        node.typ: `Outside`
 	node.expr_type = c.expr(node.expr) // type to be casted
 
+	if node.expr is ast.ComptimeSelector {
+		node.expr_type = c.get_comptime_selector_type(node.expr as ast.ComptimeSelector,
+			node.expr_type)
+	}
+
 	mut from_type := c.unwrap_generic(node.expr_type)
 	from_sym := c.table.sym(from_type)
 	final_from_sym := c.table.final_sym(from_type)

vlib/v/tests/comptime_sumtype_cast_test.v

@@ -0,0 +1,29 @@
+pub type Any = bool | int | map[string]Any | string
+
+struct StructType {
+mut:
+	val string
+}
+
+fn test_main() {
+	struct_to_map_string()
+}
+
+fn map_from[T](t T) {
+	$if T is $Struct {
+		$for field in T.fields {
+			assert Any('string').str() == "Any('string')"
+			assert t.$(field.name).str() == 'true'
+			a := t.$(field.name)
+			assert Any(a).str() == "Any('true')"
+			assert Any(t.$(field.name)).str() == "Any('true')"
+		}
+	}
+}
+
+fn struct_to_map_string() {
+	struct_type := StructType{
+		val: 'true'
+	}
+	map_from(struct_type)
+}