cgen: fix struct selector with or block (#16695)

vlib/v/gen/c/cgen.v

@@ -3591,13 +3591,20 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 	}
 
 	if node.or_block.kind != .absent && !g.is_assign_lhs && g.table.sym(node.typ).kind != .chan {
+		is_ptr := g.table.sym(g.unwrap_generic(node.expr_type)).kind in [.interface_, .sum_type]
 		stmt_str := g.go_before_stmt(0).trim_space()
 		styp := g.typ(node.typ)
 		g.empty_line = true
 		tmp_var := g.new_tmp_var()
 		g.write('${styp} ${tmp_var} = ')
+		if is_ptr {
+			g.write('*(')
+		}
 		g.expr(node.expr)
 		g.write('.${node.field_name}')
+		if is_ptr {
+			g.write(')')
+		}
 		g.or_block(tmp_var, node.or_block, node.typ)
 		g.write(stmt_str)
 		g.write(' ')

vlib/v/tests/struct_selector_or_block_test.v

@@ -0,0 +1,26 @@
+module main
+
+interface Greeting {
+	tt ?string
+}
+
+struct Hello {
+	tt    ?string = none
+	value string
+}
+
+struct Hi {
+	tt ?string = none
+}
+
+fn greet(g Greeting) string {
+	// Problematic piece of code here
+	// If I leave it unchecked, the compiler complains (as expected)
+	t := g.tt or { 'UNKNOWN' }
+	return t
+}
+
+fn test_main() {
+	assert greet(Hello{}) == 'UNKNOWN'
+	assert greet(Hello{'cool', ''}) == 'cool'
+}