checker: fix #13626, when unwrapped type is fixed array, and selector expr is 'len' (#13627)

vlib/v/checker/checker.v

@@ -1667,6 +1667,12 @@ pub fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 	if sym.kind !in [.struct_, .aggregate, .interface_, .sum_type] {
 		if sym.kind != .placeholder {
 			unwrapped_sym := c.table.sym(c.unwrap_generic(typ))
+
+			if unwrapped_sym.kind == .array_fixed && node.field_name == 'len' {
+				node.typ = ast.int_type
+				return ast.int_type
+			}
+
 			c.error('`$unwrapped_sym.name` has no property `$node.field_name`', node.pos)
 		}
 	} else {

vlib/v/tests/comptime_generic_test.v

@@ -0,0 +1,38 @@
+fn test_comptime_generic() {
+	a := [5]int{}
+	func1(&a)
+}
+
+[inline]
+pub fn func1<T>(t &T) {
+	func2<T>(t)
+}
+
+[inline]
+pub fn func2<T>(t &T) {
+	$if T is $Array {
+		unsafe {
+			for i in 0 .. t.len {
+				func2(&t[i])
+			}
+		}
+	} $else $if T is $Map {
+		// fake_map(t, df)
+	} $else $if T is $Struct {
+		$for f in T.fields {
+			$if f.typ is string {
+			}
+			// Dummy expression to generate and specify t.$(f.name)'s type
+
+			// mut attrs := get_attrs(t.$(f.name), f)
+
+			// if !attrs.skip() {
+			// 	fake_data_wdf(&(t.$(f.name)))
+			// }
+		}
+	} $else {
+		unsafe {
+			// *t = fake_primitive_value<T>(df) or { panic(err) }
+		}
+	}
+}