cgen: fix it variable casting on map call when arr is a comptime variable #18083

vlib/v/gen/c/array.v

@@ -484,6 +484,17 @@ fn (mut g Gen) gen_array_map(node ast.CallExpr) {
 			g.write('${ret_elem_type} ti = ')
 			g.expr(node.args[0].expr)
 		}
+		ast.CastExpr {
+			// value.map(Type(it)) when `value` is a comptime var
+			if expr.expr is ast.Ident && node.left is ast.Ident && g.is_comptime_var(node.left) {
+				ctyp := g.get_comptime_var_type(node.left)
+				if ctyp != ast.void_type {
+					expr.expr_type = g.table.value_type(ctyp)
+				}
+			}
+			g.write('${ret_elem_type} ti = ')
+			g.expr(node.args[0].expr)
+		}
 		else {
 			if closure_var_decl != '' {
 				g.write('${closure_var_decl} = ')

vlib/v/tests/comptime_map_it_test.v

@@ -0,0 +1,30 @@
+type Any = []Any | f64 | int | map[string]Any | string
+
+struct Arr {
+	ints   []int
+	floats []f64
+	strs   []string
+}
+
+fn encode[T](typ T) map[string]Any {
+	mut mp := map[string]Any{}
+	$for field in T.fields {
+		value := typ.$(field.name)
+		$if field.is_enum {
+			mp[field.name] = Any(int(value))
+		} $else $if field.is_array {
+			mp[field.name] = value.map(Any(it))
+		} $else {
+			mp[field.name] = Any(value)
+		}
+	}
+	return mp
+}
+
+fn test_main() {
+	a := Arr{[5], [2.0], ['asdf']}
+	r := encode[Arr](a)
+	assert r['ints'] == Any([Any(5)])
+	assert r['floats'] == Any([Any(2.0)])
+	assert r['strs'] == Any([Any('asdf')])
+}