cgen: fix typeof with comptime var (#16816)

vlib/v/gen/c/cgen.v

@@ -3549,6 +3549,22 @@ fn (mut g Gen) typeof_expr(node ast.TypeOf) {
 	}
 }
 
+fn (mut g Gen) comptime_typeof(node ast.TypeOf, default_type ast.Type) ast.Type {
+	if node.expr is ast.ComptimeSelector {
+		if node.expr.field_expr is ast.SelectorExpr {
+			if node.expr.field_expr.expr is ast.Ident {
+				key_str := '${node.expr.field_expr.expr.name}.typ'
+				return g.comptime_var_type_map[key_str] or { default_type }
+			}
+		}
+	} else if g.inside_comptime_for_field && node.expr is ast.Ident
+		&& (node.expr as ast.Ident).obj is ast.Var && ((node.expr as ast.Ident).obj as ast.Var).is_comptime_field == true {
+		// typeof(var) from T.fields
+		return g.comptime_for_field_type
+	}
+	return default_type
+}
+
 fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 	prevent_sum_type_unwrapping_once := g.prevent_sum_type_unwrapping_once
 	g.prevent_sum_type_unwrapping_once = false
@@ -3568,20 +3584,17 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 					// typeof(expr).name
 					mut name_type := node.name_type
 					if node.expr is ast.TypeOf {
-						if node.expr.expr is ast.ComptimeSelector {
+						name_type = g.comptime_typeof(node.expr, name_type)
-							if node.expr.expr.field_expr is ast.SelectorExpr {
-								if node.expr.expr.field_expr.expr is ast.Ident {
-									key_str := '${node.expr.expr.field_expr.expr.name}.typ'
-									name_type = g.comptime_var_type_map[key_str] or { name_type }
-								}
-							}
-						}
 					}
 					g.type_name(name_type)
 					return
 				} else if node.field_name == 'idx' {
+					mut name_type := node.name_type
+					if node.expr is ast.TypeOf {
+						name_type = g.comptime_typeof(node.expr, name_type)
+					}
 					// `typeof(expr).idx`
-					g.write(int(g.unwrap_generic(node.name_type)).str())
+					g.write(int(g.unwrap_generic(name_type)).str())
 					return
 				}
 				g.error('unknown generic field', node.pos)

vlib/v/tests/comptime_for_in_field_typeof_test.v

@@ -0,0 +1,41 @@
+struct Ab {
+	a []int
+}
+
+struct ComplexStruct {
+	a int
+	b []Ab
+	c string
+	d map[string]int
+}
+
+fn test_typeof_in_comptime_for_in_fields() {
+	out := encode_struct(ComplexStruct{})
+	dump(out)
+	assert out[0] == 'a'
+	assert out[1] == typeof[int]().idx.str()
+	assert out[2] == 'int'
+
+	assert out[3] == 'b'
+	assert out[4] == typeof[[]Ab]().idx.str()
+	assert out[5] == '[]Ab'
+
+	assert out[6] == 'c'
+	assert out[7] == typeof[string]().idx.str()
+	assert out[8] == 'string'
+
+	assert out[9] == 'd'
+	assert out[10] == typeof[map[string]int]().idx.str()
+	assert out[11] == 'map[string]int'
+}
+
+fn encode_struct[T](val T) []string {
+	mut out := []string{}
+	$for field in T.fields {
+		value := val.$(field.name)
+		out << field.name
+		out << typeof(value).idx.str()
+		out << typeof(value).name
+	}
+	return out
+}