checker: check the arity and the kinds of the type parameters in generic struct inits (#13429)

vlib/v/checker/struct.v

@@ -138,6 +138,20 @@ pub fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 			} else if node.generic_types.len == 0 {
 				c.error('generic struct init must specify type parameter, e.g. Foo<T>',
 					node.pos)
+			} else if node.generic_types.len > 0
+				&& node.generic_types.len != struct_sym.info.generic_types.len {
+				c.error('generic struct init expects $struct_sym.info.generic_types.len generic parameter, but got $node.generic_types.len',
+					node.pos)
+			} else if node.generic_types.len > 0 {
+				for gtyp in node.generic_types {
+					gtyp_name := c.table.sym(gtyp).name
+					if gtyp_name !in c.table.cur_fn.generic_names {
+						cur_generic_names := '(' + c.table.cur_fn.generic_names.join(',') + ')'
+						c.error('generic struct init type parameter `$gtyp_name` must be within the parameters `$cur_generic_names` of the current generic function',
+							node.pos)
+						break
+					}
+				}
 			}
 		}
 		if node.generic_types.len > 0 && struct_sym.info.generic_types != node.generic_types {

vlib/v/checker/tests/generics_struct_init_type_parameter_count_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/generics_struct_init_type_parameter_count_err.vv:6:9: error: generic struct init expects 1 generic parameter, but got 2
+    4 |
+    5 | pub fn send<A, B>(res A, b B) string {
+    6 |     msg := Response<A, B>{
+      |            ~~~~~~~~~~~~~~~
+    7 |         result: res
+    8 |     }

vlib/v/checker/tests/generics_struct_init_type_parameter_count_err.vv

@@ -0,0 +1,16 @@
+struct Response<U> {
+	result U
+}
+
+pub fn send<A, B>(res A, b B) string {
+	msg := Response<A, B>{
+		result: res
+	}
+	println(b)
+	return '$msg'
+}
+
+fn main() {
+	mut ret := send(123, 22.2)
+	println(ret)
+}

vlib/v/checker/tests/generics_struct_init_type_parameter_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/generics_struct_init_type_parameter_err.vv:6:9: error: generic struct init type parameter `U` must be within the parameters `(A,B)` of the current generic function
+    4 |
+    5 | pub fn send<A, B>(res A, b B) string {
+    6 |     msg := Response<U>{
+      |            ~~~~~~~~~~~~
+    7 |         result: res
+    8 |     }
+vlib/v/checker/tests/generics_struct_init_type_parameter_err.vv:7:3: error: cannot assign to field `result`: expected `U`, not `int`
+    5 | pub fn send<A, B>(res A, b B) string {
+    6 |     msg := Response<U>{
+    7 |         result: res
+      |         ~~~~~~~~~~~
+    8 |     }
+    9 |     println(b)

vlib/v/checker/tests/generics_struct_init_type_parameter_err.vv

@@ -0,0 +1,16 @@
+struct Response<U> {
+	result U
+}
+
+pub fn send<A, B>(res A, b B) string {
+	msg := Response<U>{
+		result: res
+	}
+	println(b)
+	return '$msg'
+}
+
+fn main() {
+	mut ret := send(123, 22.2)
+	println(ret)
+}