cgen: fix codegen for generic structinit and generic array return (fix #23916) (#23943)

vlib/v/gen/c/fn.v

@@ -991,6 +991,17 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 			if unaliased_type.has_option_or_result() {
 				ret_typ = unaliased_type
 			}
+		} else if node.return_type_generic != 0 && node.raw_concrete_types.len == 0 {
+			unwrapped_ret_typ := g.unwrap_generic(node.return_type_generic)
+			if !unwrapped_ret_typ.has_flag(.generic) {
+				ret_sym := g.table.sym(unwrapped_ret_typ)
+				if ret_sym.info is ast.Array && g.table.sym(node.return_type_generic).kind == .array {
+					// Make []T returns T type when array was supplied to T
+					if g.table.type_to_str(node.return_type_generic).count('[]') < g.table.type_to_str(unwrapped_ret_typ).count('[]') {
+						ret_typ = g.unwrap_generic(ret_sym.info.elem_type).derive(unwrapped_ret_typ)
+					}
+				}
+			}
 		}
 		mut styp := g.styp(ret_typ)
 		if gen_or && !is_gen_or_and_assign_rhs {

vlib/v/tests/generics/generic_array_ret_test.v

@@ -0,0 +1,32 @@
+module main
+
+fn decode_primitive[T]() !T {
+	$if T is int {
+		return T(1)
+	} $else $if T is u8 {
+		return T(10)
+	} $else $if T is u16 {
+		return T(100)
+	}
+	return error('decode_primitive: not found')
+}
+
+fn decode_array[T](_ []T) ![]T {
+	mut arr := []T{}
+	arr << decode_primitive[T]()!
+	return arr
+}
+
+fn decode[T]() !T {
+	$if T is $array {
+		a := decode_array(T{})!
+		return a
+	}
+	return error('decode: not found')
+}
+
+fn test_main() {
+	assert decode[[]int]()! == [1]
+	assert decode[[]u8]()! == [u8(10)]
+	assert decode[[]u16]()! == [u16(100)]
+}

vlib/v/type_resolver/generic_resolver.v

@@ -316,6 +316,18 @@ pub fn (mut t TypeResolver) resolve_args(cur_fn &ast.FnDecl, func &ast.Fn, mut n
 				ctyp = cparam_type_sym.info.elem_type
 			}
 			comptime_args[k] = ctyp
+		} else if mut call_arg.expr is ast.StructInit && call_arg.expr.typ.has_flag(.generic) {
+			mut ctyp := t.resolver.unwrap_generic(call_arg.expr.typ)
+			param_typ_sym := t.table.sym(param_typ)
+			cparam_type_sym := t.table.sym(ctyp)
+			if param_typ_sym.kind == .array && cparam_type_sym.info is ast.Array {
+				comptime_args[k] = cparam_type_sym.info.elem_type
+			} else if param_typ_sym.kind == .map && cparam_type_sym.info is ast.Map {
+				comptime_args[k] = cparam_type_sym.info.key_type
+				comptime_args[k + 1] = cparam_type_sym.info.value_type
+			} else {
+				comptime_args[k] = ctyp
+			}
 		}
 	}
 	return comptime_args