array: [].map(fn...) return type can be different than original type (#7300)

vlib/builtin/array_test.v

@@ -676,6 +676,23 @@ fn test_anon_fn_arg_map() {
 	assert a == [2,3,4]
 }
 
+fn test_anon_fn_arg_different_type_map() {
+	i_to_str := fn (i int) string {
+		return i.str()
+	}
+	a := [1, 2, 3].map(i_to_str)
+
+	assert a == ['1', '2', '3']
+}
+
+fn test_anon_fn_inline_different_type_map() {
+	a := [1, 2, 3].map(fn (i int) string {
+		return i.str()
+	})
+
+	assert a == ['1', '2', '3']
+}
+
 fn test_array_str() {
 	numbers := [1, 2, 3]
 	assert numbers == [1,2,3]

vlib/v/checker/checker.v

@@ -1086,9 +1086,8 @@ fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ table.Type, call_e
 			if arg_expr.decl.params.len > 1 {
 				c.error('function needs exactly 1 argument', arg_expr.decl.pos)
 			} else if is_map &&
-				(arg_expr.decl.return_type != elem_typ || arg_expr.decl.params[0].typ != elem_typ) {
-				c.error('type mismatch, should use `fn(a $elem_sym.name) $elem_sym.name {...}`',
-					arg_expr.decl.pos)
+				(arg_expr.decl.return_type == table.void_type || arg_expr.decl.params[0].typ != elem_typ) {
+				c.error('type mismatch, should use `fn(a $elem_sym.name) T {...}`', arg_expr.decl.pos)
 			} else if !is_map &&
 				(arg_expr.decl.return_type != table.bool_type || arg_expr.decl.params[0].typ != elem_typ) {
 				c.error('type mismatch, should use `fn(a $elem_sym.name) bool {...}`',
@@ -1103,8 +1102,9 @@ fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ table.Type, call_e
 				}
 				if func.params.len > 1 {
 					c.error('function needs exactly 1 argument', call_expr.pos)
-				} else if is_map && (func.return_type != elem_typ || func.params[0].typ != elem_typ) {
-					c.error('type mismatch, should use `fn(a $elem_sym.name) $elem_sym.name {...}`',
+				} else if is_map &&
+					(func.return_type == table.void_type || func.params[0].typ != elem_typ) {
+					c.error('type mismatch, should use `fn(a $elem_sym.name) T {...}`',
 						arg_expr.pos)
 				} else if !is_map &&
 					(func.return_type != table.bool_type || func.params[0].typ != elem_typ) {

vlib/v/checker/tests/array_map_anon_fn_err_b.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/array_map_anon_fn_err_b.vv:2:21: error: type mismatch, should use `fn(a int) int {...}`
+vlib/v/checker/tests/array_map_anon_fn_err_b.vv:2:21: error: type mismatch, should use `fn(a int) T {...}`
     1 | fn main() {
     2 |     a := [1,2,3,4].map(fn(a string) string { return a })
       |                        ~~

vlib/v/checker/tests/array_map_anon_fn_err_c.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/array_map_anon_fn_err_c.vv:2:21: error: type mismatch, should use `fn(a int) T {...}`
+    1 | fn main() {
+    2 |     a := [1,2,3,4].map(fn(a string) {})
+      |                        ~~
+    3 |     println(a)
+    4 | }

vlib/v/checker/tests/array_map_anon_fn_err_c.vv

@@ -0,0 +1,4 @@
+fn main() {
+	a := [1,2,3,4].map(fn(a string) {})
+	println(a)
+}

vlib/v/checker/tests/array_map_fn_err_b.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/array_map_fn_err_b.vv:5:21: error: type mismatch, should use `fn(a int) int {...}`
+vlib/v/checker/tests/array_map_fn_err_b.vv:5:21: error: type mismatch, should use `fn(a int) T {...}`
     3 | }
     4 | fn main() {
     5 |     a := [1,2,3,4].map(add)

vlib/v/checker/tests/array_map_fn_err_c.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/array_map_fn_err_c.vv:3:21: error: type mismatch, should use `fn(a int) T {...}`
+    1 | fn do_nothing(a string) {}
+    2 | fn main() {
+    3 |     a := [1,2,3,4].map(do_nothing)
+      |                        ~~~~~~~~~~
+    4 |     println(a)
+    5 | }

vlib/v/checker/tests/array_map_fn_err_c.vv

@@ -0,0 +1,5 @@
+fn do_nothing(a string) {}
+fn main() {
+	a := [1,2,3,4].map(do_nothing)
+	println(a)
+}