checker: improve errors for `return` count mismatch (#8686)

vlib/v/checker/checker.v

@@ -1301,7 +1301,7 @@ pub fn (mut c Checker) call_expr(mut call_expr ast.CallExpr) table.Type {
 
 fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ table.Type, call_expr ast.CallExpr) {
 	if call_expr.args.len != 1 {
-		c.error('expected 1 arguments, but got $call_expr.args.len', call_expr.pos)
+		c.error('expected 1 argument, but got $call_expr.args.len', call_expr.pos)
 		// Finish early so that it doesn't fail later
 		return
 	}
@@ -2359,12 +2359,13 @@ pub fn (mut c Checker) return_stmt(mut return_stmt ast.Return) {
 	expected_type := c.unwrap_generic(c.expected_type)
 	expected_type_sym := c.table.get_type_symbol(expected_type)
 	if return_stmt.exprs.len > 0 && c.cur_fn.return_type == table.void_type {
-		c.error('too many arguments to return, current function does not return anything',
-			return_stmt.pos)
+		c.error('unexpected argument, current function does not return anything', return_stmt.exprs[0].position())
 		return
 	} else if return_stmt.exprs.len == 0 && !(c.expected_type == table.void_type
 		|| expected_type_sym.kind == .void) {
-		c.error('too few arguments to return', return_stmt.pos)
+		stype := c.table.type_to_str(expected_type)
+		arg := if expected_type_sym.kind == .multi_return { 'arguments' } else { 'argument' }
+		c.error('expected `$stype` $arg', return_stmt.pos)
 		return
 	}
 	if return_stmt.exprs.len == 0 {
@@ -2397,7 +2398,8 @@ pub fn (mut c Checker) return_stmt(mut return_stmt ast.Return) {
 		return
 	}
 	if expected_types.len > 0 && expected_types.len != got_types.len {
-		c.error('wrong number of return arguments', return_stmt.pos)
+		arg := if expected_types.len == 1 { 'argument' } else { 'arguments' }
+		c.error('expected $expected_types.len $arg, but got $got_types.len', return_stmt.pos)
 		return
 	}
 	for i, exp_type in expected_types {

vlib/v/checker/tests/array_map_arg_mismatch.out

@@ -1,11 +1,11 @@
-vlib/v/checker/tests/array_map_arg_mismatch.vv:3:4: error: expected 1 arguments, but got 0
+vlib/v/checker/tests/array_map_arg_mismatch.vv:3:4: error: expected 1 argument, but got 0
     1 | fn main() {
     2 |     a := [1, 2, 3]
     3 |     a.map()
       |       ~~~~~
     4 |     a.map(it * 2, 3)
     5 | }
-vlib/v/checker/tests/array_map_arg_mismatch.vv:4:4: error: expected 1 arguments, but got 2
+vlib/v/checker/tests/array_map_arg_mismatch.vv:4:4: error: expected 1 argument, but got 2
     2 |     a := [1, 2, 3]
     3 |     a.map()
     4 |     a.map(it * 2, 3)

vlib/v/checker/tests/return_count_mismatch.out

@@ -0,0 +1,34 @@
+vlib/v/checker/tests/return_count_mismatch.vv:2:9: error: unexpected argument, current function does not return anything
+    1 | fn v() {
+    2 |     return 3
+      |            ^
+    3 | }
+    4 |
+vlib/v/checker/tests/return_count_mismatch.vv:7:2: error: expected `int` argument
+    5 | fn f() int
+    6 | {
+    7 |     return
+      |     ~~~~~~
+    8 | }
+    9 |
+vlib/v/checker/tests/return_count_mismatch.vv:12:2: error: expected `(int, string)` arguments
+   10 | fn g() (int, string)
+   11 | {
+   12 |     return
+      |     ~~~~~~
+   13 | }
+   14 |
+vlib/v/checker/tests/return_count_mismatch.vv:17:2: error: expected 1 argument, but got 2
+   15 | fn ff() int
+   16 | {
+   17 |     return 2, ''
+      |     ~~~~~~~~~~~~
+   18 | }
+   19 |
+vlib/v/checker/tests/return_count_mismatch.vv:22:2: error: expected 2 arguments, but got 1
+   20 | fn gg() (int, string)
+   21 | {
+   22 |     return 3
+      |     ~~~~~~~~
+   23 | }
+   24 |

vlib/v/checker/tests/return_count_mismatch.vv

@@ -0,0 +1,24 @@
+fn v() {
+	return 3
+}
+
+fn f() int
+{
+	return
+}
+
+fn g() (int, string)
+{
+	return
+}
+
+fn ff() int
+{
+	return 2, ''
+}
+
+fn gg() (int, string)
+{
+	return 3
+}
+