checker: deny multi-value in a singe-value context (#8328)

vlib/v/checker/checker.v

@@ -2358,19 +2358,25 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 	right_first := assign_stmt.right[0]
 	mut right_len := assign_stmt.right.len
 	mut right_type0 := table.void_type
-	if right_first is ast.CallExpr || right_first is ast.IfExpr || right_first is ast.MatchExpr {
+	for right in assign_stmt.right {
-		right_type0 = c.expr(right_first)
+		if right is ast.CallExpr || right is ast.IfExpr || right is ast.MatchExpr {
+			right_type0 = c.expr(right)
 			assign_stmt.right_types = [
-			c.check_expr_opt_call(right_first, right_type0),
+				c.check_expr_opt_call(right, right_type0),
 			]
 			right_type_sym0 := c.table.get_type_symbol(right_type0)
 			if right_type_sym0.kind == .multi_return {
+				if assign_stmt.right.len > 1 {
+					c.error('cannot use multi-value $right_type_sym0.name in signle-value context',
+						right.position())
+				}
 				assign_stmt.right_types = right_type_sym0.mr_info().types
 				right_len = assign_stmt.right_types.len
 			} else if right_type0 == table.void_type {
 				right_len = 0
 			}
 		}
+	}
 	if assign_stmt.left.len != right_len {
 		if right_first is ast.CallExpr {
 			c.error('assignment mismatch: $assign_stmt.left.len variable(s) but `${right_first.name}()` returns $right_len value(s)',

vlib/v/checker/tests/assign_multi_mismatch.out

@@ -0,0 +1,81 @@
+vlib/v/checker/tests/assign_multi_mismatch.vv:5:3: error: assignment mismatch: 1 variable(s) 2 value(s)
+    3 | }
+    4 | 
+    5 | _ := 0, 0
+      |   ~~
+    6 | _ := f()
+    7 | _, _ := f()
+vlib/v/checker/tests/assign_multi_mismatch.vv:6:3: error: assignment mismatch: 1 variable(s) but `f()` returns 2 value(s)
+    4 | 
+    5 | _ := 0, 0
+    6 | _ := f()
+      |   ~~
+    7 | _, _ := f()
+    8 | _, _ := 0, f()
+vlib/v/checker/tests/assign_multi_mismatch.vv:8:12: error: cannot use multi-value (int, int) in signle-value context
+    6 | _ := f()
+    7 | _, _ := f()
+    8 | _, _ := 0, f()
+      |            ~~~
+    9 | _, _ := f(), 0
+   10 | _, _, _ := 0, f()
+vlib/v/checker/tests/assign_multi_mismatch.vv:9:9: error: cannot use multi-value (int, int) in signle-value context
+    7 | _, _ := f()
+    8 | _, _ := 0, f()
+    9 | _, _ := f(), 0
+      |         ~~~
+   10 | _, _, _ := 0, f()
+   11 | _, _, _ := f(), 0
+vlib/v/checker/tests/assign_multi_mismatch.vv:10:15: error: cannot use multi-value (int, int) in signle-value context
+    8 | _, _ := 0, f()
+    9 | _, _ := f(), 0
+   10 | _, _, _ := 0, f()
+      |               ~~~
+   11 | _, _, _ := f(), 0
+   12 | _, _ := f(), f()
+vlib/v/checker/tests/assign_multi_mismatch.vv:11:12: error: cannot use multi-value (int, int) in signle-value context
+    9 | _, _ := f(), 0
+   10 | _, _, _ := 0, f()
+   11 | _, _, _ := f(), 0
+      |            ~~~
+   12 | _, _ := f(), f()
+   13 | _, _, _, _ := f(), f()
+vlib/v/checker/tests/assign_multi_mismatch.vv:12:9: error: cannot use multi-value (int, int) in signle-value context
+   10 | _, _, _ := 0, f()
+   11 | _, _, _ := f(), 0
+   12 | _, _ := f(), f()
+      |         ~~~
+   13 | _, _, _, _ := f(), f()
+   14 |
+vlib/v/checker/tests/assign_multi_mismatch.vv:13:15: error: cannot use multi-value (int, int) in signle-value context
+   11 | _, _, _ := f(), 0
+   12 | _, _ := f(), f()
+   13 | _, _, _, _ := f(), f()
+      |               ~~~
+   14 | 
+   15 | _, _ := 0, match 4 {
+vlib/v/checker/tests/assign_multi_mismatch.vv:19:3: error: assignment mismatch: 1 variable(s) 2 value(s)
+   17 |     else { 1 }
+   18 | }
+   19 | _ := match 4 {
+      |   ~~
+   20 |     1 { f() }
+   21 |     else { f() }
+vlib/v/checker/tests/assign_multi_mismatch.vv:23:12: error: cannot use multi-value (int, int) in signle-value context
+   21 |     else { f() }
+   22 | }
+   23 | _, _ := 0, match 4 {
+      |            ~~~~~~~~~
+   24 |     1 { f() }
+   25 |     else { f() }
+vlib/v/checker/tests/assign_multi_mismatch.vv:29:3: error: assignment mismatch: 1 variable(s) 2 value(s)
+   27 | 
+   28 | _, _ := 0, if true { 0 } else { 1 }
+   29 | _ := if true { f() } else { f() }
+      |   ~~
+   30 | _, _ := 0, if true { f() } else { f() }
+vlib/v/checker/tests/assign_multi_mismatch.vv:30:12: error: cannot use multi-value (int, int) in signle-value context
+   28 | _, _ := 0, if true { 0 } else { 1 }
+   29 | _ := if true { f() } else { f() }
+   30 | _, _ := 0, if true { f() } else { f() }
+      |            ~~

vlib/v/checker/tests/assign_multi_mismatch.vv

@@ -0,0 +1,30 @@
+fn f() (int, int) {
+	return 0, 0
+}
+
+_ := 0, 0
+_ := f()
+_, _ := f()
+_, _ := 0, f()
+_, _ := f(), 0
+_, _, _ := 0, f()
+_, _, _ := f(), 0
+_, _ := f(), f()
+_, _, _, _ := f(), f()
+
+_, _ := 0, match 4 {
+	1 { 0 }
+	else { 1 }
+}
+_ := match 4 {
+	1 { f() }
+	else { f() }
+}
+_, _ := 0, match 4 {
+	1 { f() }
+	else { f() }
+}
+
+_, _ := 0, if true { 0 } else { 1 }
+_ := if true { f() } else { f() }
+_, _ := 0, if true { f() } else { f() }