checker: reject method that have multi-value type receiver (#8696)

vlib/v/checker/checker.v

@@ -5753,6 +5753,8 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 			c.error('method overrides built-in array method', node.pos)
 		} else if sym.kind == .sum_type && node.name == 'type_name' {
 			c.error('method overrides built-in sum type method', node.pos)
+		} else if sym.kind == .multi_return {
+			c.error('cannot define method on multi-value', node.method_type_pos)
 		}
 		if sym.name.len == 1 {
 			// One letter types are reserved for generics.

vlib/v/checker/tests/multi_value_method_err.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/multi_value_method_err.vv:1:7: error: cannot define method on multi-value
+    1 | fn (v (int, int)) f() {}
+      |       ~~~~~~~~~~

vlib/v/checker/tests/multi_value_method_err.vv

@@ -0,0 +1 @@
+fn (v (int, int)) f() {}