Add value-returning match expressions

internal/lang/semantics.go

@@ -225,6 +225,15 @@ func (c *mutabilityChecker) checkExpr(expr Expr) error {
 				return err
 			}
 		}
+	case MatchExpr:
+		if err := c.checkExpr(e.Value); err != nil {
+			return err
+		}
+		for _, matchCase := range e.Cases {
+			if err := c.checkExpr(matchCase.Value); err != nil {
+				return err
+			}
+		}
 	case LambdaExpr:
 		bindings := map[string]bool{}
 		if e.ImplicitIt {