Fix #10009: bad overflow protection when taking out loans

src/core/overflowsafe_type.hpp

@@ -176,6 +176,9 @@ class OverflowSafeInt
 	inline constexpr bool operator <= (const int other) const { return !(*this > other); }
 
 	inline constexpr operator T () const { return this->m_value; }
+
+	static inline constexpr OverflowSafeInt<T> max() { return T_MAX; }
+	static inline constexpr OverflowSafeInt<T> min() { return T_MIN; }
 };
 
 

src/misc_cmd.cpp

@@ -60,8 +60,10 @@ CommandCost CmdIncreaseLoan(DoCommandFlag flags, LoanCommand cmd, Money amount)
 			break;
 	}
 
-	/* Overflow protection */
-	if (c->money + c->current_loan + loan < c->money) return CMD_ERROR;
+	/* In case adding the loan triggers the overflow protection of Money,
+	 * we would essentially be losing money as taking and repaying the loan
+	 * immediately would not get us back to the same bank balance anymore. */
+	if (c->money > Money::max() - loan) return CMD_ERROR;
 
 	if (flags & DC_EXEC) {
 		c->money        += loan;