some improvements

money/money.go

@@ -78,6 +78,16 @@ func ParseCents(s string) (Cents, error) {
 	if intStr == "" {
 		intStr = "0"
 	}
+	// After the leading sign-strip above, the integer part must NOT contain a
+	// second sign character. strconv.ParseInt accepts a leading '-' or '+',
+	// which is how the previous "--1" bug silently flipped the sign (strip
+	// first '-', re-parse "-1" as -1, then `if neg { c = -c }` flipped it back
+	// to +100 — the wrong sign on the wrong input). Reject any second sign
+	// anywhere in the integer part: every malformed-sign input
+	// ("--1", "+-1", "-+1", "++1") now errors with ErrParse.
+	if strings.ContainsAny(intStr, "+-") {
+		return 0, fmt.Errorf("%w: %q has multiple signs in integer part", ErrParse, s)
+	}
 	// Pad the fractional part to exactly 2 digits: "5" → "50", "" → "00".
 	switch len(fracStr) {
 	case 0:
@@ -85,6 +95,15 @@ func ParseCents(s string) (Cents, error) {
 	case 1:
 		fracStr += "0"
 	}
+	// The fractional part must also be sign-free. A leading '-' on the
+	// fractional string ("-" alone, "-0") was previously parsed by ParseInt
+	// as 0 after the pad-to-2 above, which silently accepted inputs like
+	// ".-", "1.-", "1.-0". The fractional part is conventionally
+	// non-negative (the leading sign applies to the whole number); reject
+	// any sign anywhere in fracStr.
+	if strings.ContainsAny(fracStr, "+-") {
+		return 0, fmt.Errorf("%w: %q has a sign in fractional part", ErrParse, s)
+	}
 
 	major, err := strconv.ParseInt(intStr, 10, 64)
 	if err != nil {