refactor: Remove unused ParsePrechecks from ParseIntegral

Also:
* Remove redundant {} from return statement
* Add missing failing c-string test case and "-" and "+" strings
* Add missing failing test cases for non-int32_t integral types

src/util/strencodings.cpp

@@ -281,16 +281,11 @@ std::string DecodeBase32(const std::string& str, bool* pf_invalid)
     return std::string((const char*)vchRet.data(), vchRet.size());
 }
 
-[[nodiscard]] static bool ParsePrechecks(const std::string&);
-
 namespace {
 template <typename T>
 bool ParseIntegral(const std::string& str, T* out)
 {
     static_assert(std::is_integral<T>::value);
-    if (!ParsePrechecks(str)) {
-        return false;
-    }
     // Replicate the exact behavior of strtol/strtoll/strtoul/strtoull when
     // handling leading +/- for backwards compatibility.
     if (str.length() >= 2 && str[0] == '+' && str[1] == '-') {

src/util/strencodings.h

@@ -97,7 +97,9 @@ constexpr inline bool IsSpace(char c) noexcept {
 }
 
 /**
- * Convert string to integral type T.
+ * Convert string to integral type T. Leading whitespace, a leading +, or any
+ * trailing character fail the parsing. The required format expressed as regex
+ * is `-?[0-9]+`.
  *
  * @returns std::nullopt if the entire string could not be parsed, or if the
  *   parsed value is not in the range representable by the type T.
@@ -111,7 +113,7 @@ std::optional<T> ToIntegral(const std::string& str)
     if (first_nonmatching != str.data() + str.size() || error_condition != std::errc{}) {
         return std::nullopt;
     }
-    return {result};
+    return result;
 }
 
 /**