[SUGGESTION] Cppfront could prevent implicit truncation and conversion when functions return numeric types

[bluetarpmedia]

Suggestion
Cppfront could report errors when detecting implicit numeric truncation and/or conversion in function return statements.
These would be compile-time only checks for:

• numeric literals having invalid values for the function's return type
• expressions having a type incompatible with the function's return type

Examples:

t1: () -> uchar = {
    return -1;  // Invalid literal (signed/unsigned)
}

t2: () -> char = {
    return 300;  // Invalid literal (out-of-range for type)
}

t3: () -> i32 = {
    return 4'000'000'000;  // Invalid literal (out-of-range for type)
}

t4: () -> i32 = {
    return 1.0;  // Invalid literal (not an integer literal)
}

t5_helper: () -> i32 = {
    return std::numeric_limits<int>::min();
}

t5: () -> ushort = {
    return t5_helper();  // t5_helper returns i32, incompatible with ushort
}

Will your feature suggestion eliminate X% of security vulnerabilities of a given kind in current C++ code?
Yes, see:

• CWE-197: Numeric Truncation Error
• CWE-681: Incorrect Conversion between Numeric Types
• CWE-195: Signed to Unsigned Conversion Error.

(There are various CVEs listed for these weaknesses but I couldn't find any with source code.)

Here's a Cpp2 snippet based on examples from the above CWE links:

enum {
    kError_InvalidDimensions = -1
};

get_image_data_size: (width: u32, height: u32) -> u32 = {
    if (width > 1024 && height > 1024)
    {
        return width * height;
    }
    else if (width > 512)
    {
        return width;
    }
    else if (width > 256)
    {
        return width / 2;
    }
    else
    {
        return kError_InvalidDimensions;  // Oops, programmer bug
    }
}

copy_image_data: (dst: *char, src: *const char, width: u32, height: u32) = {
    std::memcpy(dst, src, get_image_data_size(width, height));
}

Will your feature suggestion automate or eliminate X% of current C++ guidance literature?
Yes, it automates the guidance to compile at high warning levels. See:

• Sutter & Alexandrescu, C++ Coding Standards: 101 Rules, Guidelines, and Best Practices (Item 1. Compile cleanly at high warning levels)
• Jason Turner, C++ Weekly - Ep 353 - Implicit Conversions Are Evil

Using the above Cpp2 example, there are no warnings from the 3 major compilers about return kError_InvalidDimensions with default options. To produce a warning:

• MSVC 19: requires /W4
• Clang 15: requires -Wconversion

I couldn't get a warning in GCC 12.2 even with -Wall -Wextra -Wpedantic -Wconversion -Wsign-conversion
https://godbolt.org/z/M9bb46j4T

Describe alternatives you've considered.
One alternative is to provide runtime checks which would validate the actual value rather than just the type itself.

get_internal_count: () -> i32 = {
    return 5;
}

// This is fine; the value 5 returned as an i32 can fit inside uchar without any truncation or signed conversion.
get_usage_count: () -> uchar = {
    return cpp2_runtime_check<uchar>(get_internal_count());
}

However, this could possibly introduce a significant runtime overhead for functions called in hot loops, and would probably require an opt-in/out preference, whereas the suggestion above is for compile-time only checks.