Misleading or wrong error message in UTF-8 byte sequence verification

[mike-ward]

Describe the bug

Compiling the simple program below leads to a compilation error.

Reproduction Steps

fn main() {
	println('\xF0\x81\xA0\x80')
}

Expected Behavior

It should issue an error indicating the UTF-8 sequence is wrong. The real issue is as follows:

F0 81 A0 80 is not a valid UTF-8 sequence because it represents an overlong encoding, which is not allowed in UTF-8. Valid UTF-8 sequences must use the shortest form for each character.

Current Behavior

main.v:2:22: notice: invalid utf8 string, please check your file's encoding is utf8
    1 | fn main() {
    2 |     println('\xF0\x81\xA0\x80')
      |                         ~~~~~~
    3 | }
�`

Possible Solution

It should be a matter of correcting the error message. The file is a valid UTF-8 encoded. It is the byte sequence that is wrong.

Additional Information/Context

No response

V version

V 0.4.12 e07eb54

Environment details (OS name and version, etc.)

V full version	V 0.4.12 e563c8f.e07eb54
OS	macos, macOS, 26.1, 25B78
Processor	8 cpus, 64bit, little endian, Apple M2
Memory	0.12GB/8GB
V executable	/Users/mike/Documents/github/v/v
V last modified time	2025-11-30 16:14:30
V home dir	OK, value: /Users/mike/Documents/github/v
VMODULES	OK, value: /Users/mike/.vmodules
VTMP	OK, value: /tmp/v_501
Current working dir	OK, value: /Users/mike/Documents/github/bug
env VFLAGS	"-message-limit 5"
env LDFLAGS	"-L/opt/homebrew/opt/ruby/lib"
Git version	git version 2.52.0
V git status	e07eb54
.git/config present	true
cc version	Apple clang version 17.0.0 (clang-1700.4.4.1)
gcc version	Apple clang version 17.0.0 (clang-1700.4.4.1)
clang version	Apple clang version 17.0.0 (clang-1700.4.4.1)
tcc version	tcc version 0.9.28rc 2024-02-05 HEAD@105d70f7 (AArch64 Darwin)
tcc git status	thirdparty-macos-arm64 1867108f
emcc version	N/A
glibc version	N/A

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[jorgeluismireles]

I checked 1) V playground and Golang both 2) playground and 3) console and silently print ���� as \xF0\x81\xA0\x80 is invalid UTF8. At least V is trying to notify programmer the escaped text is invalid UTF8 as a notice but compiles the code finally, so this compiles but with a lot of notices:

fn main() {
	
    for valid in [
    	'\xc3\xb1'
	    '\xe2\x82\xa1'
	    '\xf0\x90\x8c\xbc'
	] {
		println('valid ${valid}')
	}

    for invalid in [
	    '\xc3\x28'
	    '\xa0\xa1'
	    '\xe2\x28\xa1'
	    '\xe2\x82\x28'
	    '\xf0\x28\x8c\xbc'
	    '\xf0\x90\x28\xbc'
	    '\xf0\x28\x8c\x28'
	    '\xf8\xa1\xa1\xa1\xa1'
	    '\xfc\xa1\xa1\xa1\xa1\xa1'
	] {
		println('invalid ${invalid}')
	}
}

Is like V is "pedantic" in acceptance UTF8 escape, but seems good to me to prevent invalid codes, except when thinks your encode is other than UTF8 that should not be even accepted I think.