overload resolution fails for user-define string literal operator templates in an inline/`using`'d namespace

[marralesfios]

Environment

• OS and Version: Arch Linux (linux 6.18.7-arch-1 kernel)
• VS Code Version: 1.108.2 commit c9d77990917f3102ada88be140d28b038d1dd7c7
• C/C++ Extension Version: 1.30.3
• If using SSH remote, specify OS of remote machine: N/A

Bug Summary and Steps to Reproduce

Bug Summary:

Whenever a user-defined string literal operator template is placed in a namespace and then that namespace is using'd or declared inline, using said template in the global namespace causes IntelliSense errors.

Steps to reproduce:

1. Create a new folder with provided c_cpp_properties.json
2. Create a new file and paste in this code:

#include<cstdint>
#include<string>
template<typename CharT,std::size_t l>
struct basic_template_string{
    CharT arr[l];
    constexpr basic_template_string(const CharT(&a)[l]){
        std::size_t i = l;
        while(i--){
            arr[i] = a[i];
        }
    }
};
template<std::size_t n>
using c_template_string = basic_template_string<char,n>;
template<std::size_t n>
using template_string = basic_template_string<char8_t,n>;
inline namespace literals{
    template<c_template_string s>
    consteval decltype(s) operator ""_ts(){
        return s;
    }
    template<template_string s>
    consteval decltype(s) operator ""_ts(){
        return s;
    }
}
template<template_string,typename ...T>
void foo(T&&...){}
int main(){
    operator ""_ts<u8"Hello!">(); // okay
    foo<u8"Hello!"_ts>(1,2); // error?
    return 0;
}

3. Scroll down to foo<u8"Hello!"_ts>(1,2); line
4. See error

Expected behavior:
No error. (Godbolt proof: https://godbolt.org/z/WvMG14TPW)

Configuration and Logs

home folder /home/... redacted with ~ in the following logs.
c_cpp_properties.json:

{
    "configurations": [
        {
            "name": "Default",
            "intelliSenseMode": "linux-gcc-x64",
            "compilerPath": "/usr/bin/g++"
        }
    ],
    "version": 4
}

Log Diagnostics result:

-------- Diagnostics - 2/3/2026, 12:17:10 PM
Version: 1.30.3
Current Configuration:
{
    "name": "Default",
    "intelliSenseMode": "linux-gcc-x64",
    "compilerPath": "/usr/bin/g++",
    "compilerPathIsExplicit": true,
    "cStandardIsExplicit": true,
    "cppStandardIsExplicit": true,
    "intelliSenseModeIsExplicit": true,
    "recursiveIncludesReduceIsExplicit": false,
    "recursiveIncludesPriorityIsExplicit": false,
    "recursiveIncludesOrderIsExplicit": false,
    "compilerPathInCppPropertiesJson": "/usr/bin/g++",
    "includePath": [
        "~/include"
    ],
    "compilerArgs": [
        "-m64"
    ],
    "cStandard": "c23",
    "cppStandard": "c++26",
    "mergeConfigurations": false,
    "recursiveIncludes": {},
    "browse": {
        "limitSymbolsToIncludedHeaders": true
    }
}
Modified Settings:
{
    "C_Cpp.intelliSenseUpdateDelay": 500,
    "C_Cpp.autocompleteAddParentheses": true,
    "C_Cpp.suggestSnippets": false,
    "C_Cpp.default.includePath": [
        "~/include"
    ],
    "C_Cpp.default.compilerPath": "/usr/bin/g++",
    "C_Cpp.default.compilerArgs": [
        "-m64"
    ],
    "C_Cpp.default.cStandard": "c23",
    "C_Cpp.default.cppStandard": "c++26",
    "C_Cpp.codeAnalysis.updateDelay": 1000,
    "C_Cpp.debugShortcut": false,
    "C_Cpp.loggingLevel": "Debug",
    "C_Cpp.copilotHover": "disabled"
}
Additional Tracked Settings:
{
    "editorTabSize": 4,
    "editorInsertSpaces": true,
    "editorAutoClosingBrackets": "languageDefined",
    "filesEncoding": "utf8",
    "filesAssociations": {
        "array": "cpp",
        "atomic": "cpp",
        "bit": "cpp",
        "cctype": "cpp",
        "charconv": "cpp",
        "chrono": "cpp",
        "clocale": "cpp",
        "cmath": "cpp",
        "codecvt": "cpp",
        "compare": "cpp",
        "complex": "cpp",
        "concepts": "cpp",
        "condition_variable": "cpp",
        "cstdarg": "cpp",
        "cstddef": "cpp",
        "cstdint": "cpp",
        "cstdio": "cpp",
        "cstdlib": "cpp",
        "cstring": "cpp",
        "ctime": "cpp",
        "cwchar": "cpp",
        "cwctype": "cpp",
        "deque": "cpp",
        "list": "cpp",
        "map": "cpp",
        "set": "cpp",
        "string": "cpp",
        "unordered_map": "cpp",
        "unordered_set": "cpp",
        "vector": "cpp",
        "exception": "cpp",
        "algorithm": "cpp",
        "functional": "cpp",
        "iterator": "cpp",
        "memory": "cpp",
        "memory_resource": "cpp",
        "numeric": "cpp",
        "optional": "cpp",
        "random": "cpp",
        "ratio": "cpp",
        "string_view": "cpp",
        "system_error": "cpp",
        "tuple": "cpp",
        "type_traits": "cpp",
        "utility": "cpp",
        "format": "cpp",
        "fstream": "cpp",
        "future": "cpp",
        "initializer_list": "cpp",
        "iomanip": "cpp",
        "iosfwd": "cpp",
        "iostream": "cpp",
        "istream": "cpp",
        "limits": "cpp",
        "mutex": "cpp",
        "new": "cpp",
        "numbers": "cpp",
        "ostream": "cpp",
        "queue": "cpp",
        "ranges": "cpp",
        "semaphore": "cpp",
        "span": "cpp",
        "sstream": "cpp",
        "stdexcept": "cpp",
        "stdfloat": "cpp",
        "stop_token": "cpp",
        "streambuf": "cpp",
        "text_encoding": "cpp",
        "thread": "cpp",
        "cinttypes": "cpp",
        "typeinfo": "cpp",
        "valarray": "cpp",
        "variant": "cpp",
        "shared_mutex": "cpp",
        "cfenv": "cpp",
        "print": "cpp",
        "any": "cpp",
        "bitset": "cpp",
        "forward_list": "cpp",
        "stack": "cpp",
        "typeindex": "cpp",
        "*.inc": "cpp",
        "hash_map": "cpp",
        "coroutine": "cpp",
        "source_location": "cpp"
    },
    "filesExclude": {
        "**/.git": true,
        "**/.svn": true,
        "**/.hg": true,
        "**/.DS_Store": true,
        "**/Thumbs.db": true
    },
    "filesAutoSaveAfterDelay": true,
    "editorInlayHintsEnabled": true,
    "editorParameterHintsEnabled": true,
    "searchExclude": {
        "**/node_modules": true,
        "**/bower_components": true,
        "**/*.code-search": true
    },
    "workbenchSettingsEditor": "ui"
}
cpptools version (native): 1.30.2.0
Current database path: ~/.cache/vscode-cpptools/3188dd32d309a36ba9b381435fa68e21/.browse.VC.db
Translation Unit Mappings:
[ ~/Projects/cppsandbox/mre.cpp - source TU]:
Translation Unit Configurations:
[ ~/Projects/cppsandbox/mre.cpp ]
    Process ID: 30682
    Memory Usage: 83 MB
    Compiler Path: /usr/bin/g++
    Include paths:
        include: ~/include
        system include: /usr/include/c++/15.2.1
        system include: /usr/include/c++/15.2.1/x86_64-pc-linux-gnu
        system include: /usr/include/c++/15.2.1/backward
        system include: /usr/lib/gcc/x86_64-pc-linux-gnu/15.2.1/include
        system include: /usr/local/include
        system include: /usr/lib/gcc/x86_64-pc-linux-gnu/15.2.1/include-fixed
        system include: /usr/include
    Standard Version: c++26
    IntelliSense Mode: linux-gcc-x64
    Other Flags:
        --g++
        --gnu_version=150201
Total Memory Usage: 83 MB

------- Workspace parsing diagnostics -------
Number of files discovered (not excluded): 46672
Number of files parsed: 5884

Language server logging:
lslog.txt

Other Extensions

No response

Additional context

No response

[sean-mcmanus]

@marralesfios It's a bug with our EDG 6.8-based parser -- repros with that on godbolt too with --c++20 --g++ --gnu_verision=150000. Reproed with 6.7 and our previous 1.29.3 too.

I filed an internal bug 2699682.

I also "reduced" the issue (removed the includes) to

using size_t = decltype(sizeof(int));

template<typename CharT,size_t l>
struct basic_template_string{
    CharT arr[l];
    constexpr basic_template_string(const CharT(&a)[l]){
        size_t i = l;
        while(i--){
            arr[i] = a[i];
        }
    }
};
template<size_t n>
using c_template_string = basic_template_string<char,n>;
template<size_t n>
using template_string = basic_template_string<char8_t,n>;
inline namespace literals{
    template<c_template_string s>
    consteval decltype(s) operator ""_ts(){
        return s;
    }
    template<template_string s>
    consteval decltype(s) operator ""_ts(){
        return s;
    }
}
template<template_string,typename ...T>
void foo(T&&...){}
int main(){
    (void)operator ""_ts<u8"Hello!">(); // okay
    foo<u8"Hello!"_ts>(1,2); // error?
    return 0;
}