deduction guides do not work through alias templates

Environment

OS and Version: x64, Arch Linux (7.2.6.arch2-1)
VS Code Version: 1.138.0 commit: 7debcd0e2acdea1c52de81bf9ee1620444407dda
C/C++ Extension Version: 1.35.2 pre-release
If using SSH remote, specify OS of remote machine: N/A

Bug Summary and Steps to Reproduce

Bug Summary: Intellisense fails to perform CTAD in the following code, when an alias template for Foo is created:

template<typename T>
struct Foo{
    Foo(auto...){}
};
template<typename ...T>
Foo(T...) -> Foo<T...[0uz]>;

template<typename T>
using Foo2 = Foo<T>;

int main(){
    Foo x{3.1f};  // OK
    Foo2 y{3.1f}; // Not OK??
    return 0;
}

GCC accepts this code.

Steps to reproduce:

Create a new folder, open it as a workspace, and copy test.cpp into it
Open file
See error

Expected behavior:
Intellisense correctly performs CTAD and does not raise an error.

Configuration and Logs

c_cpp_properties.json: (None used)

Log Diagnostics output

Language server logging

Other Extensions

No response

Additional context

In testing, a variadic deduction guide and the use of pack indexing seemed necessary to trigger the bug. Although I'm not certain, it could indicate a problem with pack indexing somehow blocking template deduction.

maf (@marralesfios)

maf (@marralesfios) It would help if you could mention if this is a regression compared to 1.35.1 or 1.34.4...although I can find this out myself later.

Sean McManus (@sean-mcmanus)

Sean McManus (@sean-mcmanus) Seems to be buggy in both 1.35.1 and 1.34.4

I might have similar problem. I didn't want to create a new issue, so i'm posting the error here:

OS: macOS 27.0.1 (Apple Silicon)
VS Code version: 1.140.0
C/C++ extension version: 1.34.4 (darwin-arm64)
Compilers that accept the code: Apple clang 21.0.0 (clang-2100.3.34.2), GCC 16.2.0 (-std=c++23 -Wall -Wextra -pedantic)

Bug Summary:

IntelliSense reports an error on a valid deduction guide whose return type is an explicit specialization of the class template. Clang and GCC compile and run the code without warnings.

Steps to reproduce:

Set the "cppStandard": "gnu++20".

#include <cstddef>
#include <span>

using std::size_t;

template< size_t i = std::dynamic_extent, size_t s = std::dynamic_extent >
struct slice
    {
    static constexpr size_t index = i, size = s;
    };

template<>
struct slice< std::dynamic_extent, std::dynamic_extent >
    {
    size_t index = 0, size = 0;

    constexpr slice() noexcept = default;

    constexpr slice( size_t i, size_t s ) noexcept : index( i ), size( s ) {}
    };

slice() -> slice<>;

slice( size_t, size_t ) -> slice<>;   // error: "the return type must directly designate a specialization of the associated class template"

template< size_t i, size_t s >
constexpr slice< i, s > const_slice;

auto s0 = slice{};              // OK
auto s1 = slice{ 1, 2 };        // error
auto s2 = slice< 3, 4 >{};      // OK
auto s3 = const_slice< 5, 6 >;  // OK
slice s5 = { 2, 4 };            // error
slice s6;                       // OK
size_t n = 1;
auto s7 = slice{ n, 3 };        // error

int main() {}

Update: switching to 1.35.3 only partly fixed the problem. Now the error occurs at these lines:

    slice() -> slice<>; // redeclaration of deduction guide "()->slice<18446744073709551615UL, 18446744073709551615UL>" (previous declaration at line 30)

    slice( size_t, size_t ) -> slice<>; // rredeclaration of deduction guide "(size_t, size_t)->slice<18446744073709551615UL, 18446744073709551615UL>" (previous declaration at line 32)

Dominik Deák (@DominikDeak)

Dominik Deák (@DominikDeak) I can reproduce your problem. Since it seems unrelated to alias templates (as well as the error message being different), you can probably report this bug separately

Thanks, separate bug report here:
#14824

Sunbelt Computer Software

PL/B Language Development and Support

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deduction guides do not work through alias templates #14794

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Sunbelt Computer Software

PL/B Language Development and Support

deduction guides do not work through alias templates #14794

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Bug Summary and Steps to Reproduce

Configuration and Logs

Other Extensions

Additional context

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sean-mcmanus commented on Sep 23, 2026

marralesfios commented on Sep 25, 2026

DominikDeak commented on Oct 8, 2026

DominikDeak commented on Oct 8, 2026

marralesfios commented on Oct 8, 2026

DominikDeak commented on Oct 9, 2026

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