[llvm-bugs] [Bug 52322] New: C++20 warning: template lambda "does not declare any constructor to initialize its non-modifiable members"

Tue Oct 26 07:37:40 PDT 2021

https://bugs.llvm.org/show_bug.cgi?id=52322

            Bug ID: 52322
           Summary: C++20 warning: template lambda "does not declare any
                    constructor to initialize its non-modifiable members"
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jake.arkinstall at gmail.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

In this toy code, clang 13 warns that a lambda function isn't initializing
captured members.

https://godbolt.org/z/hhszT4fqj

I don't know whether (1) this is a false positive warning, (2) the warning is
legitimate and the generated code is incorrect, or (3) the code is truly
invalid and the warning is a side-effect of that.

I'm leaning towards #1. I'd rule #2 out because it still produces the correct
output, and I'd (tentatively) rule #3 out because GCC and MSVC don't see any
problems with the code (Clang 12 segfaults on it, which appears to be a parsing
issue).

> <source>:9:48: warning: class '' does not declare any constructor to initialize its non-modifiable members
>             TypeList<tail...>::max(comparison, [&comparison, &callback]<typename S>() {
>     ^
>         <source>:9:48: note: in instantiation of member class '' requested here
>                                    <source>:17:31: note: in instantiation of function template specialization 'TypeList<int, char>::max((lambda at <source>:34:9) &, (lambda at <source>:9:48) &&)::(anonymous class)::operator()<char>' requested here
>                                                                    callback.template operator()<head>();
>                               ^
> <source>:9:32: note: in instantiation of function template specialization 'TypeList<char>::max<(lambda at <source>:34:9) &, (lambda at <source>:9:48)>' requested here
>             TypeList<tail...>::max(comparison, [&comparison, &callback]<typename S>() {
>                                ^
> <source>:9:32: note: in instantiation of function template specialization 'TypeList<int, char>::max<(lambda at <source>:34:9) &, (lambda at <source>:9:48)>' requested here
> <source>:33:11: note: in instantiation of function template specialization 'TypeList<long, int, char>::max<(lambda at <source>:34:9), (lambda at <source>:35:9)>' requested here
>     list::max(
>           ^
> <source>:9:50: note: reference member '' will never be initialized
>             TypeList<tail...>::max(comparison, [&comparison, &callback]<typename S>() {
>                                                  ^
> <source>:9:63: note: reference member '' will never be initialized
>             TypeList<tail...>::max(comparison, [&comparison, &callback]<typename S>() {

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