[Clang] Clang rejects anonymous struct with generic lambda while GCC/MSVC accept

[Xniao]

Clang rejects following C++ code containing generic lambda in anonymous struct's member function, while both GCC and MSVC accept the same code. This affects all forms of generic lambdas like auto&x, auto... x.

• Here is the reproducer: https://godbolt.org/z/1ehMYbsT8
• Generated by fuzzer.

Program

typedef struct {
    void foo() {
        auto f = [](auto&& x) {};
        f(f);
    }
} Test;

Clang Output

<source>:1:15: error: anonymous non-C-compatible type given name for linkage purposes by typedef declaration after its linkage was computed; add a tag name here to establish linkage prior to definition
    1 | typedef struct {
      |               ^
      |                Test
<source>:2:5: note: type is not C-compatible due to this member declaration
    2 |     void foo() {
      |     ^~~~~~~~~~~~
    3 |         auto f = [](auto&& x) {};
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~
    4 |         f(f);
      |         ~~~~~
    5 |     }
      |     ~
<source>:6:3: note: type is given name 'Test' for linkage purposes by this typedef declaration
    6 | } Test;
      |   ^
1 error generated.
Compiler returned: 1

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Jireh Guo (Xniao)

Clang rejects following C++ code containing generic lambda in anonymous struct's member function, while both GCC and MSVC accept the same code. This affects all forms of generic lambdas like `auto&x`, `auto... x`. - Here is the reproducer: https://godbolt.org/z/1ehMYbsT8 - _**Generated by fuzzer.**_

Program

typedef struct {
    void foo() {
        auto f = [](auto&& x) {};
        f(f);
    }
} Test;

Clang Output

<source>:1:15: error: anonymous non-C-compatible type given name for linkage purposes by typedef declaration after its linkage was computed; add a tag name here to establish linkage prior to definition
    1 | typedef struct {
      |               ^
      |                Test
<source>:2:5: note: type is not C-compatible due to this member declaration
    2 |     void foo() {
      |     ^~~~~~~~~~~~
    3 |         auto f = [](auto&& x) {};
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~
    4 |         f(f);
      |         ~~~~~
    5 |     }
      |     ~
<source>:6:3: note: type is given name 'Test' for linkage purposes by this typedef declaration
    6 | } Test;
      |   ^
1 error generated.
Compiler returned: 1

[frederick-vs-ja]

This is partially due to P1766R1 (defect report, should be considered even applied to C++98). MSVC rejects the example with /permissive-.

I'm inclined to close this as wontfix Issue is real, but we can't or won't fix it. Not invalid and consistently reject lambdas in this context.

[pinskia]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97475 is the GCC bug about accepting this.

[zygoloid]

I'm inclined to close this as wontfix Issue is real, but we can't or won't fix it. Not invalid and consistently reject lambdas in this context.

Shouldn't it be invalid Resolved as invalid, i.e. not a bug instead? Under P1766R1, Clang is correct to reject. And Clang only produces a hard error in the case where something's actually gone wrong -- when we triggered a linkage computation too early. We produce a warning instead in the case where we didn't happen to look at the linkage during the class definition.

Note that while GCC accepts, it also "miscompiles" ill-formed examples in this space (because it emits symbols with the wrong linkage before later finding the name for linkage purposes that changes the linkage of things it already emitted).