[clang] fix explicit incomplete enum (PR #184210)

[via cfe-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 53016 tests passed
* 993 tests skipped
* 3 tests failed

## Failed Tests
(click on a test name to see its output)

### Clang
<details>
<summary>Clang.SemaCXX/constant-expression-cxx11.cpp</summary>

```
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
c:\_work\llvm-project\llvm-project\build\bin\clang.exe -cc1 -internal-isystem C:\_work\llvm-project\llvm-project\build\lib\clang\23\include -nostdsysteminc -std=c++23 -isystem C:\_work\llvm-project\llvm-project\clang\test\SemaCXX/Inputs -fsyntax-only -verify=expected,cxx20_23,cxx23              -triple x86_64-linux -Wno-string-plus-int -Wno-pointer-arith -Wno-zero-length-array -Wno-c99-designator -fcxx-exceptions -pedantic C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\constant-expression-cxx11.cpp -Wno-comment -Wno-tautological-pointer-compare -Wno-bool-conversion
# executed command: 'c:\_work\llvm-project\llvm-project\build\bin\clang.exe' -cc1 -internal-isystem 'C:\_work\llvm-project\llvm-project\build\lib\clang\23\include' -nostdsysteminc -std=c++23 -isystem 'C:\_work\llvm-project\llvm-project\clang\test\SemaCXX/Inputs' -fsyntax-only -verify=expected,cxx20_23,cxx23 -triple x86_64-linux -Wno-string-plus-int -Wno-pointer-arith -Wno-zero-length-array -Wno-c99-designator -fcxx-exceptions -pedantic 'C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\constant-expression-cxx11.cpp' -Wno-comment -Wno-tautological-pointer-compare -Wno-bool-conversion
# .---command stderr------------
# | error: diagnostics with 'note' severity seen but not expected: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\constant-expression-cxx11.cpp Line 2154: template parameter is declared here
# | 1 error generated.
# `-----------------------------
# error: command failed with exit status: 1

--

```
</details>
<details>
<summary>Clang.SemaCXX/gh183887.cpp</summary>

```
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
c:\_work\llvm-project\llvm-project\build\bin\clang.exe -cc1 -internal-isystem C:\_work\llvm-project\llvm-project\build\lib\clang\23\include -nostdsysteminc -std=c++20 -fsyntax-only -verify C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\gh183887.cpp
# executed command: 'c:\_work\llvm-project\llvm-project\build\bin\clang.exe' -cc1 -internal-isystem 'C:\_work\llvm-project\llvm-project\build\lib\clang\23\include' -nostdsysteminc -std=c++20 -fsyntax-only -verify 'C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\gh183887.cpp'
# .---command stderr------------
# | error: 'expected-error' diagnostics expected but not seen: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\gh183887.cpp Line 4: initialization of incomplete type 'E1'
# | error: 'expected-error' diagnostics seen but not expected: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\gh183887.cpp Line 4: ISO C++ forbids forward references to 'enum' types
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\gh183887.cpp Line 4: invalid use of incomplete type 'E1'
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\gh183887.cpp Line 4: 'explicit' can only appear on non-static member functions
# | error: 'expected-note' diagnostics seen but not expected: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaCXX\gh183887.cpp Line 4: forward declaration of 'GH183887::E1'
# | 5 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

--

```
</details>
<details>
<summary>Clang.SemaTemplate/instantiate-function-params.cpp</summary>

```
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
c:\_work\llvm-project\llvm-project\build\bin\clang.exe -cc1 -internal-isystem C:\_work\llvm-project\llvm-project\build\lib\clang\23\include -nostdsysteminc -triple i686-unknown-unknown -fsyntax-only -verify C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp
# executed command: 'c:\_work\llvm-project\llvm-project\build\bin\clang.exe' -cc1 -internal-isystem 'C:\_work\llvm-project\llvm-project\build\lib\clang\23\include' -nostdsysteminc -triple i686-unknown-unknown -fsyntax-only -verify 'C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp'
# .---command stderr------------
# | error: 'expected-error' diagnostics expected but not seen: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 19: no type named 'type' in
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 19: no type named 'type' in
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 19: no type named 'type' in
# | error: 'expected-note' diagnostics expected but not seen: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 6: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 6: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 13: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 13: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 19: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 19: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 24: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 26: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 27: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 30: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 30: in instantiation
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 30: in instantiation
# | error: 'expected-note' diagnostics seen but not expected: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 4: template parameter is declared here
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 4: template parameter is declared here
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\instantiate-function-params.cpp Line 4: template parameter is declared here
# | 18 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

--

```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/184210