[clang] [clang] more useful error message for decomposition declaration missing initializer (PR #127924)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Precommit CI failures are related:
```
FAIL: Clang Tools :: clang-tidy/checkers/misc/misplaced-const-cxx17.cpp (862 of 1576)
  ******************** TEST 'Clang Tools :: clang-tidy/checkers/misc/misplaced-const-cxx17.cpp' FAILED ********************
  Exit Code: 1
  
  Command Output (stdout):
  --
  # RUN: at line 1
  C:/Python312/python.exe C:/_work/llvm-project/llvm-project/clang-tools-extra/test/../test\clang-tidy\check_clang_tidy.py -std=c++17-or-later -expect-clang-tidy-error C:\_work\llvm-project\llvm-project\clang-tools-extra\test\clang-tidy\checkers\misc\misplaced-const-cxx17.cpp misc-misplaced-const C:\_work\llvm-project\llvm-project\build\tools\clang\tools\extra\test\clang-tidy\checkers\misc\Output\misplaced-const-cxx17.cpp.tmp
  # executed command: C:/Python312/python.exe 'C:/_work/llvm-project/llvm-project/clang-tools-extra/test/../test\clang-tidy\check_clang_tidy.py' -std=c++17-or-later -expect-clang-tidy-error 'C:\_work\llvm-project\llvm-project\clang-tools-extra\test\clang-tidy\checkers\misc\misplaced-const-cxx17.cpp' misc-misplaced-const 'C:\_work\llvm-project\llvm-project\build\tools\clang\tools\extra\test\clang-tidy\checkers\misc\Output\misplaced-const-cxx17.cpp.tmp'
  # .---command stdout------------
  # | Running ['not', 'clang-tidy', '--experimental-custom-checks', 'C:\\_work\\llvm-project\\llvm-project\\build\\tools\\clang\\tools\\extra\\test\\clang-tidy\\checkers\\misc\\Output\\misplaced-const-cxx17.cpp.tmp.cpp', '-fix', '--checks=-*,misc-misplaced-const', '--config={}', '--', '-std=c++17', '-nostdinc++', '-isystem', 'C:\\_work\\llvm-project\\llvm-project\\clang-tools-extra\\test\\clang-tidy\\checkers\\Inputs\\Headers\\std']...
  # | ------------------------ clang-tidy output -----------------------
  # | 1 error generated.
  # | 
  # | Error while processing C:\_work\llvm-project\llvm-project\build\tools\clang\tools\extra\test\clang-tidy\checkers\misc\Output\misplaced-const-cxx17.cpp.tmp.cpp.
  # | 
  # | C:\_work\llvm-project\llvm-project\build\tools\clang\tools\extra\test\clang-tidy\checkers\misc\Output\misplaced-const-cxx17.cpp.tmp.cpp:7:9: error: structured binding declaration '[x]' requires an initializer; expected '=' or braced initializer list [clang-diagnostic-error]
  # |     7 | auto [x];
  # |       |         ^
  # | Input was:
  # | 
  # | <<<<<<
  # | 
  # |            1: 1 error generated.
  # |  
  # | 
  # | check:6'0    {                      search range start (exclusive)
  # | 
  # | check:6'1                           error: no match found in search range
  # | 
  # | check:6'2                           with "@LINE+1" equal to "7"
  # | 
  # |            2: Error while processing C:\_work\llvm-project\llvm-project\build\tools\clang\tools\extra\test\clang-tidy\checkers\misc\Output\misplaced-const-cxx17.cpp.tmp.cpp.
  # |  
  # | 
  # |            3: C:\_work\llvm-project\llvm-project\build\tools\clang\tools\extra\test\clang-tidy\checkers\misc\Output\misplaced-const-cxx17.cpp.tmp.cpp:7:9: error: structured binding declaration '[x]' requires an initializer; expected '=' or braced initializer list [clang-diagnostic-error] 
  # | 
  # | check:6'3                                                                                                                                            ?                                                                                                                                             possible intended match
  # | 
  # |            4:  7 | auto [x]; 
  # | 
  # |            5:  | ^ 
  # | 
  # |            6: Found compiler errors, but -fix-errors was not specified.
  # |  
  # | 
  # |            7: Fixes have NOT been applied.
  # |  
  # | 
  # |            8: 
  # |  
  # | 
  # |            9: Found compiler error(s).
  # |  
  # | 
  # | check:6'4                               } search range end (exclusive)
  # | 
  # | >>>>>>
  # | 
  # | 
  # `-----------------------------
  # .---command stderr------------
  # | Traceback (most recent call last):
  # |   File "C:\_work\llvm-project\llvm-project\clang-tools-extra\test\clang-tidy\check_clang_tidy.py", line 558, in <module>
  # |     main()
  # |   File "C:\_work\llvm-project\llvm-project\clang-tools-extra\test\clang-tidy\check_clang_tidy.py", line 554, in main
  # |     CheckRunner(args, extra_args).run()
  # |   File "C:\_work\llvm-project\llvm-project\clang-tools-extra\test\clang-tidy\check_clang_tidy.py", line 433, in run
  # |     self.check_messages(main_output)
  # |   File "C:\_work\llvm-project\llvm-project\clang-tools-extra\test\clang-tidy\check_clang_tidy.py", line 358, in check_messages
  # |     try_run(
  # |   File "C:\_work\llvm-project\llvm-project\clang-tools-extra\test\clang-tidy\check_clang_tidy.py", line 64, in try_run
  # |     process_output = subprocess.check_output(args, stderr=subprocess.STDOUT).decode(
  # |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  # |   File "c:\python312\Lib\subprocess.py", line 466, in check_output
  # |     return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  # |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  # |   File "c:\python312\Lib\subprocess.py", line 571, in run
  # |     raise CalledProcessError(retcode, process.args,
  # | subprocess.CalledProcessError: Command '['FileCheck', '-input-file=C:\\_work\\llvm-project\\llvm-project\\build\\tools\\clang\\tools\\extra\\test\\clang-tidy\\checkers\\misc\\Output\\misplaced-const-cxx17.cpp.tmp.cpp.msg', 'C:\\_work\\llvm-project\\llvm-project\\clang-tools-extra\\test\\clang-tidy\\checkers\\misc\\misplaced-const-cxx17.cpp', '-check-prefixes=CHECK-MESSAGES', '-implicit-check-not={{warning|error}}:']' returned non-zero exit status 1.
  # `-----------------------------
  # error: command failed with exit status: 1
  
  --
  
  ********************
```

https://github.com/llvm/llvm-project/pull/127924