Decimal use produces unexpected errors: "identifier "_Decimal128" is undefined"; "extra text after expected end of number"

Environment

• OS and Version: Windows 11 Pro (10.0.22621)
• VS Code Version: 1.86
• C/C++ Extension Version: 1.18.5
• If using SSH remote, specify OS of remote machine: Rocky Linux 9.3 x64

Bug Summary and Steps to Reproduce

Bug Summary:
C/C++ is reporting errors in a source project that uses GCC's _Decimal128 implementation.

Steps to reproduce:

Create new project containing single file on remote

int main (int argc, char **argv) {
  _Decimal128 v;
  v=1.25DL;
  return (int)v;
}

Result is two errors for the source file:

identifier "_Decimal128" is undefined
extra text after expected end of number

Expected behavior:

No errors

Configuration and Logs

c_cpp_properties.json
{
    "configurations": [
        {
            "name": "Linux",
            "includePath": [
                "${workspaceFolder}/**"
            ],
            "defines": [],
            "compilerPath": "/usr/bin/gcc",
            "cStandard": "c23",
            "cppStandard": "c++23",
            "intelliSenseMode": "linux-gcc-x64"
        }
    ],
    "version": 4
}

C/C++: Log Diagnostics output:

-------- Diagnostics - 08/02/2024, 14:08:12
Version: 1.18.5
Current Configuration:
{
    "name": "Linux",
    "includePath": [
        "/tmp/dectest/**"
    ],
    "defines": [],
    "compilerPath": "/usr/bin/gcc",
    "cStandard": "c23",
    "cppStandard": "c++23",
    "intelliSenseMode": "linux-gcc-x64",
    "compilerPathIsExplicit": true,
    "cStandardIsExplicit": true,
    "cppStandardIsExplicit": true,
    "intelliSenseModeIsExplicit": true,
    "compilerPathInCppPropertiesJson": "/usr/bin/gcc",
    "mergeConfigurations": false,
    "browse": {
        "path": [
            "/tmp/dectest/**",
            "${workspaceFolder}"
        ],
        "limitSymbolsToIncludedHeaders": true
    }
}
cpptools version (native): 1.18.3.0
Translation Unit Mappings:
[ /tmp/dectest/dectest.c ]:
    /tmp/dectest/dectest.c
Translation Unit Configurations:
[ /tmp/dectest/dectest.c ]:
    Process ID: 491636
    Memory Usage: 10 MB
    Compiler Path: /usr/bin/gcc
    Includes:
        /usr/lib/gcc/x86_64-redhat-linux/11/include
        /usr/local/include
        /usr/include
    Standard Version: c23
    IntelliSense Mode: linux-gcc-x64
    Other Flags:
        --gcc
        --gnu_version=110401
Total Memory Usage: 10 MB

------- Workspace parsing diagnostics -------
Number of files discovered (not excluded): 6377



Language Server Debug Logging:

loggingLevel: Debug
loggingLevel has changed to: Debug
LSP: (invoked) cpptools/didChangeCppProperties (id: 172)
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 173)
LSP: (received) cpptools/activeDocumentChange: file:///tmp/dectest/dectest.c
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 173)
LSP: (received) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/activeDocumentChange: file:///tmp/dectest/dectest.c
LSP: (invoked) cpptools/textEditorSelectionChange
LSP: (received) cpptools/getInlayHints: file:///tmp/dectest/dectest.c (id: 174)
LSP: (invoked) cpptools/getInlayHints: file:///tmp/dectest/dectest.c (id: 174)
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 175)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 175)
LSP: (received) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 176)
LSP: (invoked) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 176)
LSP: (received) cpptools/getSemanticTokens: file:///tmp/dectest/dectest.c (id: 177)
LSP: (invoked) cpptools/getSemanticTokens: file:///tmp/dectest/dectest.c (id: 177)
LSP: (received) cpptools/activeDocumentChange: file:///tmp/dectest/dectest.c
LSP: (invoked) cpptools/activeDocumentChange: file:///tmp/dectest/dectest.c
LSP: (received) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/textEditorSelectionChange
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 178)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 178)
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 179)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 179)
LSP: (received) cpptools/activeDocumentChange: file:///tmp/dectest/dectest.c
LSP: (invoked) cpptools/activeDocumentChange: file:///tmp/dectest/dectest.c
LSP: (received) cpptools/textEditorSelectionChange
LSP: (received) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/textEditorSelectionChange
LSP: (received) textDocument/hover: file:///tmp/dectest/dectest.c (id: 180)
LSP: (invoked) textDocument/hover: file:///tmp/dectest/dectest.c (id: 180)
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 181)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 181)
LSP: (received) textDocument/documentHighlight: file:///tmp/dectest/dectest.c (id: 182)
LSP: (invoked) textDocument/documentHighlight: file:///tmp/dectest/dectest.c (id: 182)
LSP: (received) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/textEditorSelectionChange
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 183)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 183)
LSP: (received) textDocument/didChange: file:///tmp/dectest/dectest.c
LSP: (invoked) textDocument/didChange: file:///tmp/dectest/dectest.c
LSP: (received) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/textEditorSelectionChange
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 184)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 184)
LSP: (received) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 185)
LSP: (invoked) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 185)
LSP: (received) cpptools/getSemanticTokens: file:///tmp/dectest/dectest.c (id: 186)
LSP: (invoked) cpptools/getSemanticTokens: file:///tmp/dectest/dectest.c (id: 186)
LSP: (received) cpptools/getDocumentSymbols: file:///tmp/dectest/dectest.c (id: 187)
LSP: (invoked) cpptools/getDocumentSymbols: file:///tmp/dectest/dectest.c (id: 187)
  tag parsing file: /tmp/dectest/dectest.c
LSP: (received) textDocument/hover: file:///tmp/dectest/dectest.c (id: 188)
LSP: (invoked) textDocument/hover: file:///tmp/dectest/dectest.c (id: 188)
sending 1 changes to server
LSP: (received) textDocument/documentHighlight: file:///tmp/dectest/dectest.c (id: 189)
LSP: (invoked) textDocument/documentHighlight: file:///tmp/dectest/dectest.c (id: 189)
LSP: (received) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/textEditorSelectionChange
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 190)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 190)
LSP: (received) textDocument/didChange: file:///tmp/dectest/dectest.c
LSP: (invoked) textDocument/didChange: file:///tmp/dectest/dectest.c
LSP: (received) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/textEditorSelectionChange
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 191)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 191)
LSP: (received) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 192)
LSP: (invoked) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 192)
LSP: (received) textDocument/hover: file:///tmp/dectest/dectest.c (id: 193)
LSP: (invoked) textDocument/hover: file:///tmp/dectest/dectest.c (id: 193)
sending 1 changes to server
LSP: (received) textDocument/documentHighlight: file:///tmp/dectest/dectest.c (id: 194)
LSP: (invoked) textDocument/documentHighlight: file:///tmp/dectest/dectest.c (id: 194)
LSP: (received) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/textEditorSelectionChange
LSP: (received) cpptools/getDocumentSymbols: file:///tmp/dectest/dectest.c (id: 195)
LSP: (invoked) cpptools/getDocumentSymbols: file:///tmp/dectest/dectest.c (id: 195)
  tag parsing file: /tmp/dectest/dectest.c
idle loop: reparsing the active document
Checking for syntax errors: /tmp/dectest/dectest.c
Queueing IntelliSense update for files in translation unit of: /tmp/dectest/dectest.c
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 196)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 196)
LSP: (received) textDocument/didChange: file:///tmp/dectest/dectest.c
LSP: (invoked) textDocument/didChange: file:///tmp/dectest/dectest.c
LSP: (received) cpptools/textEditorSelectionChange
LSP: (invoked) cpptools/textEditorSelectionChange
sending 1 changes to server
Error squiggle count: 2
LSP: Request canceled by server (cpptools/getSemanticTokens, code: -32802, id: 186)
Update IntelliSense time (sec): 0.258
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 197)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 197)
LSP: (received) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 198)
LSP: (invoked) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 198)
LSP: (received) cpptools/getSemanticTokens: file:///tmp/dectest/dectest.c (id: 199)
LSP: (invoked) cpptools/getSemanticTokens: file:///tmp/dectest/dectest.c (id: 199)
LSP: (received) cpptools/getDocumentSymbols: file:///tmp/dectest/dectest.c (id: 200)
LSP: (invoked) cpptools/getDocumentSymbols: file:///tmp/dectest/dectest.c (id: 200)
  tag parsing file: /tmp/dectest/dectest.c
LSP: (received) cpptools/getInlayHints: file:///tmp/dectest/dectest.c (id: 201)
LSP: (invoked) cpptools/getInlayHints: file:///tmp/dectest/dectest.c (id: 201)
LSP: (received) textDocument/willSaveWaitUntil: file:///tmp/dectest/dectest.c (id: 202)
LSP: (invoked) textDocument/willSaveWaitUntil: file:///tmp/dectest/dectest.c (id: 202)
willSaveWaitUntil: 0ms
LSP: (received) textDocument/didSave: file:///tmp/dectest/dectest.c
LSP: (invoked) textDocument/didSave: file:///tmp/dectest/dectest.c
  tag parsing file: /tmp/dectest/dectest.c
LSP: (received) cpptools/fileChanged: file:///tmp/dectest/dectest.c
LSP: (invoked) cpptools/fileChanged: file:///tmp/dectest/dectest.c
idle loop: reparsing the active document
Checking for syntax errors: /tmp/dectest/dectest.c
Queueing IntelliSense update for files in translation unit of: /tmp/dectest/dectest.c
Checking for syntax errors: /tmp/dectest/dectest.c
Queueing IntelliSense update for files in translation unit of: /tmp/dectest/dectest.c
Error squiggle count: 2
Update IntelliSense time (sec): 0.509
LSP: (received) cpptools/getSemanticTokens: file:///tmp/dectest/dectest.c (id: 203)
LSP: (invoked) cpptools/getSemanticTokens: file:///tmp/dectest/dectest.c (id: 203)
LSP: (received) cpptools/getInlayHints: file:///tmp/dectest/dectest.c (id: 204)
LSP: (invoked) cpptools/getInlayHints: file:///tmp/dectest/dectest.c (id: 204)
LSP: (received) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 205)
LSP: (invoked) cpptools/getCodeActions: file:///tmp/dectest/dectest.c (id: 205)
LSP: (received) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 206)
LSP: (invoked) cpptools/getFoldingRanges: file:///tmp/dectest/dectest.c (id: 206)

Other Extensions

Only extensions enabled for the workspace are the remote developent extensions.

Additional context

No response

Start with the single-file reproducer at /tmp/dectest/dectest.c and the logged C/C++ configuration using GCC, C23, and linux-gcc-x64. Reproduce the two diagnostics, then trace the IntelliSense parsing path for _Decimal128 and the 1.25DL literal. Done means both diagnostics are absent for the supplied C source.