[Bug] Wrong handling of "-std=c++26" option

[adriankwiat]

Brief Issue Summary

When I configure gcc options in Makefile to -std=c++26, correct code (that is also correct C++23) is wrongly marked as errors. This is not problem with C/C++ extension, because when I configure manually c_cpp_properties.json it behaves properly.

Example:

• c_cpp_properties.json with cppStandard set to c++23 - correct behavior:
• c_cpp_properties.json with cppStandard set to c++26 - correct behavior:
• Makefile with -std=c++23 - correct behavior:
• Makefile with -std=c++26 - wrong:

Debug Log

No current configuration is defined in the workspace state. Assuming 'Default'.
No target defined in the workspace state. Assuming 'Default'.
Dropping various extension output files at /home/adrian/.config/Code/User/workspaceStorage/5628666ef90b2fdc2d3c4aceef47215d/ms-vscode.makefile-tools
Logging level: Normal
Configurations cached at /home/adrian/.config/Code/User/workspaceStorage/5628666ef90b2fdc2d3c4aceef47215d/ms-vscode.makefile-tools/configurationCache.log
No path to the makefile is defined in the settings file.
No folder path to the makefile is defined in the settings file.
Always pre-configure: false
Always post-configure: false
Dry-run switches: '--always-make', '--keep-going', '--print-directory'
No current launch configuration is set in the workspace state.
Default launch configuration: MIMode = undefined,
                miDebuggerPath = undefined,
                stopAtEntry = undefined,
                symbolSearchPath = undefined
Configure on open: true
Configure on edit: true
Configure after command: true
Only .PHONY targets: false
Save before build or configure: true
Build before launch: true
Clear output before build: true
Ignore directory commands: true
compile_commands.json path: null
Deduced command 'make ' for configuration "Default"
Saving opened files before build.
Configuring from cache: /home/adrian/.config/Code/User/workspaceStorage/5628666ef90b2fdc2d3c4aceef47215d/ms-vscode.makefile-tools/configurationCache.log
Load configuration from cache elapsed time: 0.023
Generating in the background a new configuration cache with command: 'make --dry-run --always-make --keep-going --print-directory'
Writing the dry-run output: /home/adrian/.config/Code/User/workspaceStorage/5628666ef90b2fdc2d3c4aceef47215d/ms-vscode.makefile-tools/dryrun.log
Generating dry-run elapsed time: 0.02
Preprocessing: "/home/adrian/.config/Code/User/workspaceStorage/5628666ef90b2fdc2d3c4aceef47215d/ms-vscode.makefile-tools/dryrun.log"
Preprocess elapsed time: 0
Parsing for IntelliSense.
Updating the CppTools IntelliSense Configuration Provider.
Parsing dry-run output for CppTools Custom Configuration Provider.
Unknown C++ standard control flag: c++26
Sending configuration for file /home/adrian/Code/cpp-test/main.cpp -----------------------------------
Parsing for IntelliSense elapsed time: 0.029
Parsing for launch targets.
Found the following 1 launch targets defined in the makefile: /home/adrian/Code/cpp-test>main()
Complete list of launch targets: /home/adrian/Code/cpp-test>main()
Parsing for launch targets elapsed time: 0.016
Generating parse content for build targets.
Generating targets information with command: 'make all --print-data-base --no-builtin-variables --no-builtin-rules --question'
Writing the dry-run output: /home/adrian/.config/Code/User/workspaceStorage/5628666ef90b2fdc2d3c4aceef47215d/ms-vscode.makefile-tools/targets.log
Generating dry-run elapsed time: 0.019
Parsing for build targets from: "/home/adrian/.config/Code/User/workspaceStorage/5628666ef90b2fdc2d3c4aceef47215d/ms-vscode.makefile-tools/targets.log"
Found the following 1 build targets defined in the makefile: main
Complete list of build targets: main
Parsing build targets elapsed time: 0.003
Configure finished. The status for all the subphases that ran:
loadFromCache: return code = 0, elapsed time = 0.023
generateParseContent: return code = 0, elapsed time = 0.02
preprocessParseContent: return code = 0, elapsed time = 0
parseIntelliSense: return code = 0, elapsed time = 0.029
parseLaunch: return code = 0, elapsed time = 0.016
dryrunTargets: return code = 2, elapsed time = 0.019
parseTargets: return code = 0, elapsed time = 0.003
Configure succeeded.
Configure elapsed time: 0.215

Additional Information

No response

[yanghhhhhhh]

Hi @adriankwiat , thanks for reporting issue here! We can reproduce this issue on Visual Studio Code 1.104.1 + Makefile Tools v0.12.17.

Repro Steps:

1. Create a new folder and open it with VS Code.
2. Create a new file named main.cpp, add the following code.

consteval int f() {}

int main() {}

3. Create a Makefile file and add the following code.

main: main.cpp
	g++ -std=c++26 -o main main.cpp

4. Run command "Makefile: Configure", observe the result.

Details please see following video:

Screencast.from.2025-09-23.10-28-34.webm

FYI: @gcampbell-msft