Add example in C2154 error reference

[Rageking8]

• Add new example that generates C2154.
• In the error index page, change *trait* to __underlying_type as it seems to be the only trait that can cause C2154 (also to ensure both instances of the C2154 error message are identical).
• Remove link to Compiler Support for Type Traits as there is no mention of __underlying_type. In fact, across the entire repository, the only mention of __underlying_type is in this error topic.
• Add links to enumeration and underlying_type.

Example

// C2154.cpp
// compile with: /c

struct S {};
enum E {};
enum class EC {};

__underlying_type(S) s;     // C2154
__underlying_type(int) i;   // C2154
__underlying_type(E) e;     // OK
__underlying_type(EC) ec;   // OK

Visual Studio 2022

C:\Test>cl /c C2154.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.44.35217 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

C2154.cpp
C2154.cpp(8): error C2154: 'S': only enumeration type is allowed as an argument to compiler intrinsic type trait '__underlying_type'
C2154.cpp(9): error C2154: 'int': only enumeration type is allowed as an argument to compiler intrinsic type trait '__underlying_type'

Visual Studio 2026 Insiders

C:\Test>cl /c C2154.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35503 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

C2154.cpp
C2154.cpp(8): error C2154: 'S': only enumeration type is allowed as an argument to compiler intrinsic type trait '__underlying_type'
C2154.cpp(9): error C2154: 'int': only enumeration type is allowed as an argument to compiler intrinsic type trait '__underlying_type'

[prmerger-automator]

@Rageking8 : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 72be0ba:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/error-messages/compiler-errors-1/compiler-error-c2154.md	✅Succeeded
docs/error-messages/compiler-errors-1/compiler-errors-c2100-through-c2199.md	✅Succeeded

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[prmerger-automator]

PRMerger Results

Issue	Description
File Change Percent	This PR contains file(s) with more than 30% file change.

[v-regandowner]

@TylerMSFT - Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team