[clang] [Clang] Diagnose invalid object size kinds during constant evaluation (PR #230979)

[via cfe-commits]

keepyixiao wrote:

Alternative approaches
Option 1: Change BuiltinConstantArgRange
Change the early return to:

```c++
if (isConstantEvaluatedContext() && !RangeIsError)
  return false;
```
This allows range errors to be diagnosed directly by Sema in constant-evaluated contexts.
Advantages:
- Produces the same direct Sema diagnostic in all evaluation contexts.
- Diagnoses invalid arguments even when constant evaluation does not visit the call, for example due to short-circuiting.
Disadvantages:
- Changes the behavior of a shared helper used by many unrelated builtins.
- Can alter diagnostic ordering before builtin-specific conversions and checks have completed.
- In particular, it caused __builtin_isfpclass to emit premature “must be a constant integer” errors, masking the expected incompatible-type diagnostics.
- Therefore, it has a larger and less predictable impact.
Option 2: Diagnose the failure in the constant evaluators
Keep the original Sema behavior and replace the assertions in both the legacy and ByteCode constant evaluators with a constant-evaluation failure and a range diagnostic.
Advantages:
- Directly addresses the assertion failure at the point where it occurs.
- Limits the behavior change to __builtin_object_size and __builtin_dynamic_object_size.
- Does not affect other users of BuiltinConstantArgRange.
- Preserves existing builtin-specific conversions and diagnostic ordering.
Disadvantages:
- Both constant evaluators need equivalent checks.
- The diagnostic consists of the general constant-expression error followed by a specific note.
- An invalid call that is not actually evaluated, such as one skipped by short-circuiting, is not diagnosed through this path.
Rationale
Option 2 was chosen because changing BuiltinConstantArgRange caused observable regressions in unrelated builtins, particularly __builtin_isfpclass. The evaluator-local approach fixes the crash with a much narrower behavioral impact while preserving the existing Sema diagnostic behavior for other builtins.

https://github.com/llvm/llvm-project/pull/230979