[clang] [clang][HLSL] Fix assertion crash on forward-declared record in hasConstantBufferLayout (#198349) (PR #209692)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Macro Terra (hongtaihu)

<details>
<summary>Changes</summary>

hasConstantBufferLayout iterates fields and accesses base class info of CXXRecordDecl
without checking hasDefinition(), causing an assertion:

  Assertion `DD && "queried property of class with no definition"' failed.

This is triggered when clangd opens HLSL files whose compile_commands.json has
`-x hlsl` without a DXIL/SPIR-V target triple. In this state, HLSL builtin
types like SamplerState exist as forward-declared CXXRecordDecls without
HLSLResourceAttr, bypassing the isHLSLResourceRecord() early return.

Fix: add hasDefinition() guard before accessing record fields and bases.

Fixes #<!-- -->198349.
Assisted by: claude code.

---
Full diff: https://github.com/llvm/llvm-project/pull/209692.diff


1 Files Affected:

- (modified) clang/lib/Sema/SemaHLSL.cpp (+2) 


``````````diff
diff --git a/clang/lib/Sema/SemaHLSL.cpp b/clang/lib/Sema/SemaHLSL.cpp
index ae6cbce2f890c..3bd1f06355205 100644
--- a/clang/lib/Sema/SemaHLSL.cpp
+++ b/clang/lib/Sema/SemaHLSL.cpp
@@ -5187,6 +5187,8 @@ static bool hasConstantBufferLayout(QualType QT) {
     return false;
 
   if (const auto *RD = Ty->getAsCXXRecordDecl()) {
+    if (!RD->hasDefinition())
+      return false;
     for (const auto *FD : RD->fields()) {
       if (hasConstantBufferLayout(FD->getType()))
         return true;

``````````

</details>


https://github.com/llvm/llvm-project/pull/209692