[clang] [clang][AST] Fix assertion crash in LValue::addUnsizedArray on invalid base type (#225046) (PR #234517)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Sriraj2911

<details>
<summary>Changes</summary>

**Description**
Fixes an assertion failure in LValue::addUnsizedArray when constant evaluation encounters array subscripting on an invalid/redeclared object declaration.

**Root Cause**
Evaluating '&a[6]' on 'void a;' passes a Base object with void type into addUnsizedArray. Because void is neither a pointer nor an array type, it fails the assertion:
assert(!Base || getType(Base).getNonReferenceType()->isPointerType() || getType(Base).getNonReferenceType()->isArrayType());

**Fix**
Guarded addUnsizedArray to check for isInvalidDecl(), containsErrors(), and validated that BaseTy is an array or pointer type before updating the designator.

**Testing**
Added regression test clang/test/SemaCXX/225046.cpp and verified via llvm-lit.

---
Full diff: https://github.com/llvm/llvm-project/pull/234517.diff


2 Files Affected:

- (modified) clang/lib/AST/ExprConstant.cpp (+29) 
- (added) clang/test/SemaCXX/225046.cpp (+10) 


``````````diff
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 6d390032e2c71..17d3855f0d907 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -1538,7 +1538,36 @@ namespace {
         Designator.setInvalid();
         return;
       }
+
+      if(Base){
+        // Mark invalid and return early if Base points to an invalid decl
+        if(const auto *VD = Base.dyn_cast<const ValueDecl *>()){
+          if(VD->isInvalidDecl()){
+            Designator.setInvalid();
+            return;
+          }
+        }
+        // Mark invalid and return early if Base points to an invalid type
+        QualType BaseTy = getType(Base).getNonReferenceType();
+        if(BaseTy.isNull() || BaseTy->containsErrors() || (!BaseTy->isPointerType() && !BaseTy->isArrayType())){
+          Designator.setInvalid();
+          return;
+        }
+      }
+
       if (checkSubobject(Info, E, CSK_ArrayToPointer)) {
+        /*
+        When a user writes invalid C++ code (e.g., declaring a 
+        variable as void and then illegally re-declaring/indexing 
+        it as an array), the AST node representing Base ends up
+        with an invalid or unexpected type structure. 
+        => getType(Base) does not fulfill either conditions, causing
+        the assert statement to fail and crash the compiler.
+        */
+       /*
+       To get a clean error, add a check before entering the above
+       if clause
+       */
         assert(!Base || getType(Base).getNonReferenceType()->isPointerType() ||
                getType(Base).getNonReferenceType()->isArrayType());
         Designator.FirstEntryIsAnUnsizedArray = true;
diff --git a/clang/test/SemaCXX/225046.cpp b/clang/test/SemaCXX/225046.cpp
new file mode 100644
index 0000000000000..98c22f8b1a569
--- /dev/null
+++ b/clang/test/SemaCXX/225046.cpp
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+void a; // expected-error {{variable has incomplete type 'void'}}
+extern int a[];
+
+void test() {
+  // Force constant evaluation of &a[6].
+  // Before the fix this asserted in LValue::addUnsizedArray.
+  (void)__builtin_constant_p(&a[6]);
+}
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/234517