[clang] [clang] Fix assertion failure when initializing union with FAM (PR #77298)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

When initializing a union that constrain a struct with a flexible array member, and the initializer list is empty, we currently trigger an assertion failure. This happens because getFlexibleArrayInitChars() assumes that the initializer list is non-empty.

The relevant code is only executed in assertion-enabled builds: https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/CodeGenModule.cpp#L5318-L5324

Fixes https://github.com/llvm/llvm-project/issues/77085.

---
Full diff: https://github.com/llvm/llvm-project/pull/77298.diff


2 Files Affected:

- (modified) clang/lib/AST/Decl.cpp (+1-1) 
- (modified) clang/test/CodeGen/flexible-array-init.c (+8) 


``````````diff
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 12e0a6faa4c33d..e1440e5183a4e6 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -2835,7 +2835,7 @@ CharUnits VarDecl::getFlexibleArrayInitChars(const ASTContext &Ctx) const {
   if (!Ty || !Ty->getDecl()->hasFlexibleArrayMember())
     return CharUnits::Zero();
   auto *List = dyn_cast<InitListExpr>(getInit()->IgnoreParens());
-  if (!List)
+  if (!List || List->getNumInits() == 0)
     return CharUnits::Zero();
   const Expr *FlexibleInit = List->getInit(List->getNumInits() - 1);
   auto InitTy = Ctx.getAsConstantArrayType(FlexibleInit->getType());
diff --git a/clang/test/CodeGen/flexible-array-init.c b/clang/test/CodeGen/flexible-array-init.c
index b2cf959f7e1220..bae926da5feb07 100644
--- a/clang/test/CodeGen/flexible-array-init.c
+++ b/clang/test/CodeGen/flexible-array-init.c
@@ -20,3 +20,11 @@ struct __attribute((packed, aligned(4))) { char a; int x; char z[]; } e = { 1, 2
 
 struct { int x; char y[]; } f = { 1, { 13, 15 } };
 // CHECK: @f ={{.*}} global <{ i32, [2 x i8] }> <{ i32 1, [2 x i8] c"\0D\0F" }>
+
+union {
+  struct {
+    int a;
+    char b[];
+  } x;
+} in_union = {};
+// CHECK: @in_union ={{.*}} global %union.anon zeroinitializer

``````````

</details>


https://github.com/llvm/llvm-project/pull/77298