Fix ASan for zero-sized variables passed as function argument.

Fixes ldc-developers#4816

gen/llvmhelpers.cpp

@@ -895,10 +895,12 @@ void DtoVarDeclaration(VarDeclaration *vd) {
 
     Type *type = isSpecialRefVar(vd) ? pointerTo(vd->type) : vd->type;
 
+    // We also allocate a variable for zero-sized variables, because they are technically not `null` when loaded.
+    // The x86_64 ABI "loads" zero-sized function arguments, and without an allocation ASan will report an error (Github #4816).
     llvm::Value *allocainst;
     bool isRealAlloca = false;
     LLType *lltype = DtoType(type); // void for noreturn
-    if (lltype->isVoidTy() || gDataLayout->getTypeSizeInBits(lltype) == 0) {
+    if (lltype->isVoidTy()) {
       allocainst = getNullPtr();
     } else if (type != vd->type) {
       allocainst = DtoAlloca(type, vd->toChars());

tests/sanitizers/asan_gh4816.d

@@ -0,0 +1,20 @@
+// Test extern(C) structs (zero sized structs) and zero-sized arrays. Github #4816
+
+// REQUIRES: ASan
+
+// RUN: %ldc -g -fsanitize=address %s -of=%t%exe
+// RUN: %t%exe
+
+auto foo(void[0] bar) { }
+
+extern(C) struct S {}
+auto foo(S s) { }
+
+void main()
+{
+    void[0] bar;
+    foo(bar);
+
+    S s;
+    foo(s);
+}