stage1: fix crash on accessing an array of size zero with runtime index

src/codegen.cpp

@@ -3869,6 +3869,9 @@ static LLVMValueRef ir_render_elem_ptr(CodeGen *g, IrExecutableGen *executable,
             assert(array_type->data.pointer.child_type->id == ZigTypeIdArray);
             array_type = array_type->data.pointer.child_type;
         }
+        
+        assert(array_type->data.array.len != 0 || array_type->data.array.sentinel != nullptr);
+
         if (safety_check_on) {
             uint64_t extra_len_from_sentinel = (array_type->data.array.sentinel != nullptr) ? 1 : 0;
             uint64_t full_len = array_type->data.array.len + extra_len_from_sentinel;

src/ir.cpp

@@ -21199,6 +21199,11 @@ static IrInstGen *ir_analyze_instruction_elem_ptr(IrAnalyze *ira, IrInstSrcElemP
     }
 
     if (array_type->id == ZigTypeIdArray) {
+        if(array_type->data.array.len == 0 && array_type->data.array.sentinel == nullptr){
+            ir_add_error(ira, &elem_ptr_instruction->base.base, buf_sprintf("accessing a zero length array is not allowed"));
+            return ira->codegen->invalid_inst_gen;
+        }
+
         ZigType *child_type = array_type->data.array.child_type;
         if (ptr_type->data.pointer.host_int_bytes == 0) {
             return_type = get_pointer_to_type_extra(ira->codegen, child_type,

test/compile_errors.zig

@@ -4552,7 +4552,17 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\    const pointer = &array[0];
         \\}
     , &[_][]const u8{
-        "tmp.zig:3:27: error: index 0 outside array of size 0",
+        "tmp.zig:3:27: error: accessing a zero length array is not allowed",
+    });
+
+    cases.add("indexing an array of size zero with runtime index",
+        \\const array = [_]u8{};
+        \\export fn foo() void {
+        \\    var index: usize = 0;
+        \\    const pointer = &array[index];
+        \\}
+    , &[_][]const u8{
+        "tmp.zig:4:27: error: accessing a zero length array is not allowed",
     });
 
     cases.add("compile time division by zero",