fix codegen for union init with runtime value

see #144

1 files changed, 22 insertions, 5 deletions

diff --git a/src/codegen.cpp b/src/codegen.cpp
index eb56d26cae..680f5a9e35 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -3414,17 +3414,34 @@ static LLVMValueRef ir_render_struct_init(CodeGen *g, IrExecutable *executable,
 static LLVMValueRef ir_render_union_init(CodeGen *g, IrExecutable *executable, IrInstructionUnionInit *instruction) {
     TypeUnionField *type_union_field = instruction->field;
 
-    assert(type_has_bits(type_union_field->type_entry));
-
-    LLVMValueRef field_ptr = LLVMBuildStructGEP(g->builder, instruction->tmp_ptr, (unsigned)0, "");
-    LLVMValueRef value = ir_llvm_value(g, instruction->init_value);
+    if (!type_has_bits(type_union_field->type_entry))
+        return nullptr;
 
     uint32_t field_align_bytes = get_abi_alignment(g, type_union_field->type_entry);
-
     TypeTableEntry *ptr_type = get_pointer_to_type_extra(g, type_union_field->type_entry,
             false, false, field_align_bytes,
             0, 0);
 
+    LLVMValueRef uncasted_union_ptr;
+    // Even if safety is off in this block, if the union type has the safety field, we have to populate it
+    // correctly. Otherwise safety code somewhere other than here could fail.
+    TypeTableEntry *union_type = instruction->union_type;
+    if (union_type->data.unionation.gen_tag_index != SIZE_MAX) {
+        LLVMValueRef tag_field_ptr = LLVMBuildStructGEP(g->builder, instruction->tmp_ptr,
+                union_type->data.unionation.gen_tag_index, "");
+        LLVMValueRef tag_value = LLVMConstInt(union_type->data.unionation.tag_type->type_ref,
+                type_union_field->value, false);
+        gen_store_untyped(g, tag_value, tag_field_ptr, 0, false);
+
+        uncasted_union_ptr = LLVMBuildStructGEP(g->builder, instruction->tmp_ptr,
+                (unsigned)union_type->data.unionation.gen_union_index, "");
+    } else {
+        uncasted_union_ptr = LLVMBuildStructGEP(g->builder, instruction->tmp_ptr, (unsigned)0, "");
+    }
+
+    LLVMValueRef field_ptr = LLVMBuildBitCast(g->builder, uncasted_union_ptr, ptr_type->type_ref, "");
+    LLVMValueRef value = ir_llvm_value(g, instruction->init_value);
+
     gen_assign_raw(g, field_ptr, ptr_type, value);
 
     return instruction->tmp_ptr;