fix incorrect union const value generation

closes #1381

The union was generated as a 3 byte struct when it needed to be
4 bytes so that the packed struct bitcast could work correctly.

Now it recognizes this situation and adds padding bytes to become
the correct size so that it can fit into an array.

src/codegen.cpp

@@ -3218,7 +3218,8 @@ static LLVMValueRef ir_render_decl_var(CodeGen *g, IrExecutable *executable,
         assert(var->value->type == init_value->value.type);
         ZigType *var_ptr_type = get_pointer_to_type_extra(g, var->value->type, false, false,
                 PtrLenSingle, var->align_bytes, 0, 0);
-        gen_assign_raw(g, var->value_ref, var_ptr_type, ir_llvm_value(g, init_value));
+        LLVMValueRef llvm_init_val = ir_llvm_value(g, init_value);
+        gen_assign_raw(g, var->value_ref, var_ptr_type, llvm_init_val);
     } else {
         bool want_safe = ir_want_runtime_safety(g, &decl_var_instruction->base);
         if (want_safe) {
@@ -5863,12 +5864,20 @@ static LLVMValueRef gen_const_val(CodeGen *g, ConstExprValue *const_val, const c
                 LLVMValueRef tag_value = bigint_to_llvm_const(type_entry->data.unionation.tag_type->type_ref,
                         &const_val->data.x_union.tag);
 
-                LLVMValueRef fields[2];
+                LLVMValueRef fields[3];
                 fields[type_entry->data.unionation.gen_union_index] = union_value_ref;
                 fields[type_entry->data.unionation.gen_tag_index] = tag_value;
 
                 if (make_unnamed_struct) {
-                    return LLVMConstStruct(fields, 2, false);
+                    LLVMValueRef result = LLVMConstStruct(fields, 2, false);
+                    size_t expected_sz = LLVMStoreSizeOfType(g->target_data_ref, type_entry->type_ref);
+                    size_t actual_sz = LLVMStoreSizeOfType(g->target_data_ref, LLVMTypeOf(result));
+                    if (actual_sz < expected_sz) {
+                        unsigned pad_sz = expected_sz - actual_sz;
+                        fields[2] = LLVMGetUndef(LLVMArrayType(LLVMInt8Type(), pad_sz));
+                        result = LLVMConstStruct(fields, 3, false);
+                    }
+                    return result;
                 } else {
                     return LLVMConstNamedStruct(type_entry->type_ref, fields, 2);
                 }

test/behavior.zig

@@ -10,6 +10,7 @@ comptime {
     _ = @import("cases/bool.zig");
     _ = @import("cases/bugs/1111.zig");
     _ = @import("cases/bugs/1277.zig");
+    _ = @import("cases/bugs/1381.zig");
     _ = @import("cases/bugs/1421.zig");
     _ = @import("cases/bugs/394.zig");
     _ = @import("cases/bugs/655.zig");

test/cases/bugs/1381.zig

@@ -0,0 +1,21 @@
+const std = @import("std");
+
+const B = union(enum) {
+    D: u8,
+    E: u16,
+};
+
+const A = union(enum) {
+    B: B,
+    C: u8,
+};
+
+test "union that needs padding bytes inside an array" {
+    var as = []A{
+        A{ .B = B{ .D = 1 } },
+        A{ .B = B{ .D = 1 } },
+    };
+
+    const a = as[0].B;
+    std.debug.assertOrPanic(a.D == 1);
+}