cgen: fix assignment from comptime var (#16999)

vlib/v/gen/c/assign.v

@@ -99,6 +99,7 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 		val := node.right[i]
 		mut is_call := false
 		mut blank_assign := false
+		mut is_comptime_var := false
 		mut ident := ast.Ident{
 			scope: 0
 		}
@@ -120,16 +121,24 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 				}
 			}
 			if mut left.obj is ast.Var {
-				if val is ast.ComptimeSelector {
+				if val is ast.Ident
+					&& (val as ast.Ident).info is ast.IdentVar && (val as ast.Ident).kind == .variable && (val as ast.Ident).obj is ast.Var && ((val as ast.Ident).obj as ast.Var).is_comptime_field {
+					var_type = g.unwrap_generic(g.comptime_for_field_type)
+					val_type = var_type
+					left.obj.typ = var_type
+					is_comptime_var = true
+				} else if val is ast.ComptimeSelector {
 					key_str := g.get_comptime_selector_key_type(val)
 					if key_str != '' {
 						var_type = g.comptime_var_type_map[key_str] or { var_type }
 						left.obj.typ = var_type
+						is_comptime_var = true
 					}
 				} else if val is ast.ComptimeCall {
 					key_str := '${val.method_name}.return_type'
 					var_type = g.comptime_var_type_map[key_str] or { var_type }
 					left.obj.typ = var_type
+					is_comptime_var = true
 				}
 				is_auto_heap = left.obj.is_auto_heap
 			}
@@ -478,6 +487,9 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 							g.array_init(val, c_name(ident.name))
 						} else if val_type.has_flag(.shared_f) {
 							g.expr_with_cast(val, val_type, var_type)
+						} else if is_comptime_var && g.right_is_opt {
+							tmp_var := g.new_tmp_var()
+							g.expr_with_tmp_var(val, val_type, var_type, tmp_var)
 						} else {
 							g.expr(val)
 						}

vlib/v/tests/comptime_var_assignment_test.v

@@ -0,0 +1,31 @@
+struct FixedStruct1 {
+	a int
+	b string
+	c ?int
+	d ?string
+}
+
+fn encode_struct[T](val T) []string {
+	mut out := []string{}
+	$for field in T.fields {
+		value := val.$(field.name)
+		$if field.is_option {
+			gg := value
+			println(gg)
+			out << gg.str()
+		} $else {
+			gg := value
+			println(gg)
+			out << gg.str()
+		}
+	}
+	return out
+}
+
+fn test_main() {
+	out := encode_struct(FixedStruct1{1, 'foo', 4, 'bar'})
+	assert out[0] == '1'
+	assert out[1] == 'foo'
+	assert out[2] == 'Option(4)'
+	assert out[3] == "Option('bar')"
+}