cgen: fix treating errors as values inside if expressions (#9440)

vlib/v/gen/c/cgen.v

@@ -917,13 +917,11 @@ fn (mut g Gen) stmts_with_tmp_var(stmts []ast.Stmt, tmp_var string) {
 				g.stmt_path_pos << g.out.len
 				g.skip_stmt_pos = true
 				if stmt is ast.ExprStmt {
-					sym := g.table.get_type_symbol(stmt.typ)
-					if sym.name in ['Option2', 'Option'] || stmt.expr is ast.None {
-						tmp := g.new_tmp_var()
-						g.write('Option $tmp = (Option){.state = 0,.err = ')
+					if stmt.typ == table.error_type_idx || stmt.expr is ast.None {
+						g.writeln('${tmp_var}.state = 2;')
+						g.write('${tmp_var}.err = ')
 						g.expr(stmt.expr)
-						g.writeln('};')
-						g.writeln('memcpy(&$tmp_var, &$tmp, sizeof(Option));')
+						g.writeln(';')
 					} else {
 						mut styp := g.base_type(stmt.typ)
 						$if tinyc && x32 && windows {

vlib/v/tests/if_expr_of_optional_test.v

@@ -7,6 +7,10 @@ fn foo2() ?int {
 }
 
 fn foo3() ?int {
+    return if false { 1 } else { error('foo3 error') }
+}
+
+fn foo4() ?int {
 	return if true { 2 } else { 0 }
 }
 
@@ -19,9 +23,15 @@ fn test_if_expr_of_optional() {
 	println(a2)
 	assert a2 == 1
 
-	a3 := foo3() or { panic('error') }
-	println(a3)
-	assert a3 == 2
+	if _ := foo3() {
+		assert false
+	} else {
+		assert err.msg == 'foo3 error'
+	}
+
+	a4 := foo4() or { panic('error') }
+	println(a4)
+	assert a4 == 2
 }
 
 fn foo_complex() ?int {