gen: fix generic comp time if (#8254)

vlib/v/gen/comptime.v

@@ -213,6 +213,11 @@ fn (mut g Gen) comp_if(node ast.IfExpr) {
 			if should_create_scope {
 				g.writeln('}')
 			}
+			if !comp_if_stmts_skip && branch.cond is ast.InfixExpr {
+				if (branch.cond as ast.InfixExpr).op == .key_is {
+					break
+				}
+			}
 		}
 		g.defer_ifdef = ''
 	}

vlib/v/tests/comptime_if_expr_test.v

@@ -80,3 +80,24 @@ fn test_generic_t_is3() {
 	}
 	assert res == GenericTIsTest{}
 }
+
+fn test_generic_t_is_with_else() {
+	res := generic_t_is_with_else<GenericTIsTest>('') or {
+		assert false
+		GenericTIsTest{}
+	}
+	assert res == GenericTIsTest{}
+	str := generic_t_is_with_else<string>('test') or {
+		assert false
+		''
+	}
+	assert str == 'test'
+}
+
+fn generic_t_is_with_else<T>(raw_data string) ?T {
+	$if T is string {
+		return raw_data
+	} $else {
+		return T{}
+	}
+}