feat(zend): reject required type parameter after an optional one

Signed-off-by: azjezz <azjezz@protonmail.com>

Author: azjezz

Zend/tests/generics/errors/forward_reference_in_default.phpt

@@ -2,7 +2,7 @@
 Errors: forward reference in default of earlier parameter
 --FILE--
 <?php
-class C<U = T, T> {}
+class C<U = T, T = int> {}
 ?>
 --EXPECTF--
 Fatal error: Type parameter T referenced before declaration in %s on line %d

Zend/tests/generics/errors/optional_only_at_tail_ok.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Errors: optional type parameters at the tail are accepted
+--FILE--
+<?php
+class A<T> {}
+class B<T = int> {}
+class C<T, U = int> {}
+class D<T, U = int, V = string> {}
+class E<T = int, U = string> {}
+function f<T, U = int>(): void {}
+function g<T = int, U = string>(): void {}
+echo "ok\n";
+?>
+--EXPECT--
+ok

Zend/tests/generics/errors/required_after_optional.phpt

@@ -0,0 +1,8 @@
+--TEST--
+Errors: required type parameter cannot follow an optional one
+--FILE--
+<?php
+class C<T = int, U> {}
+?>
+--EXPECTF--
+Fatal error: Optional type parameter T cannot be declared before required type parameter U in %s on line %d

Zend/tests/generics/errors/required_after_optional_function.phpt

@@ -0,0 +1,8 @@
+--TEST--
+Errors: required type parameter cannot follow an optional one
+--FILE--
+<?php
+function f<T = int, U>(): void {}
+?>
+--EXPECTF--
+Fatal error: Optional type parameter T cannot be declared before required type parameter U in %s on line %d

Zend/tests/generics/errors/required_after_optional_three_params.phpt

@@ -0,0 +1,8 @@
+--TEST--
+Errors: required type parameter following two optional ones names the first optional
+--FILE--
+<?php
+class C<T = int, U = string, V> {}
+?>
+--EXPECTF--
+Fatal error: Optional type parameter T cannot be declared before required type parameter V in %s on line %d

Zend/zend_compile.c

@@ -512,10 +512,13 @@ static zend_generic_parameter_list *zend_compile_generic_type_parameter_list(zen
 	zend_generic_parameter_list *params =
 		zend_generic_parameter_list_alloc(list->children, /* persistent */ false);
 
+	zend_string *prev_optional_name = NULL;
+
 	for (uint32_t i = 0; i < list->children; i++) {
 		zend_ast *param_ast = list->child[i];
 		ZEND_ASSERT(param_ast->kind == ZEND_AST_GENERIC_TYPE_PARAMETER);
 		zend_string *name = zval_make_interned_string(zend_ast_get_zval(param_ast->child[0]));
+		bool has_default = param_ast->child[2] != NULL;
 
 		for (uint32_t j = 0; j < i; j++) {
 			if (zend_string_equals(params->parameters[j].name, name)) {
@@ -533,6 +536,19 @@ static zend_generic_parameter_list *zend_compile_generic_type_parameter_list(zen
 				"Type parameter %s shadows enclosing type parameter", ZSTR_VAL(dup));
 		}
 
+		if (!has_default && prev_optional_name != NULL) {
+			zend_string *cur = zend_string_copy(name);
+			zend_string *prev = zend_string_copy(prev_optional_name);
+			zend_generic_parameter_list_destroy(params);
+			zend_error_noreturn(E_COMPILE_ERROR,
+				"Optional type parameter %s cannot be declared before required type parameter %s",
+				ZSTR_VAL(prev), ZSTR_VAL(cur));
+		}
+
+		if (has_default && prev_optional_name == NULL) {
+			prev_optional_name = name;
+		}
+
 		params->parameters[i].name = zend_string_copy(name);
 		params->parameters[i].variance = (uint8_t) param_ast->attr;
 	}