fix: use PHP 8 concatenation precedence in the pretty printer

The precedence table gave "." the same precedence as "+" and "-", which is
the PHP 7 ordering. PHP 8.0 lowered "." below the arithmetic and shift
operators, so printing ('abc' . 1) + 2 produced 'abc' . 1 + 2, which PHP 8
reparses as 'abc' . (1 + 2). The same applied to "-", "<<" and ">>".

The printer already targets a specific PHP version and already varies
parenthesization by version, so select the concatenation precedence the same
way: for a target of PHP 8.0 or newer, place "." between the shift operators
and the pipe operator, matching the PHP8 block of grammar/php.y. Targets below
8.0 keep the PHP 7 table, which is correct for them and produces unchanged
output.

Fixes #1041

doc/component/Pretty_printing.markdown

@@ -67,6 +67,10 @@ The behaviors controlled by `phpVersion` (defaults to PHP 7.4) are:
   for this has been removed with the introduction of flexible heredoc/nowdoc strings.
 * For PHP >= 7.3, heredoc/nowdoc strings are indented just like regular code. This was allowed with
   the introduction of flexible heredoc/nowdoc strings.
+* For PHP >= 8.0, the concatenation operator is treated as binding less tightly than the arithmetic
+  and shift operators, matching the precedence change in PHP 8.0. For older versions `.` keeps the
+  same precedence as `+` and `-`. This determines where parentheses are required, so the printed
+  code only reparses into the same AST under the targeted version.
 
 The default pretty printer does not provide functionality for fine-grained customization of code
 formatting.

lib/PhpParser/PhpVersion.php

@@ -172,4 +172,12 @@ class PhpVersion {
     public function supportsNewDereferenceWithoutParentheses(): bool {
         return $this->id >= 80400;
     }
+
+    /**
+     * Whether the concatenation operator has lower precedence than the arithmetic and shift
+     * operators. Prior to PHP 8.0, "." had the same precedence as "+" and "-".
+     */
+    public function hasLowerConcatPrecedence(): bool {
+        return $this->id >= 80000;
+    }
 }

lib/PhpParser/PrettyPrinterAbstract.php

@@ -33,6 +33,13 @@ abstract class PrettyPrinterAbstract implements PrettyPrinter {
 
     protected const MAX_PRECEDENCE = 1000;
 
+    /**
+     * @var array{int, int, int} Precedence of the concatenation operator on PHP >= 8.0, where it
+     *                           binds less tightly than the arithmetic and shift operators, but
+     *                           more tightly than the pipe operator.
+     */
+    protected const PHP8_CONCAT_PRECEDENCE = [62, 63, 62];
+
     /** @var array<class-string, array{int, int, int}> */
     protected array $precedenceMap = [
         // [precedence, precedenceLHS, precedenceRHS]
@@ -59,7 +66,8 @@ abstract class PrettyPrinterAbstract implements PrettyPrinter {
         BinaryOp\Mod::class            => [ 40,  41,  40],
         BinaryOp\Plus::class           => [ 50,  51,  50],
         BinaryOp\Minus::class          => [ 50,  51,  50],
-        // FIXME: This precedence is incorrect for PHP 8.
+        // This is the PHP 7 precedence. For PHP >= 8.0 the constructor replaces it with
+        // PHP8_CONCAT_PRECEDENCE.
         BinaryOp\Concat::class         => [ 50,  51,  50],
         BinaryOp\ShiftLeft::class      => [ 60,  61,  60],
         BinaryOp\ShiftRight::class     => [ 60,  61,  60],
@@ -179,9 +187,13 @@ abstract class PrettyPrinterAbstract implements PrettyPrinter {
      *  * PhpVersion $phpVersion: The PHP version to target (default to PHP 7.4). This option
      *                            controls compatibility of the generated code with older PHP
      *                            versions in cases where a simple stylistic choice exists (e.g.
-     *                            array() vs []). It is safe to pretty-print an AST for a newer
-     *                            PHP version while specifying an older target (but the result will
-     *                            of course not be compatible with the older version in that case).
+     *                            array() vs []). It also selects the operator precedence table,
+     *                            which changed for the concatenation operator in PHP 8.0, so the
+     *                            printed code is only guaranteed to parse back into the same AST
+     *                            under the targeted version. It is safe to pretty-print an AST for
+     *                            a newer PHP version while specifying an older target (but the
+     *                            result will of course not be compatible with the older version in
+     *                            that case).
      *  * string $newline:        The newline style to use. Should be "\n" (default) or "\r\n".
      *  * string $indent:         The indentation to use. Should either be all spaces or a single
      *                            tab. Defaults to four spaces ("    ").
@@ -195,6 +207,9 @@ abstract class PrettyPrinterAbstract implements PrettyPrinter {
      */
     public function __construct(array $options = []) {
         $this->phpVersion = $options['phpVersion'] ?? PhpVersion::fromComponents(7, 4);
+        if ($this->phpVersion->hasLowerConcatPrecedence()) {
+            $this->precedenceMap[BinaryOp\Concat::class] = self::PHP8_CONCAT_PRECEDENCE;
+        }
 
         $this->newline = $options['newline'] ?? "\n";
         if ($this->newline !== "\n" && $this->newline != "\r\n") {

test/code/prettyPrinter/expr/concatPrecedence.test

@@ -0,0 +1,55 @@
+Precedence of the concatenation operator
+-----
+<?php
+
+echo ('abc' . 1) + 2;
+echo ('abc' . 1) - 2;
+echo 2 + ('abc' . 1);
+echo 'abc' . (1 + 2);
+echo (1 + 2) . 'abc';
+echo ('a' . 'b') << 1;
+echo 'a' . (1 << 2);
+echo 'a' . 'b' . 'c';
+echo 'a' . ('b' . 'c');
+echo ('a' . 'b') == 'c';
+echo 'a' . ('b' == 'c');
+-----
+!!version=8.0
+echo ('abc' . 1) + 2;
+echo ('abc' . 1) - 2;
+echo 2 + ('abc' . 1);
+echo 'abc' . 1 + 2;
+echo 1 + 2 . 'abc';
+echo ('a' . 'b') << 1;
+echo 'a' . 1 << 2;
+echo 'a' . 'b' . 'c';
+echo 'a' . ('b' . 'c');
+echo 'a' . 'b' == 'c';
+echo 'a' . ('b' == 'c');
+-----
+<?php
+
+echo ('abc' . 1) + 2;
+echo ('abc' . 1) - 2;
+echo 2 + ('abc' . 1);
+echo 'abc' . (1 + 2);
+echo (1 + 2) . 'abc';
+echo ('a' . 'b') << 1;
+echo 'a' . (1 << 2);
+echo 'a' . 'b' . 'c';
+echo 'a' . ('b' . 'c');
+echo ('a' . 'b') == 'c';
+echo 'a' . ('b' == 'c');
+-----
+!!version=7.4
+echo 'abc' . 1 + 2;
+echo 'abc' . 1 - 2;
+echo 2 + ('abc' . 1);
+echo 'abc' . (1 + 2);
+echo 1 + 2 . 'abc';
+echo 'a' . 'b' << 1;
+echo 'a' . (1 << 2);
+echo 'a' . 'b' . 'c';
+echo 'a' . ('b' . 'c');
+echo 'a' . 'b' == 'c';
+echo 'a' . ('b' == 'c');