Narrowing of type for array value is not inferred with expression key

[shello]

Bug report

PHPStan seems to not be inferring a narrowing of a type for an array value if that value's type is narrowed by accessing a key that is not a simple variable (arithmetic expression, and backed enum ->value are the ones I checked).

The code snippet attached employs a (somewhat unusual, for sure, but simple) Fibonacci sequence memoization strategy, by preparing a zero-indexed memoization array with null values when instantiated. Both fib1 and fib0 always return int.

• When using fib1 the access to the memoization array is done by decrementing the parameter by one (i.e. an expression): $this->mem[$n - 1]. It is guaranteed to have a return value type of int, however a return.type error is reported, stating that the type of $this->mem[$n - 1] is int|null.
• When using fib0 (the zero-indexed counterpart to fib1), the access to the memoization array is made using the given parameter (a simple variable) as key, $this->mem[$n]. In this form, PHPStan correctly infers the narrowing of the variable, and does not report any error.

Code snippet that reproduces the problem

https://phpstan.org/r/a685005a-94c9-45e2-a9b0-52f924e2d3e9

Expected output

The narrowing of types should hold for expression keys that always evaluate to the same value, such as arithmetic expressions with literals, or a backed enum's ->value (and possibly others).

Specifically, in the case of the code snippet, fib1 should not report a return.type error (much like fib0).

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[shello]

I forgot to share the example I mentioned that used a backed enum's ->value as key: https://phpstan.org/r/dfc0816f-56e6-4289-a132-d98ab262c5f9 - its structure is very similar to the first snippet.

[phpstan-bot]

@shello After the latest push in 2.0.x, PHPStan now reports different result with your code snippet:

@@ @@
 
 51: Method Foo::mem2() should return string but returns string|null.
 
-PHP 7.2 – 8.0 (2 errors)
+PHP 8.0 (2 errors)
+==========
+
+ 3: Syntax error, unexpected T_STRING on line 3
+ 4: Syntax error, unexpected '{' on line 4
+
+PHP 7.2 – 7.4 (2 errors)
 ==========
 
  3: Syntax error, unexpected T_STRING on line 3
  5: Syntax error, unexpected T_CASE on line 5

Full report

PHP 8.1 – 8.2 (1 error)

Line	Error
51	`Method Foo::mem2() should return string but returns string

PHP 8.0 (2 errors)

Line	Error
3	Syntax error, unexpected T_STRING on line 3
4	Syntax error, unexpected '{' on line 4

PHP 7.2 – 7.4 (2 errors)

Line	Error
3	Syntax error, unexpected T_STRING on line 3
5	Syntax error, unexpected T_CASE on line 5