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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