Mixed is assignable to any generic type

[klimick]

Bug report

In the process of studying PHPStan, I discovered that mixed can be passed to any generic type.

Code snippet that reproduces the problem

https://phpstan.org/r/45528d36-48b9-47f0-a6b4-7162139201df

Expected output

Parameter #1 of callable callable(A): A expects A, mixed given.

Did PHPStan help you today? Did it make you happy in any way?

No response

[klimick]

Just wondering.
Why is this a feature-request request and not a bug?

[ondrejmirtes]

Sometimes the line between those two is very thin. PHPStan currently does not report an error, but you want it to report an error. You want PHPStan to do more than it currently does, hence it's a feature request.

[Akhristenko]

If we replace mixed with explicit union we get the error
https://phpstan.org/r/4ca0c22d-465c-4cdc-98e7-d5261f6c8c2f

[klimick]

I found a more realistic false negative example:
https://phpstan.org/r/18cc0159-cd71-44ed-908a-fd0801510741
This makes me a little scary.

[ondrejmirtes]

@klimick Looks like a separate issue in your last comment. Please open a separate feature request.

[phpstan-bot]

@klimick After the latest push in 2.3.x, PHPStan now reports different result with your code snippet:

@@ @@
-PHP 8.0 – 8.5
+PHP 8.0 – 8.5 (1 error)
 ==========
 
-No errors
+33: Parameter #1 $i (Inv<object{a: 'able to accept…'}>) of method B::a() should be contravariant with parameter $i (Inv<mixed>) of method A<Y>::a()
 
-PHP 7.2 – 7.4 (1 error)
+PHP 7.2 – 7.4 (2 errors)
 ==========
 
-17: Constructor of class A has a return type.
+17: Constructor of class A has a return type.
+33: Parameter #1 $i (Inv<object{a: 'able to accept…'}>) of method B::a() should be contravariant with parameter $i (Inv<mixed>) of method A<Y>::a()

Full report

PHP 8.0 – 8.5 (1 error)

Line	Error
33	Parameter #1 $i (Inv<object{a: 'able to accept…'}>) of method B::a() should be contravariant with parameter $i (Inv<mixed>) of method A<Y>::a()

PHP 7.2 – 7.4 (2 errors)

Line	Error
17	Constructor of class A has a return type.
33	Parameter #1 $i (Inv<object{a: 'able to accept…'}>) of method B::a() should be contravariant with parameter $i (Inv<mixed>) of method A<Y>::a()