Fixes#17765

The offender for this crash appears to be this snippet:

This branch triggers when applying type args to a type that is generic with respect to a singleParamSpec. It allows double brackets to be omitted when providing a parameter specification by wrapping all of the provided type arguments into a single parameter specification argument (i.e. equatingFoo[int, int] toFoo[[int, int]]). This wrapping occursunless:

• there is only a single type argument, and it resolves toAny (e.g.Foo[Any])
• there is only a single type argument, and it's a bracketed parameter specification or aParamSpec (e.g.Foo[[int, int]])

The problem occurs when multiple type arguments provided and at least one of them is a bracketed parameter specification, as inFoo[[int, int], str].

Per the rules above, since there is more than 1 type argument, mypy attempts to wrap the arguments into a single parameter specification. This results in the attempted creation of aParameters instance that contains anotherParameters instance, which triggers this assert insideParameters.__init__:

I think a reasonable solution is to forgo wrapping the type arguments into a singleParameters ifany of the provided type arguments are aParameters/ParamSpecType. That is, don't transformFoo[A1, A2, ...] toFoo[[A1, A2, ...]] if any ofA1, A2, ... are a parameter specification.

This change brings the crash case inline with mypy's current behavior for a similar case:

# Current behaviorP=ParamSpec("P")classC(Generic[P]): ...c:C[int, [int,str],str]# E: Nested parameter specifications are not allowed

Before this change:

P=ParamSpec("P")classC(Generic[P]): ...classD(C[int, [int,str],str]): ...# !!! CRASH !!!

After this change:

P=ParamSpec("P")classC(Generic[P]): ...classD(C[int, [int,str],str]): ...# E: Nested parameter specifications are not allowed