Better error reporting is required for inlined non-immutable paths

[soronpo]

As discovered in #16804 and raised again in #17181 and #21536, it is clear that better error reporting is required to prevent further ticket opening, by suggesting to remove the inline from the dependent argument in the compiler error message.

Compiler version

v3.5.0

Minimized code

trait Bar[T]
given [T]: Bar[T] with {}
inline def foo[V](inline value: V)(using Bar[value.type]) : Unit = {} //error

Output

(value : V) is not a valid singleton type, since it is not an immutable path
inline def foo[V](inline value: V)(using Bar[value.type]) : Unit = {}

Expectation

The compiler should provide a hint to remove the inline keyword from the dependent value to resolve the error.

[soronpo]

cc @Gedochao

[mbovel]

Note for future readers: Nicolas explained why inline values cannot be stable paths in #16804 (comment).

What would you expect as a better error message? Something like the following?

(value : V) is not a valid singleton type, since it is not an immutable path.
Inline parameters are not considered immutable paths and cannot be used as
singleton types. Removing the `inline` qualifier from the `value` parameter
may help resolve this issue.

[mbovel]

To solve this issue, I suggest to special-case the NotAPath message:

scala3/compiler/src/dotty/tools/dotc/reporting/messages.scala

Lines 1819 to 1825 in b8c5ecb

	class NotAPath(tp: Type, usage: String)(using Context) extends TypeMsg(NotAPathID):
	def msg(using Context) = i"$tp is not a valid $usage, since it is not an immutable path"
	def explain(using Context) =
	i"""An immutable path is
	| - a reference to an immutable value, or
	| - a reference to `this`, or
	| - a selection of an immutable path with an immutable value."""

One can check if tp refers to a term symbol that is an inline parameter there, and display additional information if it is the case.

[soronpo]

Note for future readers: Nicolas explained why inline values cannot be stable paths in #16804 (comment).

What would you expect as a better error message? Something like the following?

(value : V) is not a valid singleton type, since it is not an immutable path.
Inline parameters are not considered immutable paths and cannot be used as
singleton types. Removing the `inline` qualifier from the `value` parameter
may help resolve this issue.

Seems good. I think the compiler started to have a standardize a format for providing hints.
So it will look better with:

(value : V) is not a valid singleton type, since it is not an immutable path.
Inline parameters are not considered immutable paths and cannot be used as
singleton types. 

Hint: Removing the `inline` qualifier from the `value` parameter
may help resolve this issue.

[mbovel]

This issue was picked for the Scala Issue Spree of tomorrow, September 24th. @HarrisL2, someone yet unknown and I will be working on it.