Error message "No ClassTag available for T" not helpful on what to do to fix it and has no reflect.ClassTag to show where it is

[bjornregnell]

Compiler version

3.7.4

Minimized example

def test[T](a: T) = collection.immutable.ArraySeq.fill(1)(a)

Output Error/Warning message

-- [E172] Type Error: -------------------------------------------------------------
1 |def test[T](a: T) = collection.immutable.ArraySeq.fill(1)(a)
  |                                                            ^
  |                                                  No ClassTag available for T
1 error found

Why this Error/Warning was not helpful

The message was unhelpful because it does not say precisely what to do to make it compile. If I have no clue about what a ClassTag is and why it's needed, then the message could be more helpful.

Suggested improvement

It could be made more helpful by showing an example of how to rewrite the type param with a context bound and include reflect.ClassTag so users do not stumble on "ClassTagf" not found in yet another error message, and not knowing in what package ClassTag resides.

For example:

-- [E172] Type Error: -------------------------------------------------------------
1 |def test[T](a: T) = collection.immutable.ArraySeq.fill(1)(a)
  |                                                            ^
  |                                            No reflect.ClassTag available for T
  | You can fix this by changing [T] to [T: reflect.ClassTag]
1 error found

[bjornregnell]

This improvement is even more important since we got ArraySeq, which is a "normal" Scala collection as opposed to Array which is primitive and perhaps not so surprising that it "does not behave" - hence even more surprising to get this error message for ArraySeq (comparing to ArrayBuffer which does not require a ClassTag...).