Improve error message

Author: odersky

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

@@ -1254,7 +1254,8 @@ import transform.SymUtils._
           |
           |I found: $tree
           |But the part corresponding to `<skolem>` is not a reference that can be generated.
-          |This might be because resolution yielded as given instance a function that is not known to be pure."""
+          |This might be because resolution yielded as given instance a function that is not
+          |known to be total and side-effect free."""
     def explain =
       em"""The part of given resolution that corresponds to `<skolem>` produced a term that
           |is not a stable reference. Therefore a given instance could not be generated.

tests/neg/i8623.check

@@ -5,6 +5,7 @@
    |
    |  I found: <skolem>.tasty.pos(unseal(given_QC[Any]))
    |  But the part corresponding to `<skolem>` is not a reference that can be generated.
-   |  This might be because resolution yielded as given instance a function that is not known to be pure.
+   |  This might be because resolution yielded as given instance a function that is not
+   |  known to be total and side-effect free.
 
 longer explanation available when compiling with `-explain`