Conformance of literals to attached arguments

I just came across this:

Error code: VUAR(2)
Type error: non-conforming actual argument in feature call. 
What to do: make sure that type of actual argument conforms to type
  of corresponding formal argument. 

Class: ST_TEST_ST_STRING
Feature: test_substring
Called feature: make_from_string (a_string: !STRING_GENERAL) from ST_STRING
Argument name: a_string
Argument position: 1
Actual argument type: STRING_8
Formal argument type: !STRING_GENERAL
Line: 57
      do
->      create l_32.make_from_string ("A test string")

ISE 6.1.

It astonishes me. Why doesn't a literal of type STRING_8 conform to
!STRING_GENERAL ?
-- 
Colin Adams
Preston Lancashire

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