Execution error when using 'max' function on foreign types in some cases.

[rdwebster]

I encountered the problem when comparing Time values, but it also happens for String (another foreign, non-primitive type).

To reproduce, add the following code to a module, compile and run it:

stringTest :: Int;
stringTest = 
    String.length (Prelude.max "abc" "def");

This gives the following run-time error:
Fatal Executor error.
Caused by: java.lang.NoClassDefFoundError: org/openquark/cal_Cal_Test_Data_DataGems__Tests/String_Test, Detail: org/openquark/cal_Cal_Test_Data_DataGems__Tests/String_Test

With LECC source generation turned on, the following error is reported at compilation time:
Error: Code generation aborted. Error generating code for: Cal.Test.Data.DataGems_Tests.stringTest Detail: Unboxed type mismatch in Cal.Test.Data.DataGems_Tests.stringTest : expression type = java.lang.Comparable, desiredType = java.lang.String. Caused by: org.openquark.cal.internal.machine.CodeGenerationException

The problem might be with the way that "deriving Ord" is implemented for foreign types (such as String or Time).
It seems to be mapping this to the Prelude.maxComparable function, which is the correct code but not the correct type signature (unless the appropriate conversions are being added).

[rdwebster]

The problem can be worked around by changing the code so that it doesn't use max, but rather greater/lessThan:
String.length (if "abc" > "def" then "abc" else "def");

[indicee-michald]

SCJavaDefn.buildUnboxedDirectCall() makes a call to verifyUnboxType() to check that the unboxed input type of length() matches the return type of max(). However, there is a type mismatch because max returns Comparable and length expects a String and we are not smart enough to realize that we can just downcast back to String since that was the original type anyway.

One fix is to catch the exception thrown by verifyUnboxType and return null which will cause a boxed version to be generated.

[indicee-michald]

Fixed: 16fd7d9