Syntax Checks for Duplicate Case Labels

[DavePearce]

Consider the following program:

function f(int x) -> (int r):
   switch(x):
       case 1:
          return 0
       case 1:
          return 1

The above should generate a "duplicate case label" error message (E509). At the moment, the job of checking for duplicate case labels is left to CodeGenerator. Realistically, it should come earlier in the pipeline.

The reason that checking for duplicate case labels comes earlier is that it's only after FlowTypeChecker that we know what the constant values resolve to. However, this does raise the question as to whether we should be looking at the source-level syntax or the underlying value. Java, for example, only looks at the souce-level syntax. So the following compiles without problem:

switch(x) {
 case 1+1:
   return 0;
 case 1-1:
   return 1;
}

Some thought is really needed as to what the right decision is here.