Allow empty sets in case/of branches.

Added support for conditional compilation using 'when' with empty sets and arrays in
'case of' branches.

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compiler/semtypes.nim

@@ -452,6 +452,7 @@ proc semCaseBranchSetElem(c: PContext, t, b: PNode,
 
 proc semCaseBranch(c: PContext, t, branch: PNode, branchIndex: int, 
                    covered: var BiggestInt) = 
+  
   for i in countup(0, sonsLen(branch) - 2): 
     var b = branch.sons[i]
     if b.kind == nkRange:
@@ -461,8 +462,11 @@ proc semCaseBranch(c: PContext, t, branch: PNode, branchIndex: int,
     else:
       # constant sets and arrays are allowed:
       var r = semConstExpr(c, b)
-      # for ``{}`` we want to trigger the type mismatch in ``fitNode``:
-      if r.kind notin {nkCurly, nkBracket} or len(r) == 0:
+      if r.kind in {nkCurly, nkBracket} and len(r) == 0  and sonsLen(branch)==2:
+        # discarding ``{}`` and ``[]`` branches silently
+        delSon(branch, 0)
+        return
+      elif r.kind notin {nkCurly, nkBracket} or len(r) == 0:
         checkMinSonsLen(t, 1)
         branch.sons[i] = skipConv(fitNode(c, t.sons[0].typ, r))
         inc(covered)

tests/casestmt/tcase_emptyset_when.nim

@@ -0,0 +1,24 @@
+discard """
+  file: "tcaseofwhen.nim"
+  outputsub: "compiles for 1\ni am always two\ndefault for 3\nset is 4 not 5\narray is 6 not 7\ndefault for 8"
+  exitcode: "0"
+"""
+
+proc whenCase(a: int) =
+  case a
+  of (when compiles(whenCase(1)): 1 else: {}): echo "compiles for 1"
+  of {}: echo "me not fail"
+  of 2: echo "i am always two"
+  of []: echo "me neither"
+  of {4,5}: echo "set is 4 not 5"
+  of [6,7]: echo "array is 6 not 7"
+  of (when compiles(neverCompilesIBet()): 3 else: {}): echo "compiles for 3"
+  #of {},[]: echo "me neither"
+  else: echo "default for ", a
+
+whenCase(1)
+whenCase(2)
+whenCase(3)
+whenCase(4)
+whenCase(6)
+whenCase(8)