Resolve converter call to constants in case arms (#8336)

Fixes #8333

compiler/semtypes.nim

@@ -557,7 +557,10 @@ proc semCaseBranch(c: PContext, t, branch: PNode, branchIndex: int,
         return
       elif r.kind notin {nkCurly, nkBracket} or len(r) == 0:
         checkMinSonsLen(t, 1, c.config)
-        branch.sons[i] = skipConv(fitNode(c, t.sons[0].typ, r, r.info))
+        var tmp = fitNode(c, t.sons[0].typ, r, r.info)
+        # the call to fitNode may introduce a call to a converter
+        if tmp.kind in {nkHiddenCallConv}: tmp = semConstExpr(c, tmp)
+        branch.sons[i] = skipConv(tmp)
         inc(covered)
       else:
         if r.kind == nkCurly:

tests/casestmt/t8333.nim

@@ -0,0 +1,10 @@
+discard """
+  output: "1"
+"""
+
+converter toInt*(x: char): int = 
+  x.int
+
+case 0
+of 'a': echo 0
+else: echo 1