add test for comptime err to int with only 1 member of set

TODO

@@ -1,10 +0,0 @@
-comptime calling fn with inferred error set should give empty error set but still you can use try
-
-comptime err to int of empty err set and of size 1 err set
-
-
-undefined in infer error 
-
-syntax - (error{}!void) as the return type
-
-

test/cases/error.zig

@@ -140,3 +140,13 @@ test "syntax: nullable operator in front of error union operator" {
         assert(?error!i32 == ?(error!i32));
     }
 }
+
+test "comptime err to int of error set with only 1 possible value" {
+    testErrToIntWithOnePossibleValue(error.A, u32(error.A));
+    comptime testErrToIntWithOnePossibleValue(error.A, u32(error.A));
+}
+fn testErrToIntWithOnePossibleValue(x: error{A}, comptime value: u32) void {
+    if (u32(x) != value) {
+        @compileError("bad");
+    }
+}