Each of the individual types named in the union type must be unique.

README.md

@@ -1067,18 +1067,18 @@ The type after the `as` must be the type of the expression.
     | }
     = ok
 
-Each of the individual types named in the union type must be unique.  (TODO)
+Each of the individual types named in the union type must be unique.
 
-    /| fun foo(a, b: integer|string) {
-    /|   print("ok")
-    /| }
-    /| fun main() {
-    /|   a = 20;
-    /|   b = 30;
-    /|   c = a + b as integer|integer|string
-    /|   foo(a, c)
-    /| }
-    /? bad union type
+    | fun foo(a, b: integer|string) {
+    |   print("ok")
+    | }
+    | fun main() {
+    |   a = 20;
+    |   b = 30;
+    |   c = a + b as integer|integer|string
+    |   foo(a, c)
+    | }
+    ? bad union type
 
 Cannot promote a union type to itself.
 

TODO.md

@@ -30,8 +30,6 @@ Other backends (Python? Java? CIL? Scheme?)
 
 ### Design ###
 
-Each of the individual types named in the union type must be unique.
-
 Promote union type to bigger union type (e.g. `string|int` => `string|int|void`)
 
 Don't output final value.  Command-line arguments passed to `main`.  (`sysmain`?)

src/castile/checker.py

@@ -151,7 +151,13 @@ def type_of(self, ast):
                 [self.type_of(c) for c in ast.children[1:]], return_type
             )
         elif ast.tag == 'UnionType':
-            ast.type = Union([self.type_of(c) for c in ast.children])
+            types = []
+            for c in ast.children:
+                type_ = self.type_of(c)
+                if type_ in types:
+                    raise CastileTypeError("bad union type")
+                types.append(type_)
+            ast.type = Union(types)
         elif ast.tag == 'StructType':
             ast.type = Struct(ast.value)
         elif ast.tag == 'VarRef':