towards support for composite type classes such as seq[Number] and SquareMatrix[T]

tests/compile/tcompositetypeclasses.nim

@@ -0,0 +1,30 @@
+template accept(e) =
+  static: assert(compiles(e))
+
+template reject(e) =
+  static: assert(not compiles(e))
+
+type
+  TFoo[T, U] = tuple
+    x: T
+    y: U
+
+  TBar[K] = TFoo[K, K]
+
+  TUserClass = int|string
+
+  # TBaz = TBar[TUserClass]
+
+var
+  vfoo: TFoo[int, string]
+  vbar: TFoo[string, string]
+
+proc foo(x: TFoo) = echo "foo"
+proc bar(x: TBar) = echo "bar"
+# proc baz(x: TBaz) = echo "baz"
+
+accept(foo(vfoo))
+accept(bar(vbar))
+# baz vbar
+reject(bar(vfoo))
+