implemented and documented the new typedesc binding rules

tests/compile/tbindtypedesc.nim

@@ -0,0 +1,87 @@
+discard """
+  msg: '''
+int
+float
+TFoo
+TFoo
+'''
+"""
+
+import typetraits
+
+type 
+  TFoo = object
+    x, y: int
+
+  TBar = tuple
+    x, y: int
+
+template good(e: expr) =
+  static: assert(compiles(e))
+
+template bad(e: expr) =
+  static: assert(not compiles(e))
+
+proc genericParamRepeated[T: typedesc](a: T, b: T) =
+  static:
+    echo a.name
+    echo b.name
+
+good(genericParamRepeated(int, int))
+good(genericParamRepeated(float, float))
+
+bad(genericParamRepeated(string, int))
+bad(genericParamRepeated(int, float))
+
+proc genericParamOnce[T: typedesc](a, b: T) =
+  static:
+    echo a.name
+    echo b.name
+
+good(genericParamOnce(int, int))
+good(genericParamOnce(TFoo, TFoo))
+
+bad(genericParamOnce(string, int))
+bad(genericParamOnce(TFoo, float))
+
+proc typePairs(A, B: type1; C, D: type2) = nil
+
+good(typePairs(int, int, TFoo, TFOO))
+good(typePairs(TBAR, TBar, TBAR, TBAR))
+good(typePairs(int, int, string, string))
+
+bad(typePairs(TBAR, TBar, TBar, TFoo))
+bad(typePairs(string, int, TBAR, TBAR))
+
+proc typePairs2[T: typedesc, U: typedesc](A, B: T; C, D: U) = nil
+
+good(typePairs2(int, int, TFoo, TFOO))
+good(typePairs2(TBAR, TBar, TBAR, TBAR))
+good(typePairs2(int, int, string, string))
+
+bad(typePairs2(TBAR, TBar, TBar, TFoo))
+bad(typePairs2(string, int, TBAR, TBAR))
+
+proc dontBind(a: typedesc, b: typedesc) =
+  static:
+    echo a.name
+    echo b.name
+
+good(dontBind(int, float))
+good(dontBind(TFoo, TFoo))
+
+proc dontBind2(a, b: typedesc) = nil
+
+good(dontBind2(int, float))
+good(dontBind2(TBar, int))
+
+proc bindArg(T: typedesc, U: typedesc, a, b: T, c, d: U) = nil
+
+good(bindArg(int, string, 10, 20, "test", "nest"))
+good(bindArg(int, int, 10, 20, 30, 40))
+
+bad(bindArg(int, string, 10, "test", "test", "nest"))
+bad(bindArg(int, int, 10, 20, 30, "test"))
+bad(bindArg(int, string, 10.0, 20, "test", "nest"))
+bad(bindArg(int, string, "test", "nest", 10, 20))
+