typetraits: fixes #6454; genericParams; added lenTuple; added tuple type get (#13064)

lib/pure/typetraits.nim

@@ -14,6 +14,7 @@
 
 export system.`$` # for backward compatibility
 
+include "system/inclrtl"
 
 proc name*(t: typedesc): string {.magic: "TypeTrait".}
   ## Returns the name of the given type.
@@ -70,6 +71,45 @@ proc distinctBase*(T: typedesc): typedesc {.magic: "TypeTrait".}
   ## Returns base type for distinct types, works only for distinct types.
   ## compile time error otherwise
 
+import std/macros
+
+macro lenTuple*(t: tuple): int {.since: (1, 1).} =
+  ## Return number of elements of `t`
+  newLit t.len
+
+macro lenTuple*(t: typedesc[tuple]): int {.since: (1, 1).} =
+  ## Return number of elements of `T`
+  newLit t.len
+
+when (NimMajor, NimMinor) >= (1, 1):
+  template get*(T: typedesc[tuple], i: static int): untyped =
+    ## Return `i`th element of `T`
+    # Note: `[]` currently gives: `Error: no generic parameters allowed for ...`
+    type(default(T)[i])
+
+macro genericParams*(T: typedesc): untyped {.since: (1, 1).} =
+  ## return tuple of generic params for generic `T`
+  runnableExamples:
+    type Foo[T1, T2]=object
+    doAssert genericParams(Foo[float, string]) is (float, string)
+  result = newNimNode(nnkTupleConstr)
+  var impl = getTypeImpl(T)
+  expectKind(impl, nnkBracketExpr)
+  impl = impl[1]
+  while true:
+    case impl.kind
+      of nnkSym:
+        impl = impl.getImpl
+        continue
+      of nnkTypeDef:
+        impl = impl[2]
+        continue
+      of nnkBracketExpr:
+        for i in 1..<impl.len:
+          result.add impl[i]
+        break
+      else:
+        error "wrong kind: " & $impl.kind
 
 when isMainModule:
   static:

lib/system/inclrtl.nim

@@ -50,5 +50,7 @@ else:
   {.pragma: benign, gcsafe.}
 
 template since(version, body: untyped) {.dirty.} =
+  ## limitation: can't be used to annotate a template (eg typetraits.get), would
+  ## error: cannot attach a custom pragma.
   when version <= (NimMajor, NimMinor):
     body