Fix sigsegv in getTypeImpl for unnamed tuple (#5440)

avoid sigsegv in getTypeImpl for unnamed tuple; fixes #4862

compiler/vmdeps.nim

@@ -218,8 +218,13 @@ proc mapTypeToAstX(t: PType; info: TLineInfo;
   of tyTuple:
     if inst:
       result = newNodeX(nkTupleTy)
-      for s in t.n.sons:
-        result.add newIdentDefs(s)
+      # only named tuples have a node, unnamed tuples don't
+      if t.n.isNil:
+        for subType in t.sons:
+          result.add mapTypeToAst(subType, info)
+      else:
+        for s in t.n.sons:
+          result.add newIdentDefs(s)
     else:
       result = mapTypeToBracket("tuple", mTuple, t, info)
   of tySet: result = mapTypeToBracket("set", mSet, t, info)

tests/macros/tgettypeinst.nim

@@ -120,3 +120,7 @@ test(Tree):
     right: ref Tree
 test(proc (a: int, b: Foo[2,float]))
 test(proc (a: int, b: Foo[2,float]): Bar[3,int])
+
+# bug #4862
+static:
+  discard typedesc[(int, int)].getTypeImpl