fixes #22049; fixes #22054; implicit conversion keeps varness (#22097)

* fixes #22054; codegen for var tuples conv

* rethink fixes

* add test cases

* templates only

* fixes var tuples

* keep varness no matter what

* fixes typ.isNil

* make it work for generics

* restore isSubrange

* add a test case as requested

(cherry picked from commit 77beb15)

compiler/sigmatch.nim

@@ -1888,7 +1888,13 @@ proc implicitConv(kind: TNodeKind, f: PType, arg: PNode, m: TCandidate,
     else:
       result.typ = errorType(c)
   else:
-    result.typ = f.skipTypes({tySink, tyVar})
+    result.typ = f.skipTypes({tySink})
+  # keep varness
+  if arg.typ != nil and arg.typ.kind == tyVar:
+    result.typ = toVar(result.typ, tyVar, c.idgen)
+  else:
+    result.typ = result.typ.skipTypes({tyVar})
+
   if result.typ == nil: internalError(c.graph.config, arg.info, "implicitConv")
   result.add c.graph.emptyNode
   result.add arg

tests/errmsgs/t22097.nim

@@ -0,0 +1,9 @@
+discard """
+  errormsg: "for a 'var' type a variable needs to be passed; but 'uint16(x)' is immutable"
+"""
+
+proc toUInt16(x: var uint16) =
+  discard
+
+var x = uint8(1)
+toUInt16 x

tests/tuples/ttuples_various.nim

@@ -171,3 +171,29 @@ block tuple_with_seq:
     echo s
     (s, 7)
   t = test(t.a)
+
+block: # bug #22049
+  type A = object
+    field: tuple[a, b, c: seq[int]]
+
+  func value(v: var A): var tuple[a, b, c: seq[int]] =
+    v.field
+  template get(v: A): tuple[a, b, c: seq[int]] = v.value
+
+  var v = A(field: (@[1], @[2], @[3]))
+  var (a, b, c) = v.get()
+
+  doAssert a == @[1]
+  doAssert b == @[2]
+  doAssert c == @[3]
+
+block: # bug #22054
+  type A = object
+    field: tuple[a: int]
+
+  func value(v: var A): var tuple[a: int] =
+    v.field
+  template get(v: A): tuple[a: int] = v.value
+
+  var v = A(field: (a: 1314))
+  doAssert get(v)[0] == 1314