fix #19149 Invalid codegen when returning var tuple from a template (#20762)

* fix #19149 Invalid codegen when returning var tuple from a template

* fix type

(cherry picked from commit bae0fb720a)

compiler/sigmatch.nim

@@ -1894,7 +1894,7 @@ proc implicitConv(kind: TNodeKind, f: PType, arg: PNode, m: TCandidate,
     else:
       result.typ = errorType(c)
   else:
-    result.typ = f.skipTypes({tySink})
+    result.typ = f.skipTypes({tySink, tyVar})
   if result.typ == nil: internalError(c.graph.config, arg.info, "implicitConv")
   result.add c.graph.emptyNode
   result.add arg
@@ -2153,7 +2153,8 @@ proc paramTypesMatchAux(m: var TCandidate, f, a: PType,
   of isEqual:
     inc(m.exactMatches)
     result = arg
-    if skipTypes(f, abstractVar-{tyTypeDesc}).kind == tyTuple or
+    let ff = skipTypes(f, abstractVar-{tyTypeDesc})
+    if ff.kind == tyTuple or
       (arg.typ != nil and skipTypes(arg.typ, abstractVar-{tyTypeDesc}).kind == tyTuple):
       result = implicitConv(nkHiddenSubConv, f, arg, m, c)
   of isNone:

tests/template/t19149.nim

@@ -0,0 +1,19 @@
+type Foo = tuple[active: bool, index: int]
+
+
+var f: Foo
+
+# template result type during match stage
+# f:var Foo
+# a:Foo
+# tyVar
+# tyTuple
+# after change to proc
+# f:Foo
+# a:Foo
+# tyTuple
+# tyTuple
+
+template cursor(): var Foo = f
+discard cursor()
+