fix generic converter regression with var/subtype args (#24902)

refs #24867,
https://github.com/nim-lang/Nim/pull/24867#issuecomment-2821315971

The argument node of the converter can be wrapped in [hidden `addr` or
subtype conversion
nodes](dc100c5caa/compiler/sigmatch.nim (L2327-L2335))
which have to be skipped when matching the type again, since the type of
the node is the uninstantiated type taken from the proc parameter.

compiler/semcall.nim

@@ -688,7 +688,9 @@ proc instGenericConvertersArg*(c: PContext, a: PNode, x: TCandidate) =
     if s.isGenericRoutineStrict:
       var src = s.typ.firstParamType
       var convMatch = newCandidate(c, src)
-      let srca = typeRel(convMatch, src, a[1].typ)
+      var arg = a[1]
+      if arg.kind in {nkHiddenAddr, nkHiddenSubConv}: arg = arg[^1]
+      let srca = typeRel(convMatch, src, arg.typ)
       if srca notin {isEqual, isGeneric, isSubtype}:
         internalError(c.config, a.info, "generic converter failed rematch")
       let finalCallee = generateInstance(c, s, convMatch.bindings, a.info)

tests/converter/tvargenericconverter.nim

@@ -0,0 +1,7 @@
+# regression test
+
+converter toPtr[T](x: var T): ptr T =
+  result = addr x
+
+var x = 123
+let y: ptr int = x