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.

(cherry picked from commit 8c9a645bdf)

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