tests: add openSym dot-RHS regression test

Tests the fix in 7148ae347: the RHS of a dot expression wrapped in
nkOpenSym by the generic prepass must use the captured symbol when nothing
is injected, while an injected symbol still overrides it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/generics/mopensymdot.nim

@@ -0,0 +1,18 @@
+{.experimental: "openSym".}
+
+import std/hashes
+
+template maxHash(t): untyped = high(t).Hash
+
+template implCaptured() {.dirty.} =
+  result = maxHash(t)
+
+template implInjected() {.dirty.} =
+  type Hash = uint8
+  result = high(t).Hash
+
+proc usesCaptured*[T](t: T): auto =
+  implCaptured()
+
+proc usesInjected*[T](t: T): auto =
+  implInjected()

tests/generics/topensymdot.nim

@@ -0,0 +1,12 @@
+# the RHS of a dot expression can be wrapped in `nkOpenSym` by the generic
+# prepass (e.g. a `x.T` type conversion expanded from a dirty template):
+# the captured symbol (`hashes.Hash`, not in scope here) must be used when
+# nothing is injected, while a symbol injected during instantiation still
+# overrides it
+
+{.experimental: "openSym".}
+
+import mopensymdot
+
+doAssert sizeof(usesCaptured(@[1, 2, 3])) == sizeof(int) # hashes.Hash
+doAssert sizeof(usesInjected(@[1, 2, 3])) == 1           # injected uint8