fix macro hygiene for @nospecialize(::T) (#40293)

(cherry picked from commit 73aa7e4)

Author: simeonschaub

src/macroexpand.scm

@@ -210,7 +210,8 @@
         ((atom? v) '())
         (else
          (case (car v)
-           ((... kw |::| =) (try-arg-name (cadr v)))
+           ((|::|) (if (length= v 2) '() (try-arg-name (cadr v))))
+           ((... kw =) (try-arg-name (cadr v)))
            ((escape) (list v))
            ((hygienic-scope) (try-arg-name (cadr v)))
            ((meta)  ;; allow certain per-argument annotations

test/syntax.jl

@@ -2686,3 +2686,9 @@ macro m_begin_hygiene(a)
 end
 
 @test @m_begin_hygiene([1, 2, 3]) == 1
+
+macro m_nospecialize_unnamed_hygiene()
+    return :(f(@nospecialize(::Any)) = Any)
+end
+
+@test @m_nospecialize_unnamed_hygiene()(1) === Any