fix #17911 rawProc for cpp (#17912)

lib/pure/hashes.nim

@@ -524,15 +524,21 @@ proc hash*[T: tuple | object | proc](x: T): Hash {.inline.} =
     proc hash(a: Obj2): Hash = hash((a.x))
     assert hash(Obj2[float](x: 520, y: "Nim")) == hash(Obj2[float](x: 520, y: "Nim2"))
   runnableExamples:
-    # proc and closure examples
+    # proc
     proc fn1() = discard
-    var a = 0
-    proc fn2() = a.inc
-    assert hash(fn1) != hash(fn2)
     const fn1b = fn1
     assert hash(fn1b) == hash(fn1)
-    let fn2b = fn2
-    assert hash(fn2b) == hash(fn2)
+
+    # closure
+    proc outer =
+      var a = 0
+      proc fn2() = a.inc
+      assert fn2 is "closure"
+      let fn2b = fn2
+      assert hash(fn2b) == hash(fn2)
+      assert hash(fn2) != hash(fn1)
+    outer()
+
   when T is "closure":
     result = hash((rawProc(x), rawEnv(x)))
   elif T is (proc):

lib/system.nim

@@ -2414,17 +2414,22 @@ when notJSnotNims:
 
   proc rawProc*[T: proc](x: T): pointer {.noSideEffect, inline.} =
     ## Retrieves the raw proc pointer of the closure `x`. This is
-    ## useful for interfacing closures with C.
+    ## useful for interfacing closures with C/C++, hash compuations, etc.
     when T is "closure":
+      #[
+      The conversion from function pointer to `void*` is a tricky topic, but this
+      should work at least for c++ >= c++11, e.g. for `dlsym` support.
+      refs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57869,
+      https://stackoverflow.com/questions/14125474/casts-between-pointer-to-function-and-pointer-to-object-in-c-and-c
+      ]#
       {.emit: """
-      `result` = `x`.ClP_0;
+      `result` = (void*)`x`.ClP_0;
       """.}
     else:
       {.error: "Only closure function and iterator are allowed!".}
 
   proc rawEnv*[T: proc](x: T): pointer {.noSideEffect, inline.} =
-    ## Retrieves the raw environment pointer of the closure `x`. This is
-    ## useful for interfacing closures with C.
+    ## Retrieves the raw environment pointer of the closure `x`. See also `rawProc`.
     when T is "closure":
       {.emit: """
       `result` = `x`.ClE_0;