new .redefine pragma for templates, warn on redefinition without it (#20211)

* test CI for template redefinitions

* adapt asyncmacro

* fix quote

* fix again

* try something else

* revert

* fix ioselectors_select, disable packages CI

* adapt more tests & simplify

* more

* more

* more

* rename to redefine, warn on implicit redefinition

* basic documentation [skip ci]

* Update compiler/lineinfos.nim

Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>

Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>

lib/pure/asyncmacro.nim

@@ -126,7 +126,9 @@ template await*(f: typed): untyped {.used.} =
     error "await expects Future[T], got " & $typeof(f)
 
 template await*[T](f: Future[T]): auto {.used.} =
-  template yieldFuture = yield FutureBase()
+  when not defined(nimHasTemplateRedefinitionPragma):
+    {.pragma: redefine.}
+  template yieldFuture {.redefine.} = yield FutureBase()
 
   when compiles(yieldFuture):
     var internalTmpFuture: FutureBase = f

lib/pure/ioselects/ioselectors_select.nim

@@ -398,18 +398,6 @@ proc contains*[T](s: Selector[T], fd: SocketHandle|int): bool {.inline.} =
       if s.fds[i].ident == fdi:
         return true
 
-when hasThreadSupport:
-  template withSelectLock[T](s: Selector[T], body: untyped) =
-    acquire(s.lock)
-    {.locks: [s.lock].}:
-      try:
-        body
-      finally:
-        release(s.lock)
-else:
-  template withSelectLock[T](s: Selector[T], body: untyped) =
-    body
-
 proc getData*[T](s: Selector[T], fd: SocketHandle|int): var T =
   s.withSelectLock():
     let fdi = int(fd)

lib/std/assertions.nim

@@ -85,7 +85,9 @@ template onFailedAssert*(msg, code: untyped): untyped {.dirty.} =
     onFailedAssert(msg):
       raise (ref MyError)(msg: msg, lineinfo: instantiationInfo(-2))
     doAssertRaises(MyError): doAssert false
-  template failedAssertImpl(msgIMPL: string): untyped {.dirty.} =
+  when not defined(nimHasTemplateRedefinitionPragma):
+    {.pragma: redefine.}
+  template failedAssertImpl(msgIMPL: string): untyped {.dirty, redefine.} =
     let msg = msgIMPL
     code
 
@@ -98,7 +100,7 @@ template doAssertRaises*(exception: typedesc, code: untyped) =
   var wrong = false
   const begin = "expected raising '" & astToStr(exception) & "', instead"
   const msgEnd = " by: " & astToStr(code)
-  template raisedForeign = raiseAssert(begin & " raised foreign exception" & msgEnd)
+  template raisedForeign {.gensym.} = raiseAssert(begin & " raised foreign exception" & msgEnd)
   when Exception is exception:
     try:
       if true: