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>

tests/template/template_various.nim

@@ -274,10 +274,10 @@ parse9:
 block gensym1:
   template x: untyped = -1
   template t1() =
-    template x: untyped {.gensym.} = 1
+    template x: untyped {.gensym, redefine.} = 1
     echo x()  # 1
   template t2() =
-    template x: untyped = 1  # defaults to {.inject.}
+    template x: untyped {.redefine.} = 1  # defaults to {.inject.}
     echo x()  # -1  injected x not available during template definition
   t1()
   t2()

tests/template/tredefinition_override.nim

@@ -0,0 +1,33 @@
+{.push warningAsError[TemplateRedefinition]: on.}
+
+doAssert not (compiles do:
+  template foo(): int = 1
+  template foo(): int = 2)
+doAssert (compiles do:
+  template foo(): int = 1
+  template foo(): int {.redefine.} = 2)
+doAssert not (compiles do:
+  block:
+    template foo() =
+      template bar: string {.gensym.} = "a"
+      template bar: string {.gensym.} = "b"
+    foo())
+doAssert (compiles do:
+  block:
+    template foo() =
+      template bar: string {.gensym.} = "a"
+      template bar: string {.gensym, redefine.} = "b"
+    foo())
+
+block:
+  template foo(): int = 1
+  template foo(): int {.redefine.} = 2
+  doAssert foo() == 2
+block:
+  template foo(): string =
+    template bar: string {.gensym.} = "a"
+    template bar: string {.gensym, redefine.} = "b"
+    bar()
+  doAssert foo() == "b"
+
+{.pop.}

tests/template/utemplates.nim

@@ -22,7 +22,7 @@ block: # templates can be redefined multiple times
     if not cond: fail(msg)
 
   template assertionFailed(body: untyped) {.dirty.} =
-    template fail(msg: string): typed =
+    template fail(msg: string): typed {.redefine.} =
       body
 
   assertionFailed: