…liaLang#50929)

This is the part of JuliaLang#50927
required to fix JuliaLang#49249.
Specifically, before this change `tmerge(Tuple{Any, Int}, Nothing)`
would be `Union{Nothing, Tuple{Any, Int}}` but `tmerge(Tuple{BIG_UNION,
Int}, Nothing)` would be `Union{Nothing, Tuple{Any, Any}}`. This feels
bad intuitively because giving the compiler more type information led it
to forget type information that it already knew about, and is especially
damaging because it led to unnecessary type instability when iterating
tuples with complex element types (because the iterator state should be
inferrable as an `Int` even if you have no idea what the tuple type is).

This is tagged for backport to 1.10 since it is a relatively unobtrusive
change and it fixes the string regression in a more proper way.

…liaLang#50929)

This is the part of JuliaLang#50927
required to fix JuliaLang#49249.
Specifically, before this change `tmerge(Tuple{Any, Int}, Nothing)`
would be `Union{Nothing, Tuple{Any, Int}}` but `tmerge(Tuple{BIG_UNION,
Int}, Nothing)` would be `Union{Nothing, Tuple{Any, Any}}`. This feels
bad intuitively because giving the compiler more type information led it
to forget type information that it already knew about, and is especially
damaging because it led to unnecessary type instability when iterating
tuples with complex element types (because the iterator state should be
inferrable as an `Int` even if you have no idea what the tuple type is).

This is tagged for backport to 1.10 since it is a relatively unobtrusive
change and it fixes the string regression in a more proper way.

…0929)

This is the part of #50927
required to fix #49249.
Specifically, before this change `tmerge(Tuple{Any, Int}, Nothing)`
would be `Union{Nothing, Tuple{Any, Int}}` but `tmerge(Tuple{BIG_UNION,
Int}, Nothing)` would be `Union{Nothing, Tuple{Any, Any}}`. This feels
bad intuitively because giving the compiler more type information led it
to forget type information that it already knew about, and is especially
damaging because it led to unnecessary type instability when iterating
tuples with complex element types (because the iterator state should be
inferrable as an `Int` even if you have no idea what the tuple type is).

This is tagged for backport to 1.10 since it is a relatively unobtrusive
change and it fixes the string regression in a more proper way.

(cherry picked from commit 6e2e6d0)

Backported PRs:
- [x] #48625 <!-- add replace(io, str, patterns...) -->
- [x] #48387 <!-- Improve documentation of sort-related functions -->
- [x] #48363 <!-- Revise sort.md and docstrings in sort.jl -->
- [x] #48977 <!-- Update SparseArrays.jl stdlib for SuiteSparse 7 -->
- [x] #50719 <!-- fix `CyclePadding(::DataType)` -->
- [x] #50694 <!-- inference: permit recursive type traits -->
- [x] #50860 <!-- Add `Base.get_extension` to docs/API -->
- [x] #50594 <!-- Disallow non-index Integer types in isassigned -->
- [x] #50802 <!-- Makes IntrusiveLinkedListSynchronized mutable to avoid
allocation on poptask -->
- [x] #50858 <!-- Add a `threadpool` parameter to `Channel` constructor
-->
- [x] #50874 <!-- Restrict COFF to a single thread when symbol count is
high -->
- [x] #50822 <!-- Add default method for setmodifiers! -->
- [x] #50730 <!-- Fix integer overflow in `isapprox` -->
- [x] #50850 <!-- Remove weird Rational dispatch and add pi functions to
list -->
- [x] #50809 <!-- Limit type-printing in MethodError -->
- [x] #50915 <!-- Add note the `Task` about sticky bit -->
- [x] #50929 <!-- when widening tuple types in tmerge, only widen the
complex parts -->
- [x] #50928 <!-- Bump JuliaSyntax to 0.4.6 -->
- [x] #50959 <!-- Update libssh2 patches -->
- [x] #50823 <!-- Make ranges more robust with unsigned indexes. -->
- [x] #48542 <!-- Add docs on task-specific buffering using
multithreading -->
- [x] #50912 <!-- Separate foreign threads into a :foreign threadpool
-->
- [x] #51010 <!-- Add ORIGIN to SuiteSparse rpath on Linux/FreeBSD -->
- [x] #50753 <!-- cat: remove unused promote_eltype methods that confuse
inference -->
- [x] #51027 <!-- Implement realloc accounting correctly -->
- [x] #51019 <!-- fix a case of potentially use of undefined variable
when handling error in distributed message processing -->
- [x] #51039 <!-- Revert "Optimize findall(f, ::AbstractArray{Bool})
(#42202)" -->
- [x] #51036 <!-- add missing invoke edge for nospecialize targets -->
- [x] #51042 <!-- inference: fix return_type_tfunc modeling of concrete
functions -->
- [x] #51114 <!-- Workaround upstream FreeBSD issue #272992 -->
- [x] #50892 <!-- Add `JL_DLLIMPORT` to `small_typeof` declaration -->
- [x] #51154 <!-- broadcast: use recursion rather than ntuple to map
over a tuple -->
- [x] #51153 <!-- fix debug typo in "add missing invoke edge for
nospecialize targets (#51036)" -->
- [x] #51222 <!-- Check again if the tty is open inside the IO lock -->
- [x] #51236 <!-- Add lock around uv_unref during init -->
- [x] #51243 <!-- GMP: Gracefully handle more overflows. -->
- [x] #51254 <!-- Ryu: make sure adding zeros does not overwrite
trailing dot -->
- [x] #51175 <!-- shorten stale_age for cachefile lock -->
- [x] #51300 <!-- fix method definition error for bad vararg -->
- [x] #51307 <!-- fix force-throw ctrl-C on Windows -->
- [x] #51303 <!-- ensure revising structs is safe -->
- [x] #51393 
- [x] #51403 

Need manual backport:
- [x] #51009 <!-- fix #50562, regression in `in` of tuple of Symbols -->
- [x] #51053 <!-- Bump Statistics stdlib -->
- [x] #51013 <!-- fix #50709, type bound error due to free typevar in
sparam env -->
- [x] #51305 <!-- reduce test time for rounding and floatfuncs -->

Contains multiple commits, manual intervention needed:
- [ ] #50663 <!-- Fix Expr(:loopinfo) codegen -->
- [ ] #51035 <!-- refactor GC scanning code to reflect jl_binding_t are
now first class -->
- [ ] #51092 <!-- inference: fix bad effects for recursion -->
- [x] #51247 <!-- Check if malloc has succeeded before incrementing gc
stats -->
- [x] #51294 <!-- use LibGit2_jll for LibGit2 library -->

Non-merged PRs with backport label:
- [ ] #51132 <!-- Handle `AbstractQ` in concatenation -->
- [x] #51029 <!-- testdefs: make sure that if a test set changes the
active project, they change it back when they're done -->
- [ ] #50919 <!-- Code loading: do the "skipping mtime check for stdlib"
check regardless of the value of `ispath(f)` -->
- [ ] #50824 <!-- Add some aliasing warnings to docstrings for mutating
functions -->
- [x] #50385 <!-- Precompile pidlocks: add to NEWS and docs -->
- [ ] #49805 <!-- Limit TimeType subtraction to AbstractDateTime -->

…0929)

This is the part of #50927
required to fix #49249.
Specifically, before this change `tmerge(Tuple{Any, Int}, Nothing)`
would be `Union{Nothing, Tuple{Any, Int}}` but `tmerge(Tuple{BIG_UNION,
Int}, Nothing)` would be `Union{Nothing, Tuple{Any, Any}}`. This feels
bad intuitively because giving the compiler more type information led it
to forget type information that it already knew about, and is especially
damaging because it led to unnecessary type instability when iterating
tuples with complex element types (because the iterator state should be
inferrable as an `Int` even if you have no idea what the tuple type is).

This is tagged for backport to 1.10 since it is a relatively unobtrusive
change and it fixes the string regression in a more proper way.

(cherry picked from commit 6e2e6d0)