From 8a971a66952cb039708124d37e7956c68501a25a Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Tue, 10 Jun 2025 15:11:39 -0400
Subject: [PATCH] restore fallback 3-arg `setprecision` method (#58586)

fixes #55899

---------

Co-authored-by: Sukera <11753998+Seelengrab@users.noreply.github.com>
(cherry picked from commit cdb158b58c9f0be4281fef1631c9c6e796532900)
---
 NEWS.md      |  5 +++++
 base/mpfr.jl | 20 ++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/NEWS.md b/NEWS.md
index f2e667378e5a0..9dee73bda6898 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -134,6 +134,8 @@ New library features
 * `Timer` now has readable `timeout` and `interval` properties, and a more descriptive `show` method ([#57081]).
 * `sort` now supports `NTuple`s ([#54494]).
 * `map!(f, A)` now stores the results in `A`, like `map!(f, A, A)` or `A .= f.(A)` ([#40632]).
+* `setprecision` with a function argument (typically a `do` block) is now thread safe. Other forms
+  should be avoided, and types should switch to an implementation using `ScopedValue` ([#51362]).
 
 Standard library changes
 ------------------------
@@ -225,8 +227,10 @@ Tooling Improvements
 [#40989]: https://github.com/JuliaLang/julia/issues/40989
 [#45793]: https://github.com/JuliaLang/julia/issues/45793
 [#49355]: https://github.com/JuliaLang/julia/issues/49355
+[#49933]: https://github.com/JuliaLang/julia/issues/49933
 [#50988]: https://github.com/JuliaLang/julia/issues/50988
 [#51149]: https://github.com/JuliaLang/julia/issues/51149
+[#51362]: https://github.com/JuliaLang/julia/issues/51362
 [#51810]: https://github.com/JuliaLang/julia/issues/51810
 [#52103]: https://github.com/JuliaLang/julia/issues/52103
 [#52999]: https://github.com/JuliaLang/julia/issues/52999
@@ -284,3 +288,4 @@ Tooling Improvements
 [#57087]: https://github.com/JuliaLang/julia/issues/57087
 [#57109]: https://github.com/JuliaLang/julia/issues/57109
 [#57253]: https://github.com/JuliaLang/julia/issues/57253
+[#57727]: https://github.com/JuliaLang/julia/issues/57727
diff --git a/base/mpfr.jl b/base/mpfr.jl
index 0d52510447b2f..2175fb56cc526 100644
--- a/base/mpfr.jl
+++ b/base/mpfr.jl
@@ -1178,9 +1178,29 @@ Often used as `setprecision(T, precision) do ... end`
 Note: `nextfloat()`, `prevfloat()` do not use the precision mentioned by
 `setprecision`.
 
+!!! warning
+    There is a fallback implementation of this method that calls `precision`
+    and `setprecision`, but it should no longer be relied on. Instead, you
+    should define the 3-argument form directly in a way that uses `ScopedValue`,
+    or recommend that callers use `ScopedValue` and `@with` themselves.
+
 !!! compat "Julia 1.8"
     The `base` keyword requires at least Julia 1.8.
 """
+function setprecision(f::Function, ::Type{T}, prec::Integer; kws...) where T
+    depwarn("""
+            The fallback `setprecision(::Function, ...)` method is deprecated. Packages overloading this method should
+            implement their own specialization using `ScopedValue` instead.
+            """, :setprecision)
+    old_prec = precision(T)
+    setprecision(T, prec; kws...)
+    try
+        return f()
+    finally
+        setprecision(T, old_prec)
+    end
+end
+
 function setprecision(f::Function, ::Type{BigFloat}, prec::Integer; base::Integer=2)
     Base.ScopedValues.@with(CURRENT_PRECISION => _convert_precision_from_base(prec, base), f())
 end