add isimmutabletype

NEWS.md

@@ -130,6 +130,9 @@ This section lists changes that do not have deprecation warnings.
     (since it is shorthand for `NTuple{N,T} where T`). To get the old behavior of matching
     any tuple, use `NTuple{N,Any}` ([#18457]).
 
+  * `isimmutable(T::Type)` now checks whether `T` is an immutable type,
+    rather than checking whether `typeof(T)` is an immutable type ([#18168]).
+
   * `FloatRange` has been replaced by `StepRangeLen`, and the internal
     representation of `LinSpace` has changed. Aside from changes in
     the internal field names, this leads to several differences in
@@ -156,7 +159,7 @@ This section lists changes that do not have deprecation warnings.
       and `step::FloatNN` to rationals and construct a `StepRangeLen`
       that internally uses twice-precision arithmetic.  These two
       outcomes exhibit differences in both precision and speed.
-
+      
 Library improvements
 --------------------
 
@@ -914,6 +917,7 @@ Language tooling improvements
 [#17758]: https://github.com/JuliaLang/julia/issues/17758
 [#17785]: https://github.com/JuliaLang/julia/issues/17785
 [#18050]: https://github.com/JuliaLang/julia/issues/18050
+[#18168]: https://github.com/JuliaLang/julia/issues/18168
 [#18330]: https://github.com/JuliaLang/julia/issues/18330
 [#18339]: https://github.com/JuliaLang/julia/issues/18339
 [#18346]: https://github.com/JuliaLang/julia/issues/18346

base/reflection.jl

@@ -195,13 +195,15 @@ datatype_fielddesc_type(dt::DataType) = dt.layout == C_NULL ? throw(UndefRefErro
     (unsafe_load(convert(Ptr{DataTypeLayout}, dt.layout)).alignment >> 30) & 3
 
 """
-    isimmutable(v)
+    isimmutable(x)
 
-Return `true` iff value `v` is immutable.  See [Immutable Composite Types](@ref)
-for a discussion of immutability. Note that this function works on values, so if you give it
-a type, it will tell you that a value of `DataType` is mutable.
+Return `true` if the value `x` is immutable; if `x` is a type, then returns
+whether `x` is an immutable type.  See [Immutable Composite Types](@ref)
+for a discussion of immutability.
 """
 isimmutable(x::ANY) = (@_pure_meta; (isa(x,Tuple) || !typeof(x).mutable))
+isimmutable(t::DataType) = (@_pure_meta; !t.mutable)
+isimmutable(::Type) = (@_pure_meta; false)
 isstructtype(t::DataType) = (@_pure_meta; nfields(t) != 0 || (t.size==0 && !t.abstract))
 isstructtype(x) = (@_pure_meta; false)
 

test/reflection.jl

@@ -159,6 +159,15 @@ not_const = 1
 
 @test isimmutable(1) == true
 @test isimmutable([]) == false
+@test isimmutable((3,4,5)) == true
+@test isimmutable(Int) == true
+@test isimmutable(Vector{Int}) == false
+@test isimmutable(Tuple{Int}) == true
+
+# should String in 0.6 be immutable?
+@test_broken isimmutable("abc") == true
+@test_broken isimmutable(String) == true
+
 
 ## find bindings tests
 @test ccall(:jl_get_module_of_binding, Any, (Any, Any), Base, :sin)==Base