Add compat annotation for NaN handling in (l|r)mul! (#30361)

stdlib/LinearAlgebra/src/generic.jl

@@ -31,7 +31,15 @@ mul!(C::AbstractArray, X::AbstractArray, s::Number) = generic_mul!(C, s, X)
 """
     rmul!(A::AbstractArray, b::Number)
 
-Scale an array `A` by a scalar `b` overwriting `A` in-place.
+Scale an array `A` by a scalar `b` overwriting `A` in-place.  Use
+[`lmul!`](@ref) to multiply scalar from left.  The scaling operation
+respects the semantics of the multiplication [`*`](@ref) between an
+element of `A` and `b`.  In particular, this also applies to
+multiplication involving non-finite numbers such as `NaN` and `±Inf`.
+
+!!! compat "Julia 1.1"
+    Prior to Julia 1.1, `NaN` and `±Inf` entries in `A` were treated
+    inconsistently.
 
 # Examples
 ```jldoctest
@@ -44,6 +52,10 @@ julia> rmul!(A, 2)
 2×2 Array{Int64,2}:
  2  4
  6  8
+
+julia> rmul!([NaN], 0.0)
+1-element Array{Float64,1}:
+ NaN
 ```
 """
 function rmul!(X::AbstractArray, s::Number)
@@ -57,7 +69,15 @@ end
 """
     lmul!(a::Number, B::AbstractArray)
 
-Scale an array `B` by a scalar `a` overwriting `B` in-place.
+Scale an array `B` by a scalar `a` overwriting `B` in-place.  Use
+[`rmul!`](@ref) to multiply scalar from right.  The scaling operation
+respects the semantics of the multiplication [`*`](@ref) between `a`
+and an element of `B`.  In particular, this also applies to
+multiplication involving non-finite numbers such as `NaN` and `±Inf`.
+
+!!! compat "Julia 1.1"
+    Prior to Julia 1.1, `NaN` and `±Inf` entries in `B` were treated
+    inconsistently.
 
 # Examples
 ```jldoctest
@@ -70,6 +90,10 @@ julia> lmul!(2, B)
 2×2 Array{Int64,2}:
  2  4
  6  8
+
+julia> lmul!(0.0, [Inf])
+1-element Array{Float64,1}:
+ NaN
 ```
 """
 function lmul!(s::Number, X::AbstractArray)

stdlib/LinearAlgebra/test/generic.jl

@@ -389,4 +389,16 @@ end
     @test LinearAlgebra.peakflops() > 0
 end
 
+@testset "NaN handling: Issue 28972" begin
+    @test all(isnan, rmul!([NaN], 0.0))
+    @test all(isnan, rmul!(Any[NaN], 0.0))
+    @test all(isnan, lmul!(0.0, [NaN]))
+    @test all(isnan, lmul!(0.0, Any[NaN]))
+
+    @test all(!isnan, rmul!([NaN], false))
+    @test all(!isnan, rmul!(Any[NaN], false))
+    @test all(!isnan, lmul!(false, [NaN]))
+    @test all(!isnan, lmul!(false, Any[NaN]))
+end
+
 end # module TestGeneric