deprecate using the value of .=. fixes #25954

src/julia-syntax.scm

@@ -1917,7 +1917,12 @@
 
    '.=
    (lambda (e)
-     (expand-fuse-broadcast (cadr e) (caddr e)))
+     `(ifvalue
+       ,(let ((temp (make-ssavalue)))
+          `(block ,(expand-forms `(= ,temp ,(caddr e)))
+                  ,(expand-fuse-broadcast (cadr e) temp)
+                  ,temp))
+       ,(expand-fuse-broadcast (cadr e) (caddr e))))
 
    '|<:|
    (lambda (e) (expand-forms `(call |<:| ,@(cdr e))))
@@ -3664,6 +3669,10 @@ f(x) = yt(x)
              (if value
                  (compile (cadr e) break-labels value tail)
                  #f))
+            ((ifvalue)
+             (if value
+                 (syntax-deprecation "using the value of `.=`" "" current-loc))
+             (compile (caddr e) break-labels value tail))
             ((if elseif)
              (let ((test `(gotoifnot ,(compile-cond (cadr e) break-labels) _))
                    (end-jump `(goto _))

stdlib/LinearAlgebra/src/diagonal.jl

@@ -242,8 +242,8 @@ end
 *(D::Transpose{<:Any,<:Diagonal}, B::Transpose{<:Any,<:Diagonal}) =
     Diagonal(transpose.(D.parent.diag) .* transpose.(B.parent.diag))
 
-rmul!(A::Diagonal, B::Diagonal) = Diagonal(A.diag .*= B.diag)
-lmul!(A::Diagonal, B::Diagonal) = Diagonal(B.diag .= A.diag .* B.diag)
+rmul!(A::Diagonal, B::Diagonal) = Diagonal((A.diag .*= B.diag; A.diag))
+lmul!(A::Diagonal, B::Diagonal) = Diagonal((B.diag .= A.diag .* B.diag; B.diag))
 
 function lmul!(adjA::Adjoint{<:Any,<:Diagonal}, B::AbstractMatrix)
     A = adjA.parent
@@ -264,13 +264,13 @@ function rmul!(A::AbstractMatrix, transB::Transpose{<:Any,<:Diagonal})
 end
 
 # Get ambiguous method if try to unify AbstractVector/AbstractMatrix here using AbstractVecOrMat
-mul!(out::AbstractVector, A::Diagonal, in::AbstractVector) = out .= A.diag .* in
-mul!(out::AbstractVector, A::Adjoint{<:Any,<:Diagonal}, in::AbstractVector) = out .= adjoint.(A.parent.diag) .* in
-mul!(out::AbstractVector, A::Transpose{<:Any,<:Diagonal}, in::AbstractVector) = out .= transpose.(A.parent.diag) .* in
+mul!(out::AbstractVector, A::Diagonal, in::AbstractVector) = (out .= A.diag .* in; out)
+mul!(out::AbstractVector, A::Adjoint{<:Any,<:Diagonal}, in::AbstractVector) = (out .= adjoint.(A.parent.diag) .* in; out)
+mul!(out::AbstractVector, A::Transpose{<:Any,<:Diagonal}, in::AbstractVector) = (out .= transpose.(A.parent.diag) .* in; out)
 
-mul!(out::AbstractMatrix, A::Diagonal, in::AbstractMatrix) = out .= A.diag .* in
-mul!(out::AbstractMatrix, A::Adjoint{<:Any,<:Diagonal}, in::AbstractMatrix) = out .= adjoint.(A.parent.diag) .* in
-mul!(out::AbstractMatrix, A::Transpose{<:Any,<:Diagonal}, in::AbstractMatrix) = out .= transpose.(A.parent.diag) .* in
+mul!(out::AbstractMatrix, A::Diagonal, in::AbstractMatrix) = (out .= A.diag .* in; out)
+mul!(out::AbstractMatrix, A::Adjoint{<:Any,<:Diagonal}, in::AbstractMatrix) = (out .= adjoint.(A.parent.diag) .* in; out)
+mul!(out::AbstractMatrix, A::Transpose{<:Any,<:Diagonal}, in::AbstractMatrix) = (out .= transpose.(A.parent.diag) .* in; out)
 
 mul!(C::AbstractMatrix, A::Diagonal, B::Adjoint{<:Any,<:AbstractVecOrMat}) = mul!(C, A, copy(B))
 mul!(C::AbstractMatrix, A::Diagonal, B::Transpose{<:Any,<:AbstractVecOrMat}) = mul!(C, A, copy(B))
@@ -292,8 +292,10 @@ mul!(C::AbstractMatrix, A::Transpose{<:Any,<:Diagonal}, B::Transpose{<:Any,<:Abs
 *(adjD::Adjoint{<:Any,<:Diagonal}, adjA::Adjoint{<:Any,<:RealHermSymComplexHerm}) = adjD * adjA.parent
 mul!(C::AbstractMatrix, A::Adjoint{<:Any,<:Diagonal}, B::Adjoint{<:Any,<:RealHermSymComplexHerm}) = mul!(C, A, B.parent)
 mul!(C::AbstractMatrix, A::Transpose{<:Any,<:Diagonal}, B::Transpose{<:Any,<:RealHermSymComplexSym}) = mul!(C, A, B.parent)
-mul!(C::AbstractMatrix, A::Adjoint{<:Any,<:Diagonal}, B::Adjoint{<:Any,<:RealHermSymComplexSym}) = C .= adjoint.(A.parent.diag) .* B
-mul!(C::AbstractMatrix, A::Transpose{<:Any,<:Diagonal}, B::Transpose{<:Any,<:RealHermSymComplexHerm}) = C .= transpose.(A.parent.diag) .* B
+mul!(C::AbstractMatrix, A::Adjoint{<:Any,<:Diagonal}, B::Adjoint{<:Any,<:RealHermSymComplexSym}) =
+    (C .= adjoint.(A.parent.diag) .* B; C)
+mul!(C::AbstractMatrix, A::Transpose{<:Any,<:Diagonal}, B::Transpose{<:Any,<:RealHermSymComplexHerm}) =
+    (C .= transpose.(A.parent.diag) .* B; C)
 
 
 (/)(Da::Diagonal, Db::Diagonal) = Diagonal(Da.diag ./ Db.diag)

test/broadcast.jl

@@ -553,10 +553,11 @@ end
 # Test that broadcasting identity where the input and output Array shapes do not match
 # yields the correct result, not merely a partial copy. See pull request #19895 for discussion.
 let N = 5
-    @test iszero(fill(1, N, N) .= zeros(N, N))
-    @test iszero(fill(1, N, N) .= zeros(N, 1))
-    @test iszero(fill(1, N, N) .= zeros(1, N))
-    @test iszero(fill(1, N, N) .= zeros(1, 1))
+    for rhs in (zeros(N, N), zeros(N, 1), zeros(1, N), zeros(1, 1))
+        local o = fill(1, N, N)
+        o .= rhs
+        @test iszero(o)
+    end
 end
 
 @testset "test broadcast for matrix of matrices" begin
@@ -612,3 +613,16 @@ let n = 1
     @test ceil.(Int, n ./ (1,)) == (1,)
     @test ceil.(Int, 1 ./ (1,)) == (1,)
 end
+
+# issue #25954, value of `.=`
+# TODO: use these if we want `.=` to return its RHS
+#let a = zeros(2, 3), b = zeros(4, 5)
+#    a .= b .= 1
+#    @test a == ones(2, 3)
+#    @test b == ones(4, 5)
+#    @test (b .= 1) === 1
+#    c = [6, 7]; d = [8, 9]
+#    x = (a .= c.+d)
+#    @test a == [14 14 14; 16 16 16]
+#    @test x == [14, 16]
+#end

test/subarray.jl

@@ -524,7 +524,7 @@ let foo = [X]
 end
 
 # test @views macro
-@views let f!(x) = x[1:end-1] .+= x[2:end].^2
+@views let f!(x) = (x[1:end-1] .+= x[2:end].^2; nothing)
     x = [1,2,3,4]
     f!(x)
     @test x == [5,11,19,4]