Better error message for deprecated scalar-fill indexed assignment, fixes #29266 (#31085)

Author: dourouc05

base/indices.jl

@@ -231,6 +231,9 @@ function setindex_shape_check(X::AbstractArray{<:Any,2}, i::Integer, j::Integer)
     end
 end
 
+setindex_shape_check(::Any...) =
+    throw(ArgumentError("indexed assignment with a single value to many locations is not supported; perhaps use broadcasting `.=` instead?"))
+
 # convert to a supported index type (array or Int)
 """
     to_index(A, i)

test/broadcast.jl

@@ -684,6 +684,15 @@ let n = 1
     @test ceil.(Int, 1 ./ (1,)) == (1,)
 end
 
+# Issue #29266
+@testset "deprecated scalar-fill .=" begin
+    a = fill(1, 10)
+    @test_throws ArgumentError a[1:5] = 0
+
+    x = randn(10)
+    @test_throws ArgumentError x[x .> 0.0] = 0.0
+end
+
 
 # lots of splatting!
 let x = [[1, 4], [2, 5], [3, 6]]