fix map! undefined value exposure

base/abstractarray.jl

@@ -3417,12 +3417,19 @@ function ith_all(i, as)
 end
 
 function map_n!(f::F, dest::AbstractArray, As) where F
-    idxs1 = LinearIndices(As[1])
-    @boundscheck LinearIndices(dest) == idxs1 && all(x -> LinearIndices(x) == idxs1, As)
-    for i = idxs1
-        @inbounds I = ith_all(i, As)
-        val = f(I...)
-        @inbounds dest[i] = val
+    idxs = LinearIndices(dest)
+    if all(x -> LinearIndices(x) == idxs, As)
+        for i in idxs
+            @inbounds as = ith_all(i, As)
+            val = f(as...)
+            @inbounds dest[i] = val
+        end
+    else
+        for (i, Is...) in zip(eachindex(dest), map(eachindex, As)...)
+            as = ntuple(j->getindex(As[j], Is[j]), length(As))
+            val = f(as...)
+            dest[i] = val
+        end
     end
     return dest
 end

test/abstractarray.jl

@@ -910,6 +910,26 @@ include("generic_map_tests.jl")
 generic_map_tests(map, map!)
 @test map!(-, [1]) == [-1]
 
+@testset "#30624" begin
+    ### unstructured
+    @test map!(+, ones(3), ones(3), ones(3), [1]) == [3, 1, 1]
+    @test map!(+, ones(3), [1], ones(3), ones(3)) == [3, 1, 1]
+    @test map!(+, [1], [1], [], []) == [1]
+    @test map!(+, [[1]], [1], [], []) == [[1]]
+
+    # TODO: decide if input axes & lengths should be validated
+    # @test_throws BoundsError map!(+, ones(1), ones(2))
+    # @test_throws BoundsError map!(+, ones(1), ones(2, 2))
+
+    @test map!(+, ones(3), view(ones(2, 3), 1:2, 2:3), ones(3)) == [2, 2, 2]
+    @test map!(+, ones(3), ones(2, 2), ones(3)) == [2, 2, 2]
+
+    ### structured (all mapped arguments are <:AbstractArray equal ndims > 1)
+    @test map!(+, ones(4), ones(2, 2), ones(2, 2)) == [2, 2, 2, 2]
+    @test map!(+, ones(4), ones(2, 2), ones(1, 2)) == [2, 2, 1, 1]
+    # @test_throws BoundsError map!(+, ones(3), ones(2, 2), ones(2, 2))
+end
+
 test_UInt_indexing(TestAbstractArray)
 test_13315(TestAbstractArray)
 test_checksquare()