more consistent findall output (#45538)

fix #45495

Author: aplavin

base/array.jl

@@ -2352,7 +2352,11 @@ julia> findall(x -> x >= 0, d)
 
 ```
 """
-findall(testf::Function, A) = collect(first(p) for p in pairs(A) if testf(last(p)))
+function findall(testf::Function, A)
+    T = eltype(keys(A))
+    gen = (first(p) for p in pairs(A) if testf(last(p)))
+    isconcretetype(T) ? collect(T, gen) : collect(gen)
+end
 
 # Broadcasting is much faster for small testf, and computing
 # integer indices from logical index using findall has a negligible cost

test/functional.jl

@@ -139,6 +139,13 @@ end
 @test findall(!iszero, x^2 for x in -1:0.5:1) == [1, 2, 4, 5]
 @test argmin(x^2 for x in -1:0.5:1) == 3
 
+# findall return type, see #45495
+let gen = (i for i in 1:3);
+    @test @inferred(findall(x -> true, gen))::Vector{Int} == [1, 2, 3]
+    @test @inferred(findall(x -> false, gen))::Vector{Int} == Int[]
+    @test @inferred(findall(x -> x < 0, gen))::Vector{Int} == Int[]
+end
+
 # inference on vararg generator of a type (see #22907 comments)
 let f(x) = collect(Base.Generator(=>, x, x))
     @test @inferred(f((1,2))) == [1=>1, 2=>2]