fix #39117, overly-specific type from default_eltype (#39130)

Author: JeffBezanson

base/array.jl

@@ -624,6 +624,20 @@ function _collect_indices(indsA, A)
     copyto!(B, CartesianIndices(axes(B)), A, CartesianIndices(indsA))
 end
 
+# NOTE: this function is not meant to be called, only inferred, for the
+# purpose of bounding the types of values generated by an iterator.
+function _iterator_upper_bound(itr)
+    x = iterate(itr)
+    while x !== nothing
+        val = getfield(x, 1)
+        if inferencebarrier(nothing)
+            return val
+        end
+        x = iterate(itr, getfield(x, 2))
+    end
+    throw(nothing)
+end
+
 # define this as a macro so that the call to Core.Compiler
 # gets inlined into the caller before recursion detection
 # gets a chance to see it, so that recursive calls to the caller
@@ -635,7 +649,7 @@ if isdefined(Core, :Compiler)
             if $I isa Generator && ($I).f isa Type
                 ($I).f
             else
-                Core.Compiler.return_type(first, Tuple{typeof($I)})
+                Core.Compiler.return_type(_iterator_upper_bound, Tuple{typeof($I)})
             end
         end
     end

test/dict.jl

@@ -159,6 +159,9 @@ end
     d = Dict(i==1 ? (1=>2) : (2.0=>3.0) for i=1:2)
     @test isa(d, Dict{Real,Real})
     @test d == Dict{Real,Real}(2.0=>3.0, 1=>2)
+
+    # issue #39117
+    @test Dict(t[1]=>t[2] for t in zip((1,"2"), (2,"2"))) == Dict{Any,Any}(1=>2, "2"=>"2")
 end
 
 @testset "type of Dict constructed from varargs of Pairs" begin