Revert changes to get! with callable and add test.

Fixes bug if for example the call to `default` mutated the dict.

base/iddict.jl

@@ -193,17 +193,16 @@ function get(default::Callable, d::IdDict{K,V}, @nospecialize(key)) where {K, V}
 end
 
 function get!(default::Callable, d::IdDict{K,V}, @nospecialize(key)) where {K, V}
-    keyindex, inserted = ht_keyindex!(d, key)
-
-    if inserted
+    val = ccall(:jl_eqtable_get, Any, (Any, Any, Any), d.ht, key, secret_table_token)
+    if val === secret_table_token
         val = default()
-        if !(val isa V) # avoid a dynamic call
+        if !isa(val, V)
             val = convert(V, val)::V
         end
-        _setindex!(d, val, key, keyindex, inserted)
-        return val::V
+        setindex!(d, val, key)
+        return val
     else
-        return @inbounds d.ht[keyindex+1]::V
+        return val::V
     end
 end
 

test/dict.jl

@@ -1610,3 +1610,13 @@ let d = Dict()
     d[1.0] = 'b'
     @test only(d) === Pair{Any,Any}(1.0, 'b')
 end
+
+# Check mutating IdDict during get! works
+let d = IdDict()
+    function f()
+        d[1] = 4
+        return -2
+    end
+    @test get!(f, d, 1) === -2
+    @test length(d) == 1
+end