Correctly un-export an env var when it is shadowed

Prior to this fix, if you exported a variable in one scope
and then unexported it in the next, it would remain exported.
Example:

    set -gx VAR 1
    function foo; set -l VAR; env; end
    foo

Here 'VAR' would be exported to 'env' because we failed to
notice that the env var is shadowed by an unexported variable.
This occurred at env var computation time, not in env_set!

Fixes #2132

env.cpp

@@ -1264,7 +1264,7 @@ wcstring_list_t env_get_names(int flags)
   Get list of all exported variables
 */
 
-static void get_exported(const env_node_t *n, std::map<wcstring, wcstring> &h)
+static void get_exported(const env_node_t *n, std::map<wcstring, wcstring> *h)
 {
     if (!n)
         return;
@@ -1279,10 +1279,19 @@ static void get_exported(const env_node_t *n, std::map<wcstring, wcstring> &h)
     {
         const wcstring &key = iter->first;
         const var_entry_t &val_entry = iter->second;
-        if (val_entry.exportv && (val_entry.val != ENV_NULL))
+
+        if (val_entry.exportv && val_entry.val != ENV_NULL)
         {
-            // Don't use std::map::insert here, since we need to overwrite existing values from previous scopes
-            h[key] = val_entry.val;
+            // Export the variable
+            // Don't use std::map::insert here, since we need to overwrite existing
+            // values from previous scopes
+            (*h)[key] = val_entry.val;
+        }
+        else
+        {
+            // We need to erase from the map if we are not exporting,
+            // since a lower scope may have exported. See #2132
+            h->erase(key);
         }
     }
 }
@@ -1333,7 +1342,7 @@ static void update_export_array_if_necessary(bool recalc)
 
         debug(4, L"env_export_arr() recalc");
 
-        get_exported(top, vals);
+        get_exported(top, &vals);
 
         if (uvars())
         {

tests/test3.in

@@ -227,6 +227,14 @@ else
   echo Test 16 fail
 end
 
+# Test that shadowing with a non-exported variable works
+set -gx __fish_test_env17 UNSHADOWED
+env | sgrep __fish_test_env17
+function __fish_test_shadow
+  set -l __fish_test_env17
+  env | sgrep __fish_test_env17 ; or echo SHADOWED
+end
+__fish_test_shadow
 
 # clear for other shells
 set -eU __fish_test_universal_variables_variable_foo

tests/test3.out

@@ -16,6 +16,8 @@ Test 15 pass
 Foo change detected
 Foo change detected
 Test 16 pass
+__fish_test_env17=UNSHADOWED
+SHADOWED
 Testing Universal Startup
 1
 1