patch 8.2.3345: some code not covered by tests

Problem:    Some code not covered by tests.
Solution:   Add a few more tests. (Dominique Pellé, closes #8757)

src/testdir/test_substitute.vim

@@ -943,4 +943,41 @@ func Test_substitute_skipped_range()
   bwipe!
 endfunc
 
+" Test using the 'gdefault' option (when on, flag 'g' is default on).
+func Test_substitute_gdefault()
+  new
+
+  " First check without 'gdefault'
+  call setline(1, 'foo bar foo')
+  s/foo/FOO/
+  call assert_equal('FOO bar foo', getline(1))
+  call setline(1, 'foo bar foo')
+  s/foo/FOO/g
+  call assert_equal('FOO bar FOO', getline(1))
+  call setline(1, 'foo bar foo')
+  s/foo/FOO/gg
+  call assert_equal('FOO bar foo', getline(1))
+
+  " Then check with 'gdefault'
+  set gdefault
+  call setline(1, 'foo bar foo')
+  s/foo/FOO/
+  call assert_equal('FOO bar FOO', getline(1))
+  call setline(1, 'foo bar foo')
+  s/foo/FOO/g
+  call assert_equal('FOO bar foo', getline(1))
+  call setline(1, 'foo bar foo')
+  s/foo/FOO/gg
+  call assert_equal('FOO bar FOO', getline(1))
+
+  " Setting 'compatible' should reset 'gdefault'
+  call assert_equal(1, &gdefault)
+  set compatible
+  call assert_equal(0, &gdefault)
+  set nocompatible
+  call assert_equal(0, &gdefault)
+
+  bw!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab