diff --git a/tests/test-issue3084.t b/tests/test-issue3084.t
--- a/tests/test-issue3084.t
+++ b/tests/test-issue3084.t
@@ -137,3 +137,221 @@ Updating from normal to largefile - no r
   large
 
   $ cd ..
+
+
+Systematic testing of merges involving largefiles:
+
+Ancestor: normal  Parent: normal=  Parent: large   result: large
+Ancestor: normal  Parent: normal2  Parent: large   result: ?
+Ancestor: large   Parent: large=   Parent: normal  result: normal
+Ancestor: large   Parent: large2   Parent: normal  result: ?
+
+All cases should try merging both ways.
+"=" means same file content.
+
+Prepare test repo:
+
+  $ hg init merges
+  $ cd merges
+  $ touch f1
+  $ hg ci -Aqm "0-root"
+
+ancestor is "normal":
+  $ echo normal > f
+  $ hg ci -Aqm "1-normal-ancestor"
+  $ touch f2
+  $ hg ci -Aqm "2-normal-unchanged"
+  $ hg tag -l "normal="
+  $ echo normal2 > f
+  $ hg ci -m "3-normal2"
+  $ hg tag -l "normal2"
+  $ hg up -qr 1
+  $ hg rm f
+  $ echo large > f
+  $ hg add --large f
+  $ hg ci -qm "4-normal-to-large"
+  $ hg tag -l "large"
+
+  $ hg up -qr null
+
+ancestor is "large":
+  $ echo large > f
+  $ hg add --large f
+  $ hg ci -qm "5-large-ancestor"
+  $ touch f2
+  $ hg ci -Aqm "6-large-unchanged"
+  $ hg tag -l "large="
+  $ echo large2 > f
+  $ hg ci -m "7-large2"
+  $ hg tag -l "large2"
+  $ hg up -qr 5
+  $ hg rm f
+  $ echo normal > f
+  $ hg ci -qAm "8-large-to-normal"
+  $ hg tag -l "normal"
+
+Ancestor: normal  Parent: normal=  Parent: large   result: large
+
+  $ hg up -Cqr normal=
+  $ hg merge -r large
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  1 largefiles updated, 0 removed
+  $ cat f
+  large
+
+swap
+
+  $ hg up -Cqr large
+  $ hg merge -r normal=
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  0 largefiles updated, 0 removed
+  $ cat f
+  large
+
+Ancestor: normal  Parent: normal2  Parent: large   result: ?
+(annoying extra prompt ... but it do not do any serious harm)
+
+  $ hg up -Cqr normal2
+  $ hg merge -r large
+  local changed f which remote deleted
+  use (c)hanged version or (d)elete? c
+  f has been turned into a largefile
+  use (l)argefile or keep as (n)ormal file? l
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  1 largefiles updated, 0 removed
+  $ cat f
+  large
+
+  $ hg up -Cqr normal2
+  $ ( echo c; echo n ) | hg merge -r large --config ui.interactive=Yes
+  local changed f which remote deleted
+  use (c)hanged version or (d)elete? f has been turned into a largefile
+  use (l)argefile or keep as (n)ormal file? 0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  0 largefiles updated, 0 removed
+  $ cat f
+  normal2
+
+  $ hg up -Cqr normal2
+  $ echo d | hg merge -r large --config ui.interactive=Yes
+  local changed f which remote deleted
+  use (c)hanged version or (d)elete? 1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  1 largefiles updated, 0 removed
+  $ cat f
+  large
+
+swap
+
+  $ hg up -Cqr large
+  $ hg merge -r normal2
+  remote changed f which local deleted
+  use (c)hanged version or leave (d)eleted? c
+  f has been turned into a normal file
+  keep as (l)argefile or use (n)ormal file? l
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  1 largefiles updated, 0 removed
+  $ cat f
+  large
+
+  $ hg up -Cqr large
+  $ ( echo c; echo n ) | hg merge -r normal2 --config ui.interactive=Yes
+  remote changed f which local deleted
+  use (c)hanged version or leave (d)eleted? f has been turned into a normal file
+  keep as (l)argefile or use (n)ormal file? 2 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  0 largefiles updated, 0 removed
+  $ cat f
+  normal2
+
+  $ hg up -Cqr large
+  $ echo d | hg merge -r normal2 --config ui.interactive=Yes
+  remote changed f which local deleted
+  use (c)hanged version or leave (d)eleted? 1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  0 largefiles updated, 0 removed
+  $ cat f
+  large
+
+Ancestor: large   Parent: large=   Parent: normal  result: normal
+
+  $ hg up -Cqr large=
+  $ hg merge -r normal
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  0 largefiles updated, 0 removed
+  $ cat f
+  normal
+
+swap
+
+  $ hg up -Cqr normal
+  $ hg merge -r large=
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ cat f
+  normal
+
+Ancestor: large   Parent: large2   Parent: normal  result: ?
+(annoying extra prompt ... but it do not do any serious harm)
+
+  $ hg up -Cqr large2
+  $ hg merge -r normal
+  local changed .hglf/f which remote deleted
+  use (c)hanged version or (d)elete? c
+  f has been turned into a normal file
+  keep as (l)argefile or use (n)ormal file? l
+  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  1 largefiles updated, 0 removed
+  $ cat f
+  large2
+
+  $ hg up -Cqr large2
+  $ echo d | hg merge -r normal --config ui.interactive=Yes
+  local changed .hglf/f which remote deleted
+  use (c)hanged version or (d)elete? 1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  0 largefiles updated, 0 removed
+  $ cat f
+  normal
+
+swap
+
+  $ hg up -Cqr normal
+  $ hg merge -r large2
+  remote changed .hglf/f which local deleted
+  use (c)hanged version or leave (d)eleted? c
+  f has been turned into a largefile
+  use (l)argefile or keep as (n)ormal file? l
+  2 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  getting changed largefiles
+  1 largefiles updated, 0 removed
+  $ cat f
+  large2
+
+  $ hg up -Cqr normal
+  $ echo d | hg merge -r large2 --config ui.interactive=Yes
+  remote changed .hglf/f which local deleted
+  use (c)hanged version or leave (d)eleted? 1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ cat f
+  normal
+
+  $ cd ..