--- a/mercurial/configitems.py	Wed Aug 15 22:24:38 2018 +0900
+++ b/mercurial/configitems.py	Wed Aug 15 22:24:50 2018 +0900
@@ -771,6 +771,9 @@
 coreconfigitem('merge', 'preferancestor',
         default=lambda: ['*'],
 )
+coreconfigitem('merge', 'strict-capability-check',
+    default=False,
+)
 coreconfigitem('merge-tools', '.*',
     default=None,
     generic=True,

--- a/mercurial/filemerge.py	Wed Aug 15 22:24:38 2018 +0900
+++ b/mercurial/filemerge.py	Wed Aug 15 22:24:50 2018 +0900
@@ -137,6 +137,8 @@
     return procutil.findexe(util.expandpath(exe))
 
 def _picktool(repo, ui, path, binary, symlink, changedelete):
+    strictcheck = ui.configbool('merge', 'strict-capability-check')
+
     def hascapability(tool, capability, strict=False):
         if strict and tool in internals:
             if internals[tool].capabilities.get(capability):
@@ -155,9 +157,9 @@
                 ui.warn(_("couldn't find merge tool %s\n") % tmsg)
             else: # configured but non-existing tools are more silent
                 ui.note(_("couldn't find merge tool %s\n") % tmsg)
-        elif symlink and not hascapability(tool, "symlink"):
+        elif symlink and not hascapability(tool, "symlink", strictcheck):
             ui.warn(_("tool %s can't handle symlinks\n") % tmsg)
-        elif binary and not hascapability(tool, "binary"):
+        elif binary and not hascapability(tool, "binary", strictcheck):
             ui.warn(_("tool %s can't handle binary\n") % tmsg)
         elif changedelete and not supportscd(tool):
             # the nomerge tools are the only tools that support change/delete
@@ -192,9 +194,13 @@
             return (hgmerge, hgmerge)
 
     # then patterns
+
+    # whether binary capability should be checked strictly
+    binarycap = binary and strictcheck
+
     for pat, tool in ui.configitems("merge-patterns"):
         mf = match.match(repo.root, '', [pat])
-        if mf(path) and check(tool, pat, symlink, False, changedelete):
+        if mf(path) and check(tool, pat, symlink, binarycap, changedelete):
             if binary and not hascapability(tool, "binary", strict=True):
                 ui.warn(_("warning: check merge-patterns configurations,"
                           " if %r for binary file %r is unintentional\n"

--- a/mercurial/help/config.txt	Wed Aug 15 22:24:38 2018 +0900
+++ b/mercurial/help/config.txt	Wed Aug 15 22:24:50 2018 +0900
@@ -1347,6 +1347,11 @@
    halted, the repository is left in a normal ``unresolved`` merge state.
    (default: ``continue``)
 
+``strict-capability-check``
+   Whether capabilities of internal merge tools are checked strictly
+   or not, while examining rules to decide merge tool to be used.
+   (default: False)
+
 ``merge-patterns``
 ------------------
 

--- a/mercurial/help/merge-tools.txt	Wed Aug 15 22:24:38 2018 +0900
+++ b/mercurial/help/merge-tools.txt	Wed Aug 15 22:24:50 2018 +0900
@@ -80,10 +80,14 @@
 ==== =============== ====== =======
 1.   --tool          o      o
 2.   HGMERGE         o      o
-3.   merge-patterns  o      x
-4.   ui.merge        x      x
+3.   merge-patterns  o (*)  x (*)
+4.   ui.merge        x (*)  x (*)
 ==== =============== ====== =======
 
+If ``merge.strict-capability-check`` configuration is true, Mercurial
+checks capabilities of internal merge tools strictly in (*) cases
+above. It is false by default for backward compatibility.
+
 .. note::
 
    After selecting a merge program, Mercurial will by default attempt

--- a/tests/test-help.t	Wed Aug 15 22:24:38 2018 +0900
+++ b/tests/test-help.t	Wed Aug 15 22:24:50 2018 +0900
@@ -1913,8 +1913,12 @@
       ----------------------------------
       1.   --tool         o      o
       2.   HGMERGE        o      o
-      3.   merge-patterns o      x
-      4.   ui.merge       x      x
+      3.   merge-patterns o (*)  x (*)
+      4.   ui.merge       x (*)  x (*)
+  
+      If "merge.strict-capability-check" configuration is true, Mercurial checks
+      capabilities of internal merge tools strictly in (*) cases above. It is
+      false by default for backward compatibility.
   
       Note:
          After selecting a merge program, Mercurial will by default attempt to

--- a/tests/test-merge-tools.t	Wed Aug 15 22:24:38 2018 +0900
+++ b/tests/test-merge-tools.t	Wed Aug 15 22:24:50 2018 +0900
@@ -1840,6 +1840,51 @@
   [1]
   $ hg merge --abort -q
 
+(for ui.merge, ignored unintentionally)
+
+  $ hg merge 9 \
+  > --config ui.merge=:other
+  tool :other (for pattern b) can't handle binary
+  tool true can't handle binary
+  tool false can't handle binary
+  no tool found to merge b
+  keep (l)ocal [working copy], take (o)ther [merge rev], or leave (u)nresolved for b? u
+  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges or 'hg merge --abort' to abandon
+  [1]
+  $ hg merge --abort -q
+
+With merge.strict-capability-check=true, binary files capability of
+internal merge tools is checked strictly.
+
+  $ f --hexdump b
+  b:
+  0000: 03 02 01 00                                     |....|
+
+(for merge-patterns)
+
+  $ hg merge 9 --config merge.strict-capability-check=true \
+  > --config merge-patterns.b=:merge-other \
+  > --config merge-patterns.re:[a-z]=:other
+  tool :merge-other (for pattern b) can't handle binary
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ f --hexdump b
+  b:
+  0000: 00 01 02 03                                     |....|
+  $ hg merge --abort -q
+
+(for ui.merge)
+
+  $ hg merge 9 --config merge.strict-capability-check=true \
+  > --config ui.merge=:other
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ f --hexdump b
+  b:
+  0000: 00 01 02 03                                     |....|
+  $ hg merge --abort -q
+
 Check that debugpicktool examines which merge tool is chosen for
 specified file as expected
 
@@ -1883,6 +1928,36 @@
   $ hg debugpickmergetool -r 6d00b3726f6e
   f = :prompt
 
+(by default, it is assumed that no internal merge tools has symlinks
+capability)
+
+  $ hg debugpickmergetool \
+  > -r 6d00b3726f6e \
+  > --config merge-patterns.f=:merge-other \
+  > --config merge-patterns.re:[f]=:merge-local \
+  > --config merge-patterns.re:[a-z]=:other
+  f = :prompt
+
+  $ hg debugpickmergetool \
+  > -r 6d00b3726f6e \
+  > --config ui.merge=:other
+  f = :prompt
+
+(with strict-capability-check=true, actual symlink capabilities are
+checked striclty)
+
+  $ hg debugpickmergetool --config merge.strict-capability-check=true \
+  > -r 6d00b3726f6e \
+  > --config merge-patterns.f=:merge-other \
+  > --config merge-patterns.re:[f]=:merge-local \
+  > --config merge-patterns.re:[a-z]=:other
+  f = :other
+
+  $ hg debugpickmergetool --config merge.strict-capability-check=true \
+  > -r 6d00b3726f6e \
+  > --config ui.merge=:other
+  f = :other
+
 #endif
 
 (--verbose shows some configurations)