findoption_len: treat viminfo/viminfofile as aliases

Ref: https://github.com/neovim/neovim/pull/10672#issuecomment-524716824

src/nvim/option.c

@@ -4643,6 +4643,15 @@ int findoption_len(const char *const arg, const size_t len)
   }
   if (s == NULL) {
     opt_idx = -1;
+  } else {
+    // Nvim: handle option aliases.
+    if (STRNCMP(options[opt_idx].fullname, "viminfo", 7) == 0) {
+      if (STRLEN(options[opt_idx].fullname) == 7) {
+        return findoption_len("shada", 5);
+      }
+      assert(STRCMP(options[opt_idx].fullname, "viminfofile") == 0);
+      return findoption_len("shadafile", 9);
+    }
   }
   return opt_idx;
 }

src/nvim/options.lua

@@ -2659,21 +2659,14 @@ return {
       defaults={if_true={vi="folds,options,cursor,curdir"}}
     },
     {
+      -- Alias for "shada".
       full_name='viminfo', abbreviation='vi',
-      type='string', list='onecomma', scope={'global'},
-      deny_duplicates=true,
-      secure=true,
-      varname='p_shada',
-      defaults={if_true={vi="", vim="!,'100,<50,s10,h"}}
+      type='string', scope={'global'}, nodefault=true,
     },
     {
+      -- Alias for "shadafile".
       full_name='viminfofile', abbreviation='vif',
-      type='string', list='onecomma', scope={'global'},
-      deny_duplicates=true,
-      vi_def=true,
-      secure=true,
-      varname='p_shadafile',
-      defaults={if_true={vi=""}}
+      type='string', scope={'global'}, nodefault=true,
     },
     {
       full_name='virtualedit', abbreviation='ve',

test/functional/options/defaults_spec.lua

@@ -178,6 +178,12 @@ describe('startup defaults', function()
     eq({ f }, eval('v:oldfiles'))
     os.remove('Xtest-foo')
     rmdir('Xtest-userdata')
+
+    -- Handles viminfo/viminfofile as alias for shada/shadafile.
+    eq('\n  shadafile=', eval('execute("set shadafile?")'))
+    eq('\n  shadafile=', eval('execute("set viminfofile?")'))
+    eq("\n  shada=!,'100,<50,s10,h", eval('execute("set shada?")'))
+    eq("\n  shada=!,'100,<50,s10,h", eval('execute("set viminfo?")'))
   end)
 
   it("'packpath'", function()