ui: change default path fallback mechanism (issue4796)

The previous paths API code always fell back to the default path. This
was wrong because if a requested path doesn't exist, that should error.
Only if no path was requested should we fall back to the default.

As part of implementing the test case for issue 4796, it was discovered
that the "repository does not exist" error message raised by
localrepository.__init__ wasn't being seen because the paths API
validates paths before localrepository.__init__ was being called.
The exception and error message from localrepository.__init__ has
been introduced to getpath(). This necessitated rewriting
expandpath() both to catch the exception and to have proper
default fallback.

This code is more complicated than I'd like. But making all tests pass
was a big chore. As more code moves to getpath(), there will likely be
opportunities to improve things a bit.

mercurial/ui.py

@@ -550,9 +550,21 b' class ui(object):'
 
     def expandpath(self, loc, default=None):
         """Return repository location relative to cwd or from [paths]"""
-        p = self.paths.getpath(loc, default=default)
+        try:
+            p = self.paths.getpath(loc)
         if p:
             return p.rawloc
+        except error.RepoError:
+            pass
+
+        if default:
+            try:
+                p = self.paths.getpath(default)
+                if p:
+                    return p.rawloc
+            except error.RepoError:
+                pass
+
         return loc
 
     @util.propertycache
@@ -1014,9 +1026,24 b' class paths(dict):'
         ``name`` can be a named path or locations. Locations are filesystem
         paths or URIs.
 
-        Returns the first of ``name`` or ``default`` that is present, or None
-        if neither is present.
+        Returns None if ``name`` is not a registered path, a URI, or a local
+        path to a repo.
         """
+        # Only fall back to default if no path was requested.
+        if name is None:
+            if default:
+                try:
+                    return self[default]
+                except KeyError:
+                    return None
+            else:
+                return None
+
+        # Most likely empty string.
+        # This may need to raise in the future.
+        if not name:
+            return None
+
         try:
             return self[name]
         except KeyError:
@@ -1024,15 +1051,10 b' class paths(dict):'
             try:
                 return path(None, rawloc=name)
             except ValueError:
-                pass
+                raise error.RepoError(_('repository %s does not exist') %
+                                        name)
 
-            if default is not None:
-                try:
-                    return self[default]
-                except KeyError:
-                    pass
-
-        return None
+        assert False
 
 class path(object):
     """Represents an individual path and its configuration."""

tests/test-default-push.t

@@ -45,3 +45,9 b" Push should push to 'default-push' when "
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
+
+Pushing to a path that isn't defined should not fall back to default
+
+  $ hg --cwd b push doesnotexist
+  abort: repository doesnotexist does not exist!
+  [255]