Allow switch with something that expands to nothing

Meaning empty variables, command substitutions that don't print
anything.

A switch without an argument

```fish
switch
   case ...
end
```

is still a syntax error, and more than one argument is still a runtime
error.

The none-argument matches either an empty-string `case ''` or a
catch-all `case '*'`.

Fixes #5677.

Fixes #4943.

CHANGELOG.md

@@ -20,6 +20,7 @@
 - Local values for `fish_complete_path` and `fish_function_path` are now ignored; only their global values are respected.
 - Empty universal variables may now be exported (#5992).
 - `string split` had a bug where empty strings would be dropped if the output was only empty strings; this has been fixed (#5987).
+- `switch` now allows arguments that expand to nothing, like empty variables (#5677).
 
 ### Syntax changes and new commands
 - Brace expansion now only takes place if the braces include a "," or a variable expansion, so things like `git reset HEAD@{0}` now work (#5869).

src/parse_execution.cpp

@@ -478,9 +478,9 @@ parse_execution_result_t parse_execution_context_t::run_switch_statement(
         }
     }
 
-    if (result == parse_execution_success && switch_values_expanded.size() != 1) {
+    if (result == parse_execution_success && switch_values_expanded.size() > 1) {
         result =
-            report_error(switch_value_n, _(L"switch: Expected exactly one argument, got %lu\n"),
+            report_error(switch_value_n, _(L"switch: Expected at most one argument, got %lu\n"),
                          switch_values_expanded.size());
     }
 
@@ -488,7 +488,7 @@ parse_execution_result_t parse_execution_context_t::run_switch_statement(
         return result;
     }
 
-    const wcstring &switch_value_expanded = switch_values_expanded.at(0).completion;
+    const wcstring &switch_value_expanded = switch_values_expanded.size() == 1 ? switch_values_expanded.at(0).completion : L"";
 
     block_t *sb = parser->push_block(block_t::switch_block());
 

tests/checks/switch.fish

@@ -0,0 +1,71 @@
+#RUN: %fish -C "set fish %fish" %s
+# Check that switch with an argument expanding to nothing still works.
+switch $foo
+    case a
+        echo a
+    case ''
+        echo very little
+    case '*'
+        echo Nothin
+    case ''
+        echo banana
+end
+#CHECK: very little
+
+switch (true)
+    case a
+        echo a
+    case ''
+        echo very little
+    case '*'
+        echo Nothin
+    case ''
+        echo banana
+end
+#CHECK: very little
+
+switch (echo)
+    case a
+        echo a
+    case ''
+        echo very little
+    case '*'
+        echo Nothin
+    case ''
+        echo banana
+end
+#CHECK: very little
+
+# Multiple arguments is still an error, we've not decided on good behavior for this.
+switch (echo; echo; echo)
+    case a
+        echo a
+    case ""
+        echo very little
+    case "*"
+        echo Nothin
+    case ""
+        echo banana
+end
+#CHECKERR: {{.*/?}}switch.fish (line {{\d+}}): switch: Expected at most one argument, got 3
+#CHECKERR:
+#CHECKERR: switch (echo; echo; echo)
+#CHECKERR:       ^
+
+# As is no argument at all.
+# Because this is a syntax error, we need to start a sub-fish or we wouldn't execute anything else.
+$fish -c '
+switch
+    case a
+        echo a
+    case ""
+        echo very little
+    case "*"
+        echo Nothin
+    case ""
+        echo banana
+end
+'
+#CHECKERR: fish: 'case' builtin not inside of switch block
+#CHECKERR:      case a
+#CHECKERR:           ^