Better errors when calling a command in a command substitution fails

CHANGELOG.rst

@@ -20,7 +20,8 @@ Scripting improvements
 - ``echo`` no longer writes its output one byte at a time, improving performance and allowing use with linux' special API files (``/proc``, ``/sys`` and such) (:issue:`7836`).
 - fish should now better handle ``cd`` on filesystems with broken ``stat(3)`` responses (:issue:`7577`).
 - Builtins now properly report a ``$status`` of 1 upon unsuccessful writes (:issue:`7857`).
-- `string match` with unmatched capture groups and without the `--all` flag now sets an empty variable instead of a variable containing the empty string, matching the documentation.
+- ``string match`` with unmatched capture groups and without the ``--all`` flag now sets an empty variable instead of a variable containing the empty string, matching the documentation.
+- Better errors when a command in a command substitution wasn't found or is not allowed.
 
 Interactive improvements
 -------------------------

src/expand.cpp

@@ -652,6 +652,15 @@ static expand_result_t expand_cmdsubst(wcstring input, const operation_context_t
             case STATUS_CMD_ERROR:
                 err = L"Too many active file descriptors";
                 break;
+            case STATUS_CMD_UNKNOWN:
+                err = L"Unknown command";
+                break;
+            case STATUS_ILLEGAL_CMD:
+                err = L"Commandname was invalid";
+                break;
+            case STATUS_NOT_EXECUTABLE:
+                err = L"Command not executable";
+                break;
             default:
                 err = L"Unknown error while evaluating command substitution";
                 break;

tests/checks/switch.fish

@@ -100,3 +100,20 @@ switch $smurf
         echo Test 3 pass
 end
 #CHECK: Test 3 pass
+
+begin
+    set -l PATH
+    switch (doesnotexist)
+        case '*'
+            echo Matched!
+    end
+    # CHECKERR: fish: Unknown command: doesnotexist
+    # CHECKERR: checks/switch.fish (line {{\d+}}):
+    # CHECKERR: doesnotexist
+    # CHECKERR: ^
+    # CHECKERR: in command substitution
+    # CHECKERR: {{\t}}called on line {{\d+}} of file checks/switch.fish
+    # CHECKERR: checks/switch.fish (line {{\d+}}): Unknown command
+    # CHECKERR: switch (doesnotexist)
+    # CHECKERR: ^
+end