formula: conditionally use executable name as completion name

Library/Homebrew/formula.rb

@@ -2120,23 +2120,29 @@ def extract_macho_slice_from(file, arch = Hardware::CPU.arch)
   # @param commands
   #   the path to the executable and any passed subcommand(s) to use for generating the completion scripts.
   # @param base_name
-  #   the base name of the generated completion script. Defaults to the formula name.
+  #   the base name of the generated completion script. Defaults to the name of the executable if installed
+  #   within formula's bin or sbin. Otherwise falls back to the formula name.
   # @param shells
   #   the shells to generate completion scripts for. Defaults to `[:bash, :zsh, :fish]`.
   # @param shell_parameter_format
   #   specify how `shells` should each be passed to the `executable`. Takes either a String representing a
   #   prefix, or one of `[:flag, :arg, :none, :click]`. Defaults to plainly passing the shell.
   sig {
     params(
-      commands: T.any(Pathname, String),
-      base_name: String, shells: T::Array[Symbol],
-      shell_parameter_format: T.nilable(T.any(Symbol, String))
+      commands:               T.any(Pathname, String),
+      base_name:              T.nilable(String),
+      shells:                 T::Array[Symbol],
+      shell_parameter_format: T.nilable(T.any(Symbol, String)),
     ).void
   }
   def generate_completions_from_executable(*commands,
-                                           base_name: name,
+                                           base_name: nil,
                                            shells: [:bash, :zsh, :fish],
                                            shell_parameter_format: nil)
+    executable = commands.first.to_s
+    base_name ||= File.basename(executable) if executable.start_with?(bin.to_s, sbin.to_s)
+    base_name ||= name
+
     completion_script_path_map = {
       bash: bash_completion/base_name,
       zsh:  zsh_completion/"_#{base_name}",
@@ -2155,7 +2161,7 @@ def generate_completions_from_executable(*commands,
       elsif shell_parameter_format == :none
         nil
       elsif shell_parameter_format == :click
-        prog_name = File.basename(commands.first.to_s).upcase.tr("-", "_")
+        prog_name = File.basename(executable).upcase.tr("-", "_")
         popen_read_env["_#{prog_name}_COMPLETE"] = "#{shell}_source"
         nil
       else

Library/Homebrew/test/formula_spec.rb

@@ -1897,9 +1897,9 @@ def install
 
     it "generates completion scripts" do
       f.brew { f.install }
-      expect(f.bash_completion/"testball").to be_a_file
-      expect(f.zsh_completion/"_testball").to be_a_file
-      expect(f.fish_completion/"testball.fish").to be_a_file
+      expect(f.bash_completion/"foo").to be_a_file
+      expect(f.zsh_completion/"_foo").to be_a_file
+      expect(f.fish_completion/"foo.fish").to be_a_file
     end
   end