fix: completion generation inside $()

Command substitution completion was incorrectly setting the `prev`
variable due to the `_comp_initialize` function reassigning it from
the original completion context after processing the substitution.

The fix ensures that `words` and `cword` variables are also updated
to match the command substitution context, preventing the later
reassignment from overriding the correct `prev` value.

Unit test added to validate command substitution correctly sets
`COMP_LINE`, `COMP_CWORD`, `cur`, and `prev` variables for both single
commands and commands with arguments.

Author: richardkmichael

bash_completion

@@ -1483,6 +1483,27 @@ _comp_initialize()
     local redir='@(?(+([0-9])|{[a-zA-Z_]*([a-zA-Z_0-9])})@(>?([>|&])|<?([>&])|<<?([-<]))|&>?(>))'
     _comp_get_words -n "$exclude<>&" cur prev words cword
 
+    # If the current word is a command substitution $(, reset the completion context to be the
+    # command line *inside* the substitution.
+    if [[ $cur == '$('* ]]; then
+        local inner_line=${cur#\$\(}
+
+        # Replace the completion variables with ones that reflect the inner context.
+        COMP_LINE=$inner_line
+        read -ra COMP_WORDS <<< "$inner_line"
+        COMP_CWORD=$(( ${#COMP_WORDS[@]} - 1 ))
+        cur=${COMP_WORDS[COMP_CWORD]}
+        # Also update words and cword used by the rest of _comp_initialize
+        words=("${COMP_WORDS[@]}")
+        cword=$COMP_CWORD
+        # Handle case where there's only one word in the substitution
+        if (( COMP_CWORD > 0 )); then
+            prev=${COMP_WORDS[COMP_CWORD-1]}
+        else
+            prev=""
+        fi
+    fi
+
     # Complete variable names.
     _comp_compgen_variables && return 1
 

test/t/unit/test_unit_command_substitution.py

@@ -0,0 +1,45 @@
+import pytest
+
+from conftest import TestUnitBase
+
+
+@pytest.mark.bashcomp(
+    cmd=None,
+    ignore_env=r"^[+-](COMP(_(WORDS|CWORD|LINE|POINT)|REPLY)|"
+    r"cur|prev|cword|words)=",
+)
+class TestUnitCommandSubstitution(TestUnitBase):
+    def test_command_substitution_completion(self, bash):
+        """Test that command substitution completion works correctly"""
+        # Test basic command substitution: $(echo
+        output = self._test_unit(
+            "_comp_initialize %s; echo $COMP_LINE,$COMP_CWORD,$cur,$prev",
+            bash,
+            "(echo '$(echo')",
+            1,
+            "echo '$(echo'",
+            12,
+        )
+        assert output == "echo,0,echo,"
+
+        # Test command substitution with arguments: $(ls -l
+        output = self._test_unit(
+            "_comp_initialize %s; echo $COMP_LINE,$COMP_CWORD,$cur,$prev",
+            bash,
+            "(echo '$(ls -l')",
+            1,
+            "echo '$(ls -l'",
+            13,
+        )
+        assert output == "ls -l,1,-l,ls"
+
+        # Test that normal completion is not affected
+        output = self._test_unit(
+            "_comp_initialize %s; echo $COMP_LINE,$COMP_CWORD,$cur,$prev",
+            bash,
+            "(echo hello)",
+            1,
+            "echo hello",
+            10,
+        )
+        assert output == "echo hello,1,hello,echo"