[jihtsan]

Fixes #61.

An explicit empty operand was left as a nil node, so ${FOO:=} with -no-unset was evaluated like ${FOO} and failed. Initialize an empty text operand when the parser reads a substitution operator; a supplied operand still replaces it normally.

This covers all six existing operators without changing their documented semantics. The regression matrix covers unset, empty and non-empty variables under all four restriction modes, including preservation of -no-empty errors for ${EMPTY-} and ${EMPTY=}. Bare variables still obey restrictions.

Validation on macOS arm64:

• New regression cases fail against the original parser and pass with the fix.
• Go 1.25.5 (the version declared by go.mod): go test -count=1 -vet=off ./... and go test -count=1 -race -vet=off ./... pass.
• Built CLI reproducer passes with -no-unset, with -no-unset -fail-fast, and with -no-unset -no-empty; output is value: plus a newline, exit 0.
• Separate Go 1.27.2 harness: 158 boundary cases pass, including Quick/AllErrors, all operators/restrictions, concatenation and escaping.
• gofmt -d parse/parse.go parse/parse_test.go and git diff --check are clean.

go test ./... with vet enabled fails on both this branch and unchanged base 795eb83 at existing non-constant format strings in cmd/envsubst/main.go:36 and :100. Those lines are unchanged; tests above explicitly disable vet and do not claim that check passes. Linux, Windows and the remaining CI toolchains were not run locally.

[Copilot]

🟢 Approval recommended

The focused fix preserves existing operator semantics, with regression coverage and no unresolved findings.

No open issues 🎉

What changed in this PR

Fixes #61 by distinguishing explicit empty substitution operands from missing operands, allowing ${FOO:=} under -no-unset.

Changes:

• Initializes an empty text operand when parsing a substitution operator.
• Adds regression cases for all six operators across variable states and restriction modes.

File	Description
parse/​parse.go	Preserves explicit empty substitution operands.
parse/​parse_test.go	Adds empty-operand regression coverage.

_{Review effort: Balanced}

[a8m]

Thanks for the contribution, @jihtsan.