Environment.updating(_:) on a .custom environment causes inheritance

[maxchuquimia]

Describe the bug
When using the instance function Environment.updating(_:) on any existing instance of Environment, Subprocess simply adds the new values to the .inherit environment rather than the instance itself.

To Reproduce

Consider this program that uses a .custom environment and the env command to print the subprocess' perceived environment:

let pristineEnv: Environment = .custom(["A": "1"])

let result1 = try await run(
    .name("env"),
    environment: pristineEnv,
    output: .string(limit: 4096)
)

// Print 1
print(result1.standardOutput ?? "")

let result2 = try await run(
    .name("env"),
    environment: pristineEnv.updating(["B": "2"]),
    output: .string(limit: 4096)
)

// Print 2
print(result2.standardOutput ?? "")

Expected behavior

• At Print 1
  • ✅ The printed environment is exactly "A=1\n"
    • This is expected as the environment is .custom (i.e. not .inherited from the calling process)
• At Print 2
  • ✅ The printed environment includes "B=2"
  • ❌ The printed environment does not include "A=1"
  • ❌ The printed environment includes the entire inherited environment (e.g. USER, HOME etc)

Environment:

• macOS 15.6
• swiftlang-6.1.0.110.21 clang-1700.0.13.3

Additional context
This report is based on my assumption about how the .updating function should work based on it's name, apologies if this behaviour is intended.