`deepMerge` types is broken

[NamesMT]

Describe the bug

Output types of deepMerge is broken, observed with plain simple primitive and typed variable.

Reproduction

https://github.com/NamesMT/starter-ts/tree/antfu/utils_repro

System Info

System:
    OS: Linux 6.6 Arch Linux (WSL2)
    CPU: (24) x64 AMD Ryzen AI 9 HX 370 w/ Radeon 890M
    Memory: 22.71 GB / 27.21 GB
    Container: Yes
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.19.0 - /run/user/1000/fnm_multishells/48943_1763653676545/bin/node
    npm: 10.9.3 - /run/user/1000/fnm_multishells/48943_1763653676545/bin/npm
    pnpm: 10.20.0 - /run/user/1000/fnm_multishells/48943_1763653676545/bin/pnpm
    bun: 1.3.2 - /home/mt/.bun/bin/bun

Used Package Manager

pnpm

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A, please open a GitHub Discussion instead.
• The provided reproduction is a minimal reproducible of the bug.

Contributions

• I am willing to submit a PR to fix this issue
• I am willing to submit a PR with failing tests (actually just go ahead and do it, thanks!)

[NamesMT]

Base code

Output type of strMerge:

Output type of numMerge:

Output type of typedInterfaceMerge: