If I create an empty QBS project with just one C file (not C++!), Creator will show the following error:

error: invalid argument '-std=gnu++11' not allowed with 'C'

 QBS file:

import qbs

Product {
    type: ["application"]
    Depends { name: "cpp" }
    files: [
        "main.c",
    ]
}

See the screenshot for the rest.

Note: Recently I cannot attach a screenshot when posting the initial bug. It says, "Jira could not attach the file as there was a missing token. Please try attaching the file again."  Screenshot coming in first comment.