The gcc compiler outputs error messages like:

../SpeedTest/main.cpp:57:48: error: no matching function for call to 'QVector<NoPimpl>::insert(int&, SimplePimpl)'
    vecNP.insert(foundId, SimplePimpl(val,0.));
                                             ^

using ASCII art for the '^' position taking monospaced font into account.

A shortcut to see the error output of the compiler is to hover the mouse pointer over the "red exclamation mark". This pops up a tooltip showing the error text of the compiler.
However, this tooltip text is rendered with proportional font, destroying the ASCII art of the text: the '^' indicator points to the wrong place ....

Using the same font as in the output panel should solve the issue.