If my C++ code has an error detected by clang parsing, and the error message includes a type with template parameters, the parameters were missing; for example saying "no conversion from 'std::unique_ptr' to 'pointer'" but not including the type passed to unique_ptr.

Since the tooltip uses HTML-based rich text formatting, I suspected it could be because the angle brackets are not escaped, and sure enough that was exactly the problem. Here is a test case:

template<typename T>
class SmartPtr {};

class u{};

void foo(int);
void bar() {
    SmartPtr<u> ptr;
    foo(ptr);
}

Clang will complain that 'SmartPtr<u>' cannot be converted to 'int' when calling 'foo', but since the message is not escaped, <u> will be interpreted as the 'underline' HTML tag, it won't be visible as text, and the tooltip contents after that will appear underlined.