false positives of bugprone-sizeof-expression with a plain template type argument

[sfc-gh-sgiesecke]

When a template like

template <typename T>
void unalignedStore(void* const dst, const T& src) noexcept {
  std::memcpy(dst, &src, sizeof(T));
}

is instantiated with Foo* such that Foo is an aggregate, then this produces a diagnostic of bugprone-sizeof-expression:

error: suspicious usage of 'sizeof(A*)' on pointer-to-aggregate type; did you mean 'sizeof(A)'?

I'd say that it generally does not make sense to emit a warning if the sizeof expression is of the form sizeof(T) where T is a template type argument.

In addition, even if it makes sense, the diagnostic is not too helpful, as it doesn't mention the context of instantiation or the actual template type argument. A*/A is just a placeholder, which is generally confusing.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Simon Giesecke (sfc-gh-sgiesecke)

When a template like ``` template <typename T> void unalignedStore(void* const dst, const T& src) noexcept { std::memcpy(dst, &src, sizeof(T)); } ``` is instantiated with `Foo*` such that `Foo` is an aggregate, then this produces a diagnostic of `bugprone-sizeof-expression`: ``` error: suspicious usage of 'sizeof(A*)' on pointer-to-aggregate type; did you mean 'sizeof(A)'? ```

I'd say that it generally makes sense to emit a warning if the sizeof expression is of the form sizeof(T) where T is a template type argument.

In addition, even if it makes sense, the diagnostic is not too helpful, as it doesn't mention the context of instantiation or the actual template type argument. A*/A is just a placeholder, which is generally confusing.