martong added inline comments.
================
Comment at:
clang-tools-extra/clang-tidy/bugprone/EasilySwappableParametersCheck.cpp:657-667
+ // Certain kinds unfortunately need to be side-stepped for canonical type
+ // matching.
+ if (LType->getAs<FunctionProtoType>() || RType->getAs<FunctionProtoType>()) {
+ // Unfortunately, the canonical type of a function pointer becomes the
+ // same even if exactly one is "noexcept" and the other isn't, making us
+ // give a false positive report irrespective of implicit conversions.
+ LLVM_DEBUG(llvm::dbgs()
----------------
aaron.ballman wrote:
> whisperity wrote:
> > @aaron.ballman Getting ahead of the curve here. I understand this is ugly.
> > Unfortunately, no matter how much I read the standard, I don't get anything
> > of "canonical type" mentioned, it seems to me this concept is something
> > inherent to the model of Clang.
> >
> > Basically why this is here: imagine a `void (*)() noexcept` and a `void
> > (*)()`. One's `noexcept`, the other isn't. Inside the AST, this is a
> > `ParenType` of a `PointerType` to a `FunctionProtoType`. There exists a
> > //one-way// implicit conversion from the `noexcept` to the non-noexcept
> > ("noexceptness can be discarded", similarly to how "constness can be
> > gained")
> > Unfortunately, because this is a one-way implicit conversion, it won't
> > return on the code path earlier for implicit conversions.
> >
> > Now because of this, the recursive descent in our code will reach the point
> > when the two innermost `FunctionProtoType`s are in our hands. As a fallback
> > case, we simply ask Clang "Hey, do //you// think these two are the same?".
> > And for some weird reason, Clang will say "Yes."... While one of them is a
> > `noexcept` function and the other one isn't.
> >
> > I do not know the innards of the AST well enough to even have a glimpse of
> > whether or not this is a bug. So that's the reason why I went ahead and
> > implemented this side-stepping logic. Basically, as the comment says,
> > it'lll **force** the information of "No matter what you do, do NOT consider
> > these mixable!" back up the recursion chain, and handle it appropriately
> > later.
> > Unfortunately, no matter how much I read the standard, I don't get anything
> > of "canonical type" mentioned, it seems to me this concept is something
> > inherent to the model of Clang.
>
> It is more of a Clang-centric concept. Basically, a canonical type is one
> that's had all of the typedefs stripped off it.
>
> > Now because of this, the recursive descent in our code will reach the point
> > when the two innermost FunctionProtoTypes are in our hands. As a fallback
> > case, we simply ask Clang "Hey, do you think these two are the same?". And
> > for some weird reason, Clang will say "Yes."... While one of them is a
> > noexcept function and the other one isn't.
>
> I think a confounding factor in this area is that `noexcept` did not used to
> be part of the function type until one day it started being a part of the
> function type. So my guess is that this is fallout from this sort of thing:
> https://godbolt.org/z/EYfj8z (which may be worth keeping in mind when working
> on the check).
About `noexcept`: we've faced a similar problem in the `ASTImporter` library.
We could not import a noexcept function's definition if we already had one
without the noexcept specifier.
Thus, in `ASTStructuralEquivalence.cpp` we do differentiate the function types
based on their noexcept specifier (and we even check the noexcept expression).:
```
TEST_F(StructuralEquivalenceFunctionTest, Noexcept) {
auto t = makeNamedDecls("void foo();",
"void foo() noexcept;", Lang_CXX11);
EXPECT_FALSE(testStructuralMatch(t));
}
TEST_F(StructuralEquivalenceFunctionTest, NoexceptNonMatch) {
auto t = makeNamedDecls("void foo() noexcept(false);",
"void foo() noexcept(true);", Lang_CXX11);
EXPECT_FALSE(testStructuralMatch(t));
}
```
May be in these special cases it would worth to reuse the
ASTStructuralEquivalenceContext class?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D75041/new/
https://reviews.llvm.org/D75041
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