[PATCH] D75041: [clang-tidy] Extend 'bugprone-easily-swappable-parameters' with mixability because of implicit conversions

[Gabor Marton via Phabricator via cfe-commits]

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?


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https://reviews.llvm.org/D75041