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

[Whisperity via Phabricator via cfe-commits]

whisperity 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()
----------------
whisperity wrote:
> martong wrote:
> > 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?
> Definitely, good catch, @martong, thank you very much! @aaron.ballman, what do you think? If I see this right, `StructuralEquivalenceContext` is part of `libClangAST` so should be readily available.
> 
> The only issue I'm seeing is that this class takes non-**`const`** `ASTContext` and `Decl` nodes...
Well, I started looking into putting up `const` whereever possible into the aforementioned type, but I hit a hurdle. When checking equivalence of records, the algorithm tries to ASTImport-complete the definition of a record if it's not fully defined yet... which is **not** a `const` method... I'll look into it some more, maybe we can do an overload on whether you're Structual equivalence checking in a const context or a non-const context.


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