[PATCH] D56731: Add -Wimplicit-ctad warning to diagnose CTAD on types with no user defined deduction guides.

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

I like @Quuxplusone's suggestions for more heuristics here, but I think they should probably live under a different warning flag (collectively grouped under `-Wctad` as @lebedev.ri suggested). Concretely, I think we could warn if, during template instantiation from a type-dependent initializer, we select the copy deduction candidate and there was another viable candidate (the `optional o(x)` case, the `tuple t{args...}`case, etc), maybe under a new `-Wctad-unwrap` or similar.

I don't see a good general way to catch and warn on bugs like `vector<string> v("foo", "bar")` (this problem isn't really specific to CTAD), but we could certainly add some special-case heuristics to detect cases where a pair of string literals is passed to a constructor of a container-like type (for example, we could check whether the constructor looks like it can be called with arbitrary iterators as arguments, the class also has an `initializer_list<T>` constructor where a string literal can be implicitly converted to `T`, and the class has `begin` and `end` member functions).



================
Comment at: include/clang/Basic/DiagnosticGroups.td:1054
+
+def ImplicitCTADUsage : DiagGroup<"implicit-ctad">;
----------------
lebedev.ri wrote:
> Should this be in some group?
> Alternatively, would it make sense to add it to new `-Wctad` group?
Let's not add a `-Wctad` until we have more than one thing to put in it. I expect that to happen relatively soon :)


================
Comment at: include/clang/Basic/DiagnosticSemaKinds.td:2129
+def warn_class_template_argument_deduction_no_user_defined_guides : Warning<
+  "using class template argument deduction for %0 that has no user-defined deduction guides" >,
+  InGroup<ImplicitCTADUsage>, DefaultIgnore;
----------------
gromer wrote:
> I'd prefer a phrasing more along the lines of "using class template argument deduction for %0 that might not intentionally support it". That gives us more room to do things like add an attribute later if necessary, and it helps the user understand why this warning indicates a potential problem.
I like that approach; something like "using class template argument deduction for %0 that might not intend to support it" -- or perhaps more directly "%0 might not intend to support class template argument deduction" -- along with a note describing how to syntactically suppress the warning (w"add a deduction guide to suppress this warning" or "use the [[clang::ctad]] attribute to suppress this warning" or whatever we decide is best).


================
Comment at: lib/Sema/SemaInit.cpp:9268
+
+  // Record if at least one user-defined deduction guide was considered
+  bool HasUserDefinedDeductionGuideCandidate = false;
----------------
Nit: LLVM / Clang style wants a period at the end of a comment.


================
Comment at: lib/Sema/SemaInit.cpp:9287
 
+      HasUserDeclaredDeductionGuideCandidate |= !GD->isImplicit();
+
----------------
Quuxplusone wrote:
> Nitpick: I don't know if this is LLVM style, but I wish this were written as
> 
>     if (!GD->isImplicit())
>       HasUserDeclaredDeductionGuideCandidate = true;
> 
> for clarity. Also, is it "user-defined" (per the error message) or "user-declared" (per the name of this variable)?
Formally, it's actually just "any deduction guides". Constructors aren't transformed into deduction guides; rather, deduction guides and constructors both form candidates for CTAD. So `HasAnyDeductionGuides` would work. (I also think we should omit the "candidates", because we don't care whether any deduction guides were candidates for this particular overload resolution, only whether there are any at all -- if we're excluding explicit deduction guides and the only deduction guides are explicit, we still want to (and do!) suppress the warning.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D56731/new/

https://reviews.llvm.org/D56731