[cfe-commits] [PATCH] Add smarter sorting of overload candidates that failed template deduction

[Douglas Gregor]

On Sep 8, 2011, at 4:43 PM, Kaelyn Uhrain wrote:

> Given a file tmp.cpp containing:
> 
> [snip example]
> 
> When Clang emits the error about not finding a match for that call to f because exit is an overloaded function, Clang currently emits notes for all of the function template candidates in lexical order, to where the "closest" failing match is the 6th note of 9 with 3 candidates that require a different number of arguments listed before it:
> 
> [snip example]
> 
> 
> This small patch adds rankings to the different types of template deduction failure (all of the OverloadCandidates above have a base FailureKind of ovl_fail_bad_deduction), so that e.g. a candidate where a template argument could not be inferred is listed before a candidate that requires the wrong number of arguments. So for the above code example, the output becomes:
> 
> [snip much-improved example]
> 
> While the text of the note still isn't particularly useful for figuring out what failed, it at least gives a little better clue as to where the problem lies (in this example, the problem is rooted in there being two different single-argument declarations of "exit"). Feedback is both welcome and desired, especially on the ranking of the different types of deduction failures.


I like this a lot! We could bike-shed on the specific rankings a lot, so I'll just make a few comments about the ranking:

	- TDK_InvalidExplicitArguments should, IMO, have a fairly poor ranking (4 or 5), because users are being very explicit about these arguments and the you generally have to be *very* wrong for template arguments to not match their template parameters (e.g., providing an expression where a type was expected)
	- I suggest that TDK_SubstitutionFailure and TDK_NonDeducedMismatch should move up to rank 3, because these can often mean some little failure at the call site


+  default:
+    return UINT_MAX;
+  }

Why do we need a default here?

	- Doug