[clang] [clang] Diagnose default arguments defined in different scopes (PR #124844)

[Vlad Serebrennikov via cfe-commits]

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@@ -10960,6 +10960,10 @@ OverloadCandidateSet::BestViableFunction(Sema &S, SourceLocation Loc,
     S.diagnoseEquivalentInternalLinkageDeclarations(Loc, Best->Function,
                                                     EquivalentCands);
 
+  // [over.match.best]/4 is checked for in Sema::ConvertArgumentsForCall,
+  // because not every function call goes through our overload resolution
+  // machinery, even if the Standard says it supposed to.
+
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Endilll wrote:

We have `f(0)` test in DR tests, but I don't understand how `void f(int) {} // best candidate` would bring additional coverage. I also don't understand why we'd return `OR_Ambiguous`.

I think it's worth pointing out that when there is a redeclaration chain, only one declaration out of it is considered a candidate function for overload resolution. At least as far as I saw in the debugger while working on this.

https://github.com/llvm/llvm-project/pull/124844