[clang] [Clang][RFC] Do not eat SFINAE diagnostics for explicit template arguments (PR #139066)

[Corentin Jabot via cfe-commits]

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@@ -12166,6 +12174,15 @@ static void DiagnoseBadDeduction(Sema &S, NamedDecl *Found, Decl *Templated,
              diag::note_ovl_candidate_explicit_arg_mismatch_unnamed)
           << (index + 1);
     }
+
+    if (PartialDiagnosticAt *PDiag = DeductionFailure.getSFINAEDiagnostic()) {
+      unsigned DiagID =
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cor3ntin wrote:

No, I think this patch should emit the diagnostic as-is and add a `//fixme: attach this diagnostic to the parent error`

(alternatively we can wait for @Sirraide's patch to go in first, and rebase this one on top of it)

https://github.com/llvm/llvm-project/pull/139066