[clang] [Clang] Check constraints for an explicit instantiation of a member function (PR #104438)

[via cfe-commits]

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@@ -5663,6 +5663,8 @@ def err_explicit_instantiation_internal_linkage : Error<
 def err_explicit_instantiation_not_known : Error<
   "explicit instantiation of %0 does not refer to a function template, "
   "variable template, member function, member class, or static data member">;
+def err_explicit_instantiation_no_candidate : Error<
+    "no candidate for explicit instantiation of %0">;
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cor3ntin wrote:

I think we should the new diagnostic in more places. We did find a matching entity, saying it doesn't exist is not super useful

https://github.com/llvm/llvm-project/pull/104438