[clang] [Clang] prevent null explicit object argument from being deduced (PR #104328)

[Matheus Izvekov via cfe-commits]

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@@ -4462,6 +4462,9 @@ TemplateDeductionResult Sema::DeduceTemplateArguments(
       ParamTypesForArgChecking.push_back(ParamType);
 
       if (ParamIdx == 0 && HasExplicitObject) {
+        if (ObjectType.isNull())
+          return TemplateDeductionResult::InvalidExplicitArguments;
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mizvekov wrote:

Also, if we already diagnosed the error that led to the null template argument, the most appropriate return code would be `TemplateDeductionResult::AlreadyDiagnosed`.

https://github.com/llvm/llvm-project/pull/104328