[clang] [clang] Implement CWG1878 "`operator auto` template" (PR #78103)

[via cfe-commits]

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@@ -686,3 +686,19 @@ auto f(auto x) { // cxx14-error {{'auto' not allowed in function prototype}}
 }
 
 }
+
+struct DeducedTargetTypeOfConversionFunction {
+  operator auto() const { return char(); }
+  operator const auto() const { return float(); }
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cor3ntin wrote:

can you add tests with references?

https://github.com/llvm/llvm-project/pull/78103