[clang] [Clang] Fix crash when ill-formed code is treated as a deduction guide (PR #67373)

[Richard Smith via cfe-commits]

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@@ -171,3 +171,12 @@ namespace CtorTemplateBeatsNonTemplateConversionFn {
   Foo f(Derived d) { return d; } // expected-error {{invokes a deleted function}}
   Foo g(Derived d) { return Foo(d); } // ok, calls constructor
 }
+
+namespace GH65522 {
+template<typename A3>
+class B3 : A3 {
+  template<bool = C3<B3>()> // expected-warning 2{{use of function template name with no prior declaration in function call with explicit}}
+  B3();
+}; B3(); // expected-error {{deduction guide declaration without trailing return type}} \
+         // expected-note {{while building deduction guide here}}
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zygoloid wrote:

or maybe https://godbolt.org/z/sEjTM5z31

https://github.com/llvm/llvm-project/pull/67373