[Clang][Sema] Check null after ExtractTypeForDeductionGuide (#165776)

Author: ecnelises

clang/docs/ReleaseNotes.rst

@@ -505,6 +505,7 @@ Bug Fixes to C++ Support
   nontrivial member when another member has an initializer. (#GH81774)
 - Fixed a template depth issue when parsing lambdas inside a type constraint. (#GH162092)
 - Diagnose unresolved overload sets in non-dependent compound requirements. (#GH51246) (#GH97753)
+- Fix a crash when extracting unavailable member type from alias in template deduction. (#GH165560)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -659,7 +659,8 @@ struct ConvertConstructorToDeductionGuideTransform {
                 SemaRef, MaterializedTypedefs, NestedPattern,
                 TransformingOuterPatterns ? &Args : nullptr)
                 .transform(NewDI);
-
+    if (!NewDI)
+      return nullptr;
     // Resolving a wording defect, we also inherit default arguments from the
     // constructor.
     ExprResult NewDefArg;

clang/test/SemaTemplate/ctad.cpp

@@ -104,3 +104,15 @@ namespace ConvertDeducedTemplateArgument {
 
   auto x = C(D<A::B>());
 }
+
+namespace pr165560 {
+template <class T, class> struct S {
+  using A = T;
+  template <class> struct I { // expected-note{{candidate function template not viable: requires 1 argument, but 0 were provided}} \
+                              // expected-note{{implicit deduction guide declared as 'template <class> I(pr165560::S<int, int>::I<type-parameter-0-0>) -> pr165560::S<int, int>::I<type-parameter-0-0>'}}
+    I(typename A::F) {} // expected-error{{type 'A' (aka 'int') cannot be used prior to '::' because it has no members}}
+  };
+};
+S<int, int>::I i; // expected-error{{no viable constructor or deduction guide for deduction of template arguments of 'S<int, int>::I'}} \
+                  // expected-note{{while building implicit deduction guide first needed here}}
+}