[Clang] Fix a crash when using ctad with a template template parameter. (llvm#161488)

This fixes the crash reported in llvm#130604.
It does not try to improve diagnostics or resolve CWG3003 (this will be
explored separately).

Fixes llvm#130604

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -437,6 +437,7 @@ Bug Fixes to C++ Support
 - Fix the result of `__builtin_is_implicit_lifetime` for types with a user-provided constructor. (#GH160610)
 - Correctly deduce return types in ``decltype`` expressions. (#GH160497) (#GH56652) (#GH116319) (#GH161196)
 - Fixed a crash in the pre-C++23 warning for attributes before a lambda declarator (#GH161070).
+- Fix a crash when attempting to deduce a deduction guide from a non deducible template template parameter. (#130604)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -1428,10 +1428,13 @@ void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
     DeclareImplicitDeductionGuidesForTypeAlias(*this, AliasTemplate, Loc);
     return;
   }
-  if (CXXRecordDecl *DefRecord =
-          cast<CXXRecordDecl>(Template->getTemplatedDecl())->getDefinition()) {
+  CXXRecordDecl *DefRecord =
+      dyn_cast_or_null<CXXRecordDecl>(Template->getTemplatedDecl());
+  if (!DefRecord)
+    return;
+  if (const CXXRecordDecl *Definition = DefRecord->getDefinition()) {
     if (TemplateDecl *DescribedTemplate =
-            DefRecord->getDescribedClassTemplate())
+            Definition->getDescribedClassTemplate())
       Template = DescribedTemplate;
   }
 

clang/test/SemaCXX/cxx20-ctad-type-alias.cpp

@@ -586,3 +586,18 @@ Baz a{};
 static_assert(__is_same(decltype(a), A<A<int>>));
 
 } // namespace GH133132
+
+namespace GH130604 {
+template <typename T> struct A {
+    A(T);
+};
+
+template <typename T, template <typename> class TT = A> using Alias = TT<T>; // #gh130604-alias
+template <typename T> using Alias2 = Alias<T>;
+
+Alias2 a(42);
+// expected-error@-1 {{no viable constructor or deduction guide for deduction of template arguments of 'Alias2'}}
+Alias  b(42);
+// expected-error@-1 {{alias template 'Alias' requires template arguments; argument deduction only allowed for class templates or alias template}}
+// expected-note@#gh130604-alias {{template is declared here}}
+}