[Clang] Fix a crash when using ctad with a template template parameter.

[cor3ntin]

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

Fixes #130604

[llvmbot]

@llvm/pr-subscribers-clang

Author: Corentin Jabot (cor3ntin)

Changes

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

Fixes #130604

---

Full diff: https://github.com/llvm/llvm-project/pull/161488.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/Sema/SemaTemplateDeductionGuide.cpp (+5-2)
• (modified) clang/test/SemaCXX/cxx20-ctad-type-alias.cpp (+15)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index e8deae50e4cb0..4aedcddc1146d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -433,6 +433,7 @@ Bug Fixes to C++ Support
   object type. (#GH151531)
 - Suppress ``-Wdouble-promotion`` when explicitly asked for with C++ list initialization (#GH33409).
 - Fix the result of `__builtin_is_implicit_lifetime` for types with a user-provided constructor. (#GH160610)
+- Fix a crash when attempting to deduce a deduction guide from a non deducible template template parameter. (#130604)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateDeductionGuide.cpp b/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
index 3d54d1eb4373a..4cad4a02c190f 100644
--- a/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
+++ b/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
@@ -1428,8 +1428,11 @@ void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
     DeclareImplicitDeductionGuidesForTypeAlias(*this, AliasTemplate, Loc);
     return;
   }
-  if (CXXRecordDecl *DefRecord =
-          cast<CXXRecordDecl>(Template->getTemplatedDecl())->getDefinition()) {
+  CXXRecordDecl *DefRecord =
+      llvm::dyn_cast_or_null<CXXRecordDecl>(Template->getTemplatedDecl());
+  if (!DefRecord)
+    return;
+  if (DefRecord->getDefinition()) {
     if (TemplateDecl *DescribedTemplate =
             DefRecord->getDescribedClassTemplate())
       Template = DescribedTemplate;
diff --git a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
index 2f1817d0ca7eb..fd1a5c01233d5 100644
--- a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
+++ b/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}}
+}