[clang] Fix a "!CodeSynthesisContexts.empty()" assertion failure when…

… constructing aggregate deduction guides. (llvm#89227)

We were missing to push an record to the instantiation stack in
`DeclareAggregateDeductionGuideForTypeAlias`. This patch fixes that.

clang/lib/Sema/SemaTemplate.cpp

@@ -3044,6 +3044,11 @@ FunctionTemplateDecl *DeclareAggregateDeductionGuideForTypeAlias(
     return nullptr;
 
   LocalInstantiationScope Scope(SemaRef);
+  Sema::InstantiatingTemplate BuildingDeductionGuides(
+      SemaRef, AliasTemplate->getLocation(), RHSDeductionGuide,
+      Sema::InstantiatingTemplate::BuildingDeductionGuidesTag{});
+  if (BuildingDeductionGuides.isInvalid())
+    return nullptr;
 
   // Build a new template parameter list for the synthesized aggregate deduction
   // guide by transforming the one from RHSDeductionGuide.

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

@@ -279,3 +279,13 @@ Bar t = Foo<K<Container>>();
 
 Bar s = 1; // expected-error {{no viable constructor or deduction guide for deduction of template arguments of}}
 } // namespace test20
+
+namespace test21 {
+template <typename T, unsigned N>
+struct Array { const T member[N]; };
+template <unsigned N>
+using String = Array<char, N>;
+
+// Verify no crash on constructing the aggregate deduction guides.
+String s("hello");
+} // namespace test21