[clang] Add tracking source deduction guide for the explicitly-written

deduction guide.

We miss this case in the original f94c481 commit.

Author: hokein

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -1227,11 +1227,14 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
         NewParam->setScopeInfo(0, I);
         FPTL.setParam(I, NewParam);
       }
-      auto *Transformed = cast<FunctionDecl>(buildDeductionGuide(
+      auto *Transformed = cast<CXXDeductionGuideDecl>(buildDeductionGuide(
           SemaRef, AliasTemplate, /*TemplateParams=*/nullptr,
           /*Constructor=*/nullptr, DG->getExplicitSpecifier(), FunctionType,
           AliasTemplate->getBeginLoc(), AliasTemplate->getLocation(),
           AliasTemplate->getEndLoc(), DG->isImplicit()));
+      Transformed->setSourceDeductionGuide(DG);
+      Transformed->setSourceDeductionGuideKind(
+          CXXDeductionGuideDecl::SourceDeductionGuideKind::Alias);
 
       // FIXME: Here the synthesized deduction guide is not a templated
       // function. Per [dcl.decl]p4, the requires-clause shall be present only

clang/unittests/AST/ASTImporterTest.cpp

@@ -8193,6 +8193,29 @@ TEST_P(ImportFunctions, CTADAliasTemplate) {
   EXPECT_TRUE(ToD->getSourceDeductionGuide());
 }
 
+TEST_P(ImportFunctions, CTADAliasTemplateWithExplicitSourceDeductionGuide) {
+  Decl *TU = getTuDecl(
+      R"(
+      template <typename T> struct A {
+        A(T);
+      };
+      template<typename T>
+      using B = A<T>;
+      A(int) -> A<double>; // explicit
+      B b{(int)0};
+      )",
+      Lang_CXX20, "input.cc");
+  auto *FromD = FirstDeclMatcher<CXXDeductionGuideDecl>().match(
+      TU, cxxDeductionGuideDecl(hasParameter(0, hasType(asString("int"))),
+                                hasName("<deduction guide for B>"),
+                                hasReturnTypeLoc(loc(asString("A<double>")))));
+  auto *ToD = Import(FromD, Lang_CXX20);
+  ASSERT_TRUE(ToD);
+  EXPECT_TRUE(ToD->getSourceDeductionGuideKind() ==
+              CXXDeductionGuideDecl::SourceDeductionGuideKind::Alias);
+  EXPECT_TRUE(ToD->getSourceDeductionGuide());
+}
+
 TEST_P(ImportFunctions, ParmVarDeclDeclContext) {
   constexpr auto FromTUCode = R"(
       void f(int P);