[clang] return first Decl for CanonicalDecl in TranslationUnitDecl (llvm#110101)

Return the first `Decl` when using
`TranslationUnitDecl::getCanonicalDecl`

Author: Vipul-Cariappa

clang/include/clang/AST/Decl.h

@@ -133,6 +133,10 @@ class TranslationUnitDecl : public Decl,
   static TranslationUnitDecl *castFromDeclContext(const DeclContext *DC) {
     return static_cast<TranslationUnitDecl *>(const_cast<DeclContext*>(DC));
   }
+
+  /// Retrieves the canonical declaration of this translation unit.
+  TranslationUnitDecl *getCanonicalDecl() override { return getFirstDecl(); }
+  const TranslationUnitDecl *getCanonicalDecl() const { return getFirstDecl(); }
 };
 
 /// Represents a `#pragma comment` line. Always a child of

clang/unittests/Interpreter/InterpreterTest.cpp

@@ -381,4 +381,26 @@ TEST_F(InterpreterTest, Value) {
   EXPECT_TRUE(V9.isManuallyAlloc());
 }
 
+TEST_F(InterpreterTest, TranslationUnit_CanonicalDecl) {
+  std::vector<const char *> Args;
+  std::unique_ptr<Interpreter> Interp = createInterpreter(Args);
+
+  Sema &sema = Interp->getCompilerInstance()->getSema();
+
+  llvm::cantFail(Interp->ParseAndExecute("int x = 42;"));
+
+  TranslationUnitDecl *TU =
+      sema.getASTContext().getTranslationUnitDecl()->getCanonicalDecl();
+
+  llvm::cantFail(Interp->ParseAndExecute("long y = 84;"));
+
+  EXPECT_EQ(TU,
+            sema.getASTContext().getTranslationUnitDecl()->getCanonicalDecl());
+
+  llvm::cantFail(Interp->ParseAndExecute("char z = 'z';"));
+
+  EXPECT_EQ(TU,
+            sema.getASTContext().getTranslationUnitDecl()->getCanonicalDecl());
+}
+
 } // end anonymous namespace