diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index e8deae50e4cb0..6521fc3e9a9da 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)
+- Correctly deduce return types in ``decltype`` expressions. (#GH160497) (#GH56652) (#GH116319) (#GH161196)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 3b267c1b1693d..3302bfce193a2 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -20108,8 +20108,9 @@ static void DoMarkVarDeclReferenced(
   bool NeededForConstantEvaluation =
       isPotentiallyConstantEvaluatedContext(SemaRef) && UsableInConstantExpr;
 
-  bool NeedDefinition =
-      OdrUse == OdrUseContext::Used || NeededForConstantEvaluation;
+  bool NeedDefinition = OdrUse == OdrUseContext::Used ||
+                        NeededForConstantEvaluation ||
+                        Var->getType()->isUndeducedType();
 
   assert(!isa<VarTemplatePartialSpecializationDecl>(Var) &&
          "Can't instantiate a partial template specialization.");
diff --git a/clang/test/CodeGenCXX/gh56652.cpp b/clang/test/CodeGenCXX/gh56652.cpp
new file mode 100644
index 0000000000000..06a496e320bfc
--- /dev/null
+++ b/clang/test/CodeGenCXX/gh56652.cpp
@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-elf-gnu %s -emit-llvm -o - | FileCheck %s
+
+namespace GH56652{
+
+struct foo {};
+
+template <typename T> struct bar {
+    using type = T;
+
+    template <foo> inline static constexpr auto b = true;
+};
+
+template <typename T>
+concept C = requires(T a) { T::template b<foo{}>; };
+
+template <typename T> auto fn(T) {
+    if constexpr (!C<T>)
+        return foo{};
+    else
+        return T{};
+}
+
+auto a = decltype(fn(bar<int>{})){};
+
+}
+
+namespace GH116319 {
+
+template <int = 0> struct a {
+template <class> static constexpr auto b = 2;
+template <class> static void c() noexcept(noexcept(b<int>)) {}
+};
+
+void test() { a<>::c<int>(); }
+
+
+}
+
+// CHECK: %"struct.GH56652::bar" = type { i8 }
+// CHECK: $_ZN8GH1163191aILi0EE1cIiEEvv = comdat any
+// CHECK: @_ZN7GH566521aE = global %"struct.GH56652::bar" undef
diff --git a/clang/test/SemaCXX/decltype.cpp b/clang/test/SemaCXX/decltype.cpp
index 739485b57a3ec..45a4c4cf1ac86 100644
--- a/clang/test/SemaCXX/decltype.cpp
+++ b/clang/test/SemaCXX/decltype.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify -Wno-c99-designator %s
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify -Wno-c99-designator %s
 
 // PR5290
 int const f0();
@@ -156,6 +157,8 @@ struct A {
   }
 };
 
+
+
 // This shouldn't crash.
 static_assert(A<int>().f<int>() == 0, "");
 // The result should not be dependent.
@@ -163,6 +166,81 @@ static_assert(A<int>().f<int>() != 0, ""); // expected-error {{static assertion
                                            // expected-note@-1 {{expression evaluates to '0 != 0'}}
 }
 
+
+#if __cplusplus >= 201703L
+namespace GH160497 {
+
+template <class> struct S {
+    template <class>
+    inline static auto mem =
+    [] { static_assert(false); // expected-error {{static assertion failed}} \
+        // expected-note {{while substituting into a lambda expression here}}
+        return 42;
+    }();
+};
+
+using T = decltype(S<void>::mem<void>);
+ // expected-note@-1 {{in instantiation of static data member 'GH160497::S<void>::mem<void>' requested here}}
+
+
+template <class> struct S2 {
+    template <class>
+    inline static auto* mem =
+    [] { static_assert(false); // expected-error {{static assertion failed}} \
+        // expected-note {{while substituting into a lambda expression here}}
+        return static_cast<int*>(nullptr);
+    }();
+};
+
+using T2 = decltype(S2<void>::mem<void>);
+//expected-note@-1 {{in instantiation of static data member 'GH160497::S2<void>::mem<void>' requested here}}
+
+template <class> struct S3 {
+    template <class>
+    inline static int mem =    // Check we don't instantiate when the type is not deduced.
+    [] { static_assert(false);
+        return 42;
+    }();
+};
+
+using T = decltype(S3<void>::mem<void>);
+}
+
+namespace N1 {
+
+template<class>
+struct S {
+  template<class>
+  inline static auto mem = 42;
+};
+
+using T = decltype(S<void>::mem<void>);
+
+T y = 42;
+
+}
+
+namespace GH161196 {
+
+template <typename> struct A {
+  static constexpr int digits = 0;
+};
+
+template <typename> struct B {
+  template <int, typename MaskInt = int, int = A<MaskInt>::digits>
+  static constexpr auto XBitMask = 0;
+};
+
+struct C {
+  using ReferenceHost = B<int>;
+  template <int> static decltype(ReferenceHost::XBitMask<0>) XBitMask;
+};
+
+void test() { (void)C::XBitMask<0>; }
+
+}
+#endif
+
 template<typename>
 class conditional {
 };