[clang] [Clang] Instantiate variables referenced in `decltype` with an undeduced type. (PR #161231)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin created https://github.com/llvm/llvm-project/pull/161231

Fixes #160497

>From 6c4bd440fb13335cf07ac69ed67890640e5e8323 Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Mon, 29 Sep 2025 18:55:22 +0200
Subject: [PATCH] [Clang] Instantiate variables referenced in `decltype` with
 an undeduced type.

Fixes #160497
---
 clang/docs/ReleaseNotes.rst     |  1 +
 clang/lib/Sema/SemaExpr.cpp     |  5 +++--
 clang/test/SemaCXX/decltype.cpp | 30 ++++++++++++++++++++++++++++++
 3 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 270b5d336eba7..3ff40f98334ca 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -432,6 +432,7 @@ Bug Fixes to C++ Support
 - Fix an assertion failure when taking the address on a non-type template parameter argument of
   object type. (#GH151531)
 - Suppress ``-Wdouble-promotion`` when explicitly asked for with C++ list initialization (#GH33409).
+- Correctly deduced return types in ``decltype`` expressions. (#GH160497)
 
 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/SemaCXX/decltype.cpp b/clang/test/SemaCXX/decltype.cpp
index 739485b57a3ec..971cf5132d4d5 100644
--- a/clang/test/SemaCXX/decltype.cpp
+++ b/clang/test/SemaCXX/decltype.cpp
@@ -170,3 +170,33 @@ class conditional {
 // FIXME: The diagnostics here are produced twice.
 void foo(conditional<decltype((1),int>) {  // expected-note 2 {{to match this '('}} expected-error {{expected ')'}} expected-note 2{{to match this '<'}}
 } // expected-error {{expected function body after function declarator}} expected-error 2 {{expected '>'}} expected-error {{expected ')'}}
+
+
+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 at -1 {{in instantiation of static data member 'GH160497::S<void>::mem<void>' requested here}}
+
+namespace N1 {
+
+template<class>
+struct S {
+  template<class>
+  inline static auto mem = 42;
+};
+
+using T = decltype(S<void>::mem<void>);
+
+T y = 42;
+
+}
+}