[llvm-branch-commits] [clang] release/21.x: [clang] return type not correctly deduced for discarded lambdas (#153921) (PR #154080)

[via llvm-branch-commits]

https://github.com/llvmbot updated https://github.com/llvm/llvm-project/pull/154080

>From 8254aea007d0ebfe1f853938a20900183dd80949 Mon Sep 17 00:00:00 2001
From: Oliver Hunt <oliver at apple.com>
Date: Mon, 18 Aug 2025 02:07:27 -0700
Subject: [PATCH] [clang] return type not correctly deduced for discarded
 lambdas (#153921)

The early return for lamda expressions with deduced return types in
Sema::ActOnCapScopeReturnStmt meant that we were not actually perform
the required return type deduction for such lambdas when in a discarded
context.

This PR removes that early return allowing the existing return type
deduction steps to be performed.

Fixes #153884

Fix developed by, and

Co-authored-by: Corentin Jabot <corentinjabot at gmail.com>
(cherry picked from commit bcab8ac126c0b4c439caa3104d66d33d0f70f86f)
---
 clang/lib/Sema/SemaTemplateInstantiateDecl.cpp   |  2 +-
 .../CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp     | 16 ++++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index e2c3cdcd536bc..d2b87f2702a9c 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -5685,7 +5685,7 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
   };
   Function->setDeclarationNameLoc(NameLocPointsToPattern());
 
-  EnterExpressionEvaluationContext EvalContext(
+  EnterExpressionEvaluationContextForFunction EvalContext(
       *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
 
   Qualifiers ThisTypeQuals;
diff --git a/clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp b/clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp
index abb42447d3e0b..05830de9891fe 100644
--- a/clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp
+++ b/clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp
@@ -239,5 +239,21 @@ void f2() {
 
 }
 
+namespace GH153884 {
+  bool f1() {
+    auto f = [](auto) { return true; };
+    if constexpr (0)
+      return f(1);
+    return false;
+  }
+  bool f2() {
+    auto f = [](auto x) { if (x) return 1.5; else return "wat"; };
+    // expected-error at -1 {{'auto' in return type deduced as 'const char *' here but deduced as 'double' in earlier return statement}}
+    if constexpr (0)
+      return f(1);
+    // expected-note at -1 {{in instantiation of function template specialization 'GH153884::f2()}}
+    return false;
+  }
+}
 
 #endif