[clang] [clang] return type not correctly deduced for discarded lambdas (PR #153921)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt updated https://github.com/llvm/llvm-project/pull/153921

>From cee36b9d023f97b55bf01220ccd6f404311db339 Mon Sep 17 00:00:00 2001
From: Oliver Hunt <oliver at apple.com>
Date: Fri, 15 Aug 2025 21:04:07 -0700
Subject: [PATCH 1/2] [clang] return type not correctly deduced for discarded
 lambdas

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 #GH153884

Fix developed by and

Co-authored-by: Corentin Jabot <corentinjabot at gmail.com>
---
 .../CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp     | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

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

>From 1d4875dcc2a5ac673492cc80768e82f17b39fc19 Mon Sep 17 00:00:00 2001
From: Oliver Hunt <oliver at apple.com>
Date: Sun, 17 Aug 2025 16:12:28 -0700
Subject: [PATCH 2/2] Make the correct fix

---
 clang/lib/Sema/SemaTemplateInstantiateDecl.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index f02a295220efb..6b423ce06523a 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -5669,7 +5669,7 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
   };
   Function->setDeclarationNameLoc(NameLocPointsToPattern());
 
-  EnterExpressionEvaluationContext EvalContext(
+  EnterExpressionEvaluationContextForFunction EvalContext(
       *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
 
   Qualifiers ThisTypeQuals;