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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Oliver Hunt (ojhunt)

<details>
<summary>Changes</summary>

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

---
Full diff: https://github.com/llvm/llvm-project/pull/153921.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaStmt.cpp (-2) 
- (modified) clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp (+16) 


``````````diff
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index bc1ddb80961a2..4bde0e54568dd 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -3533,8 +3533,6 @@ StmtResult Sema::ActOnCapScopeReturnStmt(SourceLocation ReturnLoc,
         return StmtError();
       RetValExp = ER.get();
     }
-    return ReturnStmt::Create(Context, ReturnLoc, RetValExp,
-                              /* NRVOCandidate=*/nullptr);
   }
 
   if (HasDeducedReturnType) {
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

``````````

</details>


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