[clang] [Clang][Sema] Fix crash when type used in return statement contains errors (PR #79788)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Shafik Yaghmour (shafik)

<details>
<summary>Changes</summary>

In Sema in `BuildReturnStmt(...)` when we try to determine is the type is move eligable or copy elidable we don't currently check of the init of the `VarDecl` contain errors or not. This can lead to a crash since we may send a type that is not complete into `getTypeInfo(...)` which does not allow this.

This fixes: https://github.com/llvm/llvm-project/issues/63244
https://github.com/llvm/llvm-project/issues/79745

---
Full diff: https://github.com/llvm/llvm-project/pull/79788.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaStmt.cpp (+2) 
- (modified) clang/test/SemaCXX/deduced-return-type-cxx14.cpp (+9) 


``````````diff
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index 9e7c8c7e4e8c12..5d5a29b825ae7d 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -3391,6 +3391,8 @@ Sema::NamedReturnInfo Sema::getNamedReturnInfo(Expr *&E,
   const auto *VD = dyn_cast<VarDecl>(DR->getDecl());
   if (!VD)
     return NamedReturnInfo();
+  if (VD->getInit() && VD->getInit()->containsErrors())
+    return NamedReturnInfo();
   NamedReturnInfo Res = getNamedReturnInfo(VD);
   if (Res.Candidate && !E->isXValue() &&
       (Mode == SimplerImplicitMoveMode::ForceOn ||
diff --git a/clang/test/SemaCXX/deduced-return-type-cxx14.cpp b/clang/test/SemaCXX/deduced-return-type-cxx14.cpp
index c0d43911b8c717..415bbbf1a0bc50 100644
--- a/clang/test/SemaCXX/deduced-return-type-cxx14.cpp
+++ b/clang/test/SemaCXX/deduced-return-type-cxx14.cpp
@@ -724,3 +724,12 @@ struct DeducedTargetTypeOfConversionFunction {
   // since-cxx20-error at -1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
 #endif
 };
+
+namespace GH79745 {
+template <typename = int> struct a; // expected-note {{template is declared here}}
+auto f() {
+  a c; // cxx20_23-error {{implicit instantiation of undefined template}} \
+       // cxx14-error {{use of class template 'a' requires template arguments}}
+  return c;
+}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/79788