[AST] Fix an undefine behavior when creating an empty recovery expr.

Summary:
We forgot to initialize the NumExpr member in one of the constructors,
which leads crashes in preamble serialization.

Reviewers: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78284

clang/include/clang/AST/Expr.h

@@ -6080,7 +6080,8 @@ public:
 private:
   RecoveryExpr(ASTContext &Ctx, SourceLocation BeginLoc, SourceLocation EndLoc,
                ArrayRef<Expr *> SubExprs);
-  RecoveryExpr(EmptyShell Empty) : Expr(RecoveryExprClass, Empty) {}
+  RecoveryExpr(EmptyShell Empty, unsigned NumSubExprs)
+      : Expr(RecoveryExprClass, Empty), NumExprs(NumSubExprs) {}
 
   size_t numTrailingObjects(OverloadToken<Stmt *>) const { return NumExprs; }
 

clang/lib/AST/Expr.cpp

@@ -4652,7 +4652,7 @@ RecoveryExpr *RecoveryExpr::Create(ASTContext &Ctx, SourceLocation BeginLoc,
 RecoveryExpr *RecoveryExpr::CreateEmpty(ASTContext &Ctx, unsigned NumSubExprs) {
   void *Mem = Ctx.Allocate(totalSizeToAlloc<Expr *>(NumSubExprs),
                            alignof(RecoveryExpr));
-  return new (Mem) RecoveryExpr(EmptyShell());
+  return new (Mem) RecoveryExpr(EmptyShell(), NumSubExprs);
 }
 
 void OMPArrayShapingExpr::setDimensions(ArrayRef<Expr *> Dims) {

clang/test/PCH/cxx-recovery-expr.cpp

@@ -0,0 +1,13 @@
+// Test with pch.
+// RUN: %clang_cc1 -emit-pch -frecovery-ast -fallow-pch-with-compiler-errors -o %t %s
+// RUN: %clang_cc1 -include-pch %t -fno-validate-pch -emit-llvm -o - %s
+
+#ifndef HEADER
+#define HEADER
+
+int func(int);
+int s = func();
+
+#else
+
+#endif