Index: clang-tools-extra/clang-tidy/bugprone/BranchCloneCheck.cpp
===================================================================
--- clang-tools-extra/clang-tidy/bugprone/BranchCloneCheck.cpp
+++ clang-tools-extra/clang-tidy/bugprone/BranchCloneCheck.cpp
@@ -19,6 +19,17 @@
 /// Returns true when the statements are Type I clones of each other.
 static bool areStatementsIdentical(const Stmt *LHS, const Stmt *RHS,
                                    const ASTContext &Context) {
+  if (isa<Expr>(LHS) && isa<Expr>(RHS)) {
+    // If we have errors in expressions, we will be unable
+    // to accurately profile and compute hashes for each
+    // of the left and right statements.
+    const auto *LHSExpr = llvm::cast<Expr>(LHS);
+    const auto *RHSExpr = llvm::cast<Expr>(RHS);
+    if (LHSExpr->containsErrors() && RHSExpr->containsErrors()) {
+      return false;
+    }
+  }
+
   llvm::FoldingSetNodeID DataLHS, DataRHS;
   LHS->Profile(DataLHS, Context, false);
   RHS->Profile(DataRHS, Context, false);
Index: clang-tools-extra/docs/ReleaseNotes.rst
===================================================================
--- clang-tools-extra/docs/ReleaseNotes.rst
+++ clang-tools-extra/docs/ReleaseNotes.rst
@@ -233,6 +233,10 @@
 
   - Don't emit an erroneous warning on self-moves.
 
+- Fixed a false positive in :doc:`bugprone-branch-clone
+  <clang-tidy/checks/bugprone-branch-clone>` when the branches
+  involve unknown expressions.
+
 Removed checks
 ^^^^^^^^^^^^^^
 
Index: clang-tools-extra/test/clang-tidy/checkers/bugprone/branch-clone-unknown-expr.cpp
===================================================================
--- /dev/null
+++ clang-tools-extra/test/clang-tidy/checkers/bugprone/branch-clone-unknown-expr.cpp
@@ -0,0 +1,9 @@
+// RUN: %check_clang_tidy -fix-errors %s bugprone-branch-clone %t
+
+int test_unknown_expression() {
+  if (unknown_expression_1) {        // CHECK-MESSAGES: :[[@LINE]]:7: error: use of undeclared identifier 'unknown_expression_1' [clang-diagnostic-error]
+    function1(unknown_expression_2); // CHECK-MESSAGES: :[[@LINE]]:15: error: use of undeclared identifier 'unknown_expression_2' [clang-diagnostic-error]
+  } else {
+    function2(unknown_expression_3); // CHECK-MESSAGES: :[[@LINE]]:15: error: use of undeclared identifier 'unknown_expression_3' [clang-diagnostic-error]
+  }
+}