[clang] ff8aadf - [clang][diagnostics] Don't warn about unreachable code in constexpr if

[Alan Zhao via cfe-commits]

Author: Alan Zhao
Date: 2022-08-15T15:24:39-04:00
New Revision: ff8aadf58d1a0ea7d8fa2b9a7a17ff0c6059baa5

URL: https://github.com/llvm/llvm-project/commit/ff8aadf58d1a0ea7d8fa2b9a7a17ff0c6059baa5
DIFF: https://github.com/llvm/llvm-project/commit/ff8aadf58d1a0ea7d8fa2b9a7a17ff0c6059baa5.diff

LOG: [clang][diagnostics] Don't warn about unreachable code in constexpr if

The point of a constexpr if statement is to determine which branch to
take at compile time, so warning on unreachable code is meaningless in
these situations.

Fixes #57123.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D131818

Added: 
    

Modified: 
    clang/lib/Analysis/ReachableCode.cpp
    clang/test/SemaCXX/unreachable-code.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/Analysis/ReachableCode.cpp b/clang/lib/Analysis/ReachableCode.cpp
index 652bef34cebb7..ce0f4f83ba04f 100644
--- a/clang/lib/Analysis/ReachableCode.cpp
+++ b/clang/lib/Analysis/ReachableCode.cpp
@@ -299,6 +299,12 @@ static bool shouldTreatSuccessorsAsReachable(const CFGBlock *B,
     if (isa<BinaryOperator>(Term)) {
       return isConfigurationValue(Term, PP);
     }
+    // Do not treat constexpr if statement successors as unreachable in warnings
+    // since the point of these statements is to determine branches at compile
+    // time.
+    if (const auto *IS = dyn_cast<IfStmt>(Term);
+        IS != nullptr && IS->isConstexpr())
+      return true;
   }
 
   const Stmt *Cond = B->getTerminatorCondition(/* stripParens */ false);

diff  --git a/clang/test/SemaCXX/unreachable-code.cpp b/clang/test/SemaCXX/unreachable-code.cpp
index 6a95f767bef02..410ed9ae8861b 100644
--- a/clang/test/SemaCXX/unreachable-code.cpp
+++ b/clang/test/SemaCXX/unreachable-code.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -Wunreachable-code-aggressive -fblocks -verify %s
+// RUN: %clang_cc1 -std=c++17 -fcxx-exceptions -fexceptions -fsyntax-only -Wunreachable-code-aggressive -fblocks -verify %s
 
 int j;
 int bar();
@@ -99,3 +99,34 @@ void f(int a) {
 }
 
 }
+
+namespace gh57123 {
+  bool foo() {
+    if constexpr (true) {
+      if (true)
+        return true;
+      else
+        return false; // expected-warning {{will never be executed}}
+    }
+    else
+      return false; // no-warning
+  }
+
+  bool bar() {
+    if (true)
+      return true;
+    else
+      return false; // expected-warning {{will never be executed}}
+  }
+
+  bool baz() {
+    if constexpr (true)
+      return true;
+    else {
+      if (true)
+        return true;
+      else
+        return false; // expected-warning {{will never be executed}}
+    }
+  }
+}