[PATCH] D131818: [clang][diagnostics] Don't warn about unreachable code in constexpr if

[Alan Zhao via Phabricator via cfe-commits]

ayzhao created this revision.
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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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D131818

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


Index: clang/test/SemaCXX/unreachable-code.cpp
===================================================================
--- clang/test/SemaCXX/unreachable-code.cpp
+++ 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 +99,12 @@
 }
 
 }
+
+namespace gh57123 {
+  bool foo() {
+    if constexpr (true)
+      return true;
+    else
+      return false; // no-warning
+  }
+}
Index: clang/lib/Analysis/ReachableCode.cpp
===================================================================
--- clang/lib/Analysis/ReachableCode.cpp
+++ clang/lib/Analysis/ReachableCode.cpp
@@ -299,6 +299,12 @@
     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);


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