[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

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);

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}}
+    }
+  }
+}