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

[ayzhao]

Summary

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.

[ayzhao]

Add a test for non-constexpr if statements

[thakis]

Thanks!

Can you add a test for non-constexpr unreachable code within the constexpr if and test that that still warns?

[ayzhao]

Add tests for nested non-constexpr if

[ayzhao]

In D131818#3723227, @thakis wrote:

Thanks!

Can you add a test for non-constexpr unreachable code within the constexpr if and test that that still warns?

Done

[thakis]

Thanks!

clang/lib/Analysis/ReachableCode.cpp

static bool shouldTreatSuccessorsAsReachable(const CFGBlock *B,
                                              Preprocessor &PP) {
   if (const Stmt *Term = B->getTerminatorStmt()) {
     if (isa<SwitchStmt>(Term))
       return true;
     // Specially handle '||' and '&&'.
     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);
   return isConfigurationValue(Cond, PP);
 }
 
 static unsigned scanFromBlock(const CFGBlock *Start,
                               llvm::BitVector &Reachable,

clang/test/SemaCXX/unreachable-code.cpp

-// 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();
 int test1() {
   for (int i = 0;
        i != 10;
        ++i) {  // expected-warning {{loop will run at most once (loop increment never executed)}}
     if (j == 23) // missing {}'s
void f(int a) {
   }
 
   [[clang::musttail]] return g(a); // no-warning
 
   [[clang::musttail]] return g(a); // expected-warning {{will never be executed}}
 }
 
 }
+
+namespace gh57123 {
+  bool foo() {
+    if constexpr (true)
+      return true;
+    else
+      return false; // no-warning
+  }
+}