[Clang] avoid adding consteval condition as the last statement to preserve valid CFG

clang/docs/ReleaseNotes.rst

@@ -555,6 +555,8 @@ Improvements to Clang's diagnostics
       getS(); // Now diagnoses "Reason 2", previously diagnoses "Reason 1"
     }
 
+- Clang now diagnoses missing return value in functions containing ``if consteval`` (#GH116485).
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Analysis/CFG.cpp

@@ -3177,11 +3177,14 @@ CFGBlock *CFGBuilder::VisitIfStmt(IfStmt *I) {
     if (!I->isConsteval())
       KnownVal = tryEvaluateBool(I->getCond());
 
-    // Add the successors.  If we know that specific branches are
+    // Add the successors. If we know that specific branches are
     // unreachable, inform addSuccessor() of that knowledge.
     addSuccessor(Block, ThenBlock, /* IsReachable = */ !KnownVal.isFalse());
     addSuccessor(Block, ElseBlock, /* IsReachable = */ !KnownVal.isTrue());
 
+    if (I->isConsteval())
+      return Block;
+
     // Add the condition as the last statement in the new block.  This may
     // create new blocks as the condition may contain control-flow.  Any newly
     // created blocks will be pointed to be "Block".

clang/test/SemaCXX/constexpr-return-non-void-cxx2b.cpp

@@ -5,3 +5,9 @@ static_assert(__is_same(decltype([] constexpr -> int { }( )), int)); // expected
 
 consteval int g() { } // expected-warning {{non-void function does not return a value}}
 static_assert(__is_same(decltype([] consteval -> int { }( )), int)); // expected-warning {{non-void lambda does not return a value}}
+
+namespace GH116485 {
+int h() {
+    if consteval { }
+} // expected-warning {{non-void function does not return a value}}
+}