[Clang] avoid adding consteval condition as the last statement to preserve valid CFG (#116513)

Fixes #116485

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -559,6 +559,8 @@ Improvements to Clang's diagnostics
 
 - Clang now diagnoses ``= delete("reason")`` extension warnings only in pedantic mode rather than on by default. (#GH109311).
 
+- 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

@@ -1,7 +1,36 @@
-// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -Wimplicit-fallthrough -verify %s
 
 constexpr int f() { } // expected-warning {{non-void function does not return a value}}
 static_assert(__is_same(decltype([] constexpr -> int { }( )), int)); // expected-warning {{non-void lambda does not return a value}}
 
 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}}
+
+void i(int x) {
+  if consteval {
+  }
+  switch (x) {
+  case 1:
+    i(1);
+  case 2: // expected-warning {{unannotated fall-through between switch labels}} \
+          // expected-note {{insert 'break;' to avoid fall-through}}
+    break;
+  }
+}
+
+constexpr bool j()  {
+    if !consteval { return true; }
+} // expected-warning {{non-void function does not return a value in all control paths}} \
+  // expected-note {{control reached end of constexpr function}}
+
+bool k = j();
+constinit bool l = j(); // expected-error {{variable does not have a constant initializer}} \
+                        // expected-note {{required by 'constinit' specifier here}} \
+                        // expected-note {{in call to 'j()'}}
+
+}