fix(codegen): raise a runtime error when a FUNCTION ends without RETURN

Report the function and its header line, then throw, instead of silently
returning zero. FUNCTION MAIN keeps its implicit result.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/CodeGen/FunctionGen.cc b/lib/CodeGen/FunctionGen.cc
index 48c9978..c233205 100644
--- a/lib/CodeGen/FunctionGen.cc
+++ b/lib/CodeGen/FunctionGen.cc
@@ -444,6 +444,10 @@ Function *FunctionGen::emitFunctionDef(FunctionDefAST *FuncAST,
     if (!Builder.GetInsertBlock()->hasTerminator()) {
         if (sameIdentifier(Proto->getReturnType(), "VOID")) {
             Exceptions.recordExit(Builder.CreateRetVoid());
+        } else if (MissingReturn && !(!IsMethod && sameIdentifier(Proto->getName(), "MAIN"))) {
+            // Falling off the end of a FUNCTION is a runtime error, not a zero value.
+            // The handler ends the block with a throw (unreachable).
+            MissingReturn(FuncAST->getLine(), Proto->getName());
         } else {
             Exceptions.recordExit(Builder.CreateRet(Constant::getNullValue(TheFunction->getReturnType())));
         }