Merge pull request #36529 from nate-chandler/rdar75547146

Sema: Stop checking @main if no decl synthesized.

Author: nate-chandler

lib/AST/Module.cpp

@@ -1419,6 +1419,7 @@ bool ModuleDecl::isBuiltinModule() const {
 }
 
 bool SourceFile::registerMainDecl(Decl *mainDecl, SourceLoc diagLoc) {
+  assert(mainDecl);
   if (mainDecl == MainDecl)
     return false;
 

lib/Sema/TypeCheckAttr.cpp

@@ -2012,6 +2012,9 @@ void AttributeChecker::visitMainTypeAttr(MainTypeAttr *attr) {
                                  SynthesizeMainFunctionRequest{D},
                                  nullptr);
 
+  if (!func)
+    return;
+
   // Register the func as the main decl in the module. If there are multiples
   // they will be diagnosed.
   if (file->registerMainDecl(func, attr->getLocation()))

validation-test/compiler_crashers_2_fixed/rdar75547146.swift

@@ -0,0 +1,10 @@
+// RUN: not %target-swift-frontend %s -c -parse-as-library
+
+@main struct M1 {
+  static func main() {}
+}
+
+@main struct M2<T> {
+  static func main() {}
+}
+