[Diags] Add check for null when printing type for diags

IIUC we should never have `ParenType`s that wrap a `nullptr` type. Adding an assertion to catch that situation. In the meantime check for null types in the `DiagnosticsEngine` to prevent a crash.

Fixes rdar://75740683

Author: ahoppen

lib/AST/DiagnosticEngine.cpp

@@ -556,13 +556,27 @@ static void formatDiagnosticArgument(StringRef Modifier,
 
     if (Arg.getKind() == DiagnosticArgumentKind::Type) {
       type = Arg.getAsType()->getWithoutParens();
+      if (type.isNull()) {
+        // FIXME: We should never receive a nullptr here, but this is causing
+        // crashes (rdar://75740683). Remove once ParenType never contains
+        // nullptr as the underlying type.
+        Out << "<null>";
+        break;
+      }
       if (type->getASTContext().TypeCheckerOpts.PrintFullConvention)
         printOptions.PrintFunctionRepresentationAttrs =
             PrintOptions::FunctionRepresentationMode::Full;
       needsQualification = typeSpellingIsAmbiguous(type, Args, printOptions);
     } else {
       assert(Arg.getKind() == DiagnosticArgumentKind::FullyQualifiedType);
       type = Arg.getAsFullyQualifiedType().getType()->getWithoutParens();
+      if (type.isNull()) {
+        // FIXME: We should never receive a nullptr here, but this is causing
+        // crashes (rdar://75740683). Remove once ParenType never contains
+        // nullptr as the underlying type.
+        Out << "<null>";
+        break;
+      }
       if (type->getASTContext().TypeCheckerOpts.PrintFullConvention)
         printOptions.PrintFunctionRepresentationAttrs =
             PrintOptions::FunctionRepresentationMode::Full;

lib/AST/Type.cpp

@@ -1443,6 +1443,11 @@ ParenType::ParenType(Type baseType, RecursiveTypeProperties properties,
   : SugarType(TypeKind::Paren,
               flags.isInOut() ? InOutType::get(baseType) : baseType,
               properties) {
+  // In some situations (rdar://75740683) we appear to end up with ParenTypes
+  // that contain a nullptr baseType. Once this is eliminated, we can remove
+  // the checks for `type.isNull()` in the `DiagnosticArgumentKind::Type` case
+  // of `formatDiagnosticArgument`.
+  assert(baseType && "A ParenType should always wrap a non-null type");
   Bits.ParenType.Flags = flags.toRaw();
   if (flags.isInOut())
     assert(!baseType->is<InOutType>() && "caller did not pass a base type");