Verify that a GenericTypeParamDecl's depth and index are correct (#20100)

(with respect to the containing GenericParamList)

Author: jrose-apple

lib/AST/ASTVerifier.cpp

@@ -2738,6 +2738,37 @@ class Verifier : public ASTWalker {
       verifyCheckedBase(nominal);
     }
 
+    void verifyCheckedAlways(GenericTypeParamDecl *GTPD) {
+      PrettyStackTraceDecl debugStack("verifying GenericTypeParamDecl", GTPD);
+
+      const DeclContext *DC = GTPD->getDeclContext();
+      if (!GTPD->getDeclContext()->isInnermostContextGeneric()) {
+        Out << "DeclContext of GenericTypeParamDecl does not have "
+               "generic params";
+        abort();
+      }
+
+      GenericParamList *paramList =
+          static_cast<const GenericContext *>(DC)->getGenericParams();
+      if (!paramList) {
+        Out << "DeclContext of GenericTypeParamDecl does not have "
+               "generic params";
+        abort();
+      }
+
+      if (paramList->size() <= GTPD->getIndex() ||
+          paramList->getParams()[GTPD->getIndex()] != GTPD) {
+        Out << "GenericTypeParamDecl has incorrect index";
+        abort();
+      }
+      if (paramList->getDepth() != GTPD->getDepth()) {
+        Out << "GenericTypeParamDecl has incorrect depth";
+        abort();
+      }
+
+      verifyCheckedBase(GTPD);
+    }
+
     void verifyChecked(ExtensionDecl *ext) {
       // Make sure that the protocol conformances are complete.
       for (auto conformance : ext->getLocalConformances()) {