Verify that a GenericTypeParamDecl's depth and index are correct (#20445)

Re-applied after fixing a bug in Serialization where the DeclContext
wasn't being set correctly for extensions with nested parameters. I
was unable to come up with a reduced test case, but the CoreStore
project in the source compatibility suite was failing without the
Serialization change, and now it isn't.

Author: jrose-apple

lib/AST/ASTVerifier.cpp

@@ -2738,6 +2738,48 @@ 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\n";
+        abort();
+      }
+
+      GenericParamList *paramList =
+          static_cast<const GenericContext *>(DC)->getGenericParams();
+      if (!paramList) {
+        Out << "DeclContext of GenericTypeParamDecl does not have "
+               "generic params\n";
+        abort();
+      }
+
+      unsigned currentDepth = paramList->getDepth();
+      if (currentDepth < GTPD->getDepth()) {
+        Out << "GenericTypeParamDecl has incorrect depth\n";
+        abort();
+      }
+      while (currentDepth > GTPD->getDepth()) {
+        paramList = paramList->getOuterParameters();
+        --currentDepth;
+      }
+      assert(paramList && "this is guaranteed by the parameter list's depth");
+
+      if (paramList->size() <= GTPD->getIndex() ||
+          paramList->getParams()[GTPD->getIndex()] != GTPD) {
+        if (llvm::is_contained(paramList->getParams(), GTPD))
+          Out << "GenericTypeParamDecl has incorrect index\n";
+        else
+          Out << "GenericTypeParamDecl not found in GenericParamList; "
+                 "incorrect depth or wrong DeclContext\n";
+        abort();
+      }
+
+      verifyCheckedBase(GTPD);
+    }
+
     void verifyChecked(ExtensionDecl *ext) {
       // Make sure that the protocol conformances are complete.
       for (auto conformance : ext->getLocalConformances()) {

lib/Serialization/Deserialization.cpp

@@ -2702,7 +2702,7 @@ ModuleFile::getDeclCheckedImpl(DeclID DID) {
                                                         index);
 
     // Always create GenericTypeParamDecls in the associated module;
-    // maybeReadGenericParams() will reparent them.
+    // the real context will reparent them.
     auto DC = getAssociatedModule();
     auto genericParam = createDecl<GenericTypeParamDecl>(DC,
                                                          getIdentifier(nameID),
@@ -3964,9 +3964,12 @@ ModuleFile::getDeclCheckedImpl(DeclID DID) {
     // Generic parameter lists are written from outermost to innermost.
     // Keep reading until we run out of generic parameter lists.
     GenericParamList *outerParams = nullptr;
-    while (auto *genericParams = maybeReadGenericParams(DC, outerParams))
+    while (auto *genericParams = maybeReadGenericParams(DC, outerParams)) {
+      // We do this repeatedly to set up the correct DeclContexts for the
+      // GenericTypeParamDecls in the list.
+      extension->setGenericParams(genericParams);
       outerParams = genericParams;
-    extension->setGenericParams(outerParams);
+    }
 
     configureGenericEnvironment(extension, genericEnvID);