[ConstraintSystem] Delay adding contextual requirements until parent type is opened

`openUnboundGenericType` eagerly tries to add conditional requirements
associated with chain of parents of the given type if type has been
declared inside of constrained extension. But one of the parent types
might be unbound e.g. `A.B` which means it has to be opened, which
by itself, would add such requirements.

Resolves: rdar://problem/49371608

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -490,47 +490,64 @@ static void checkNestedTypeConstraints(ConstraintSystem &cs, Type type,
     // info than that, unlike a typealias
   }
 
+  if (!parentTy)
+    return;
+
   // If this decl is generic, the constraints are handled when the generic
   // parameters are applied, so we don't have to handle them here (which makes
   // getting the right substitution maps easier).
-  if (decl && !decl->isGeneric()) {
-    auto extension = dyn_cast<ExtensionDecl>(decl->getDeclContext());
-    if (parentTy && extension && extension->isConstrainedExtension()) {
-      auto contextSubMap = parentTy->getContextSubstitutionMap(
-          extension->getParentModule(),
-          extension->getSelfNominalTypeDecl());
-      if (!subMap) {
-        // The substitution map wasn't set above, meaning we should grab the map
-        // for the extension itself.
-        subMap = parentTy->getContextSubstitutionMap(
-            extension->getParentModule(), extension);
-      }
+  if (!decl || decl->isGeneric())
+    return;
 
-      if (auto *signature = decl->getGenericSignature()) {
-        cs.openGenericRequirements(
-            extension, signature, /*skipProtocolSelfConstraint*/ true, locator,
-            [&](Type type) {
-              // Why do we look in two substitution maps? We have to use the
-              // context substitution map to find types, because we need to
-              // avoid thinking about them when handling the constraints, or all
-              // the requirements in the signature become tautologies (if the
-              // extension has 'T == Int', subMap will map T -> Int, so the
-              // requirement becomes Int == Int no matter what the actual types
-              // are here). However, we need the conformances for the extension
-              // because the requirements might look like `T: P, T.U: Q`, where
-              // U is an associated type of protocol P.
-              return type.subst(QuerySubstitutionMap{contextSubMap},
-                                LookUpConformanceInSubstitutionMap(subMap),
-                                SubstFlags::UseErrorType);
-            });
-      }
+  // struct A<T> {
+  //   let foo: [T]
+  // }
+  //
+  // extension A : Codable where T: Codable {
+  //   enum CodingKeys: String, CodingKey {
+  //     case foo = "foo"
+  //   }
+  // }
+  //
+  // Reference to `A.CodingKeys.foo` would point to `A` as an
+  // unbound generic type. Conditional requirements would be
+  // added when `A` is "opened". Les delay this check until then.
+  if (parentTy->hasUnboundGenericType())
+    return;
+
+  auto extension = dyn_cast<ExtensionDecl>(decl->getDeclContext());
+  if (extension && extension->isConstrainedExtension()) {
+    auto contextSubMap = parentTy->getContextSubstitutionMap(
+        extension->getParentModule(), extension->getSelfNominalTypeDecl());
+    if (!subMap) {
+      // The substitution map wasn't set above, meaning we should grab the map
+      // for the extension itself.
+      subMap = parentTy->getContextSubstitutionMap(extension->getParentModule(),
+                                                   extension);
     }
 
-    // And now make sure sure the parent is okay, for things like X<T>.Y.Z.
-    if (parentTy) {
-      checkNestedTypeConstraints(cs, parentTy, locator);
+    if (auto *signature = decl->getGenericSignature()) {
+      cs.openGenericRequirements(
+          extension, signature, /*skipProtocolSelfConstraint*/ true, locator,
+          [&](Type type) {
+            // Why do we look in two substitution maps? We have to use the
+            // context substitution map to find types, because we need to
+            // avoid thinking about them when handling the constraints, or all
+            // the requirements in the signature become tautologies (if the
+            // extension has 'T == Int', subMap will map T -> Int, so the
+            // requirement becomes Int == Int no matter what the actual types
+            // are here). However, we need the conformances for the extension
+            // because the requirements might look like `T: P, T.U: Q`, where
+            // U is an associated type of protocol P.
+            return type.subst(QuerySubstitutionMap{contextSubMap},
+                              LookUpConformanceInSubstitutionMap(subMap),
+                              SubstFlags::UseErrorType);
+          });
     }
   }
+
+  // And now make sure sure the parent is okay, for things like X<T>.Y.Z.
+  checkNestedTypeConstraints(cs, parentTy, locator);
 }
 
 Type ConstraintSystem::openUnboundGenericType(

validation-test/compiler_crashers_2_fixed/0189-rdar49371608.swift

@@ -0,0 +1,11 @@
+// RUN: %target-typecheck-verify-swift
+
+struct A<T> {
+  let foo: [T]
+}
+
+extension A : Codable where T: Codable {
+  enum CodingKeys: String, CodingKey {
+    case foo = "foo"
+  }
+}