[Type checker] Make sure we wire up all generic parameters for extensions.

Fixes a type checker crash found while working on detangling to metadata.

Author: DougGregor

lib/Sema/TypeChecker.cpp

@@ -300,6 +300,28 @@ static void prepareGenericParamList(GenericParamList *genericParams) {
     prepareGenericParamList(outerGenericParams);
 }
 
+/// Ensure that the outer generic parameters of the given generic
+/// context have been configured.
+static void configureOuterGenericParams(const GenericContext *dc) {
+  auto genericParams = dc->getGenericParams();
+  if (!genericParams) return;
+
+  if (genericParams->getOuterParameters()) return;
+
+  DeclContext *outerDC = dc->getParent();
+  while (!outerDC->isModuleScopeContext()) {
+    if (auto outerDecl = outerDC->getAsDecl()) {
+      if (auto outerGenericDC = outerDecl->getAsGenericContext()) {
+        genericParams->setOuterParameters(outerGenericDC->getGenericParams());
+        configureOuterGenericParams(outerGenericDC);
+        return;
+      }
+    }
+
+    outerDC = outerDC->getParent();
+  }
+}
+
 /// Bind the given extension to the given nominal type.
 static void bindExtensionToNominal(ExtensionDecl *ext,
                                    NominalTypeDecl *nominal) {
@@ -313,10 +335,7 @@ static void bindExtensionToNominal(ExtensionDecl *ext,
     ext->setGenericParams(genericParams);
   } else if (auto genericParams = nominal->getGenericParamsOfContext()) {
     // Make sure the generic parameters are set up.
-    if (auto nominalGenericParams = nominal->getGenericParams()) {
-      nominalGenericParams->setOuterParameters(
-        nominal->getDeclContext()->getGenericParamsOfContext());
-    }
+    configureOuterGenericParams(nominal);
 
     // Clone the generic parameter list of a generic type.
     prepareGenericParamList(genericParams);

test/decl/ext/generic.swift

@@ -175,3 +175,20 @@ func foo() {
     func foo() -> Element.AssocType {}
   }
 }
+
+// Deeply nested
+protocol P6 {
+  associatedtype Assoc1
+  associatedtype Assoc2
+}
+
+struct A<T, U, V> {
+  struct B<W, X, Y> {
+    struct C<Z: P6> {
+    }
+  }
+}
+
+extension A.B.C where T == V, X == Z.Assoc2 {
+  func f() { }
+}