[IDE][Sema] Fix code completion unreachable/hang in TypeBase::getContextSubtitutions

There are paths from Code completion giving it an UnboundGenericType, which it
doesn't expect.

This patch:
1) Updates the code completion paths to not attempt substitution on
   UnboundGenericTypes
2) Updates getContextSubstitutions to assert and break out of the loop when
   given an unhandled type to avoid hanging release builds if a similar bug
   occurs in future.

Resolves rdar://problem/53959978

Author: Nathan Hawes

lib/AST/Type.cpp

@@ -3481,7 +3481,9 @@ TypeBase::getContextSubstitutions(const DeclContext *dc,
       continue;
     }
 
-    llvm_unreachable("Bad base type");
+    // Assert and break to avoid hanging if we get an unexpected baseTy.
+    assert(0 && "Bad base type");
+    break;
   }
 
   while (n > 0) {

lib/Sema/LookupVisibleDecls.cpp

@@ -410,9 +410,14 @@ static void lookupDeclsFromProtocolsBeingConformedTo(
       continue;
 
     // Skip unsatisfied conditional conformances.
-    if (Conformance->getConditionalRequirementsIfAvailable() &&
-        !Module->conformsToProtocol(BaseTy, Proto))
-      continue;
+    // We can't check them if this type has an UnboundGenericType or if they
+    // couldn't be computed, so assume they conform in such cases.
+    if (!BaseTy->hasUnboundGenericType()) {
+      if (auto res = Conformance->getConditionalRequirementsIfAvailable()) {
+        if (!res->empty() && !Module->conformsToProtocol(BaseTy, Proto))
+          continue;
+      }
+    }
 
     DeclVisibilityKind ReasonForThisProtocol;
     if (Reason == DeclVisibilityKind::MemberOfCurrentNominal)
@@ -793,6 +798,7 @@ class OverrideFilteringConsumer : public VisibleDeclConsumer {
     // don't substitute either.
     bool shouldSubst = (Reason != DeclVisibilityKind::DynamicLookup &&
                         !BaseTy->isAnyObject() && !BaseTy->hasTypeVariable() &&
+                        !BaseTy->hasUnboundGenericType() &&
                         (BaseTy->getNominalOrBoundGenericNominal() ||
                          BaseTy->is<ArchetypeType>()) &&
                         VD->getDeclContext()->isTypeContext());

test/IDE/complete_type.swift

@@ -397,6 +397,8 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GENERIC_ARGS_LOCAL_RETURN | %FileCheck %s -check-prefix=WITH_GLOBAL_TYPES
 
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=PROTOCOL_DOT_1 | %FileCheck %s -check-prefix=PROTOCOL_DOT_1
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=UNBOUND_DOT | %FileCheck %s -check-prefix=UNBOUND_DOT
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=UNBOUND_DOT_2 | %FileCheck %s -check-prefix=UNBOUND_DOT_2
 
 //===--- Helper types that are used in this test
 
@@ -1093,3 +1095,37 @@ func testProtocol() {
 // PROTOCOL_DOT_1-DAG: Keyword/None:                       Type[#FooProtocol.Type#]; name=Type
 // PROTOCOL_DOT_1: End completions
 }
+
+//===---
+//===--- Test we can complete unbound generic types
+//===---
+
+public final class Task<Success> {
+  public enum Inner {
+    public typealias Failure = Int
+    case success(Success)
+    case failure(Failure)
+  }
+}
+extension Task.Inner {
+  public init(left error: Failure) {
+    fatalError()
+  }
+}
+extension Task.Inner.#^UNBOUND_DOT^# {}
+func testUnbound(x: Task.Inner.#^UNBOUND_DOT^#) {}
+// UNBOUND_DOT: Begin completions
+// UNBOUND_DOT-DAG: Decl[TypeAlias]/CurrNominal:        Failure[#Int#]; name=Failure
+// UNBOUND_DOT-DAG: Keyword/None:                       Type[#Task.Inner.Type#]; name=Type
+// UNBOUND_DOT: End completions
+
+
+protocol MyProtocol {}
+struct OuterStruct<U>  {
+  class Inner<V>: MyProtocol {}
+}
+
+func testUnbound2(x: OuterStruct<Int>.Inner.#^UNBOUND_DOT_2^#) {}
+// UNBOUND_DOT_2: Begin completions
+// UNBOUND_DOT_2-DAG: Keyword/None:                       Type[#OuterStruct<Int>.Inner.Type#]; name=Type
+// UNBOUND_DOT_2: End completions