Sema: Fix crash when substitution into generic typealias fails

Fixes <rdar://problem/29060933>.

Author: slavapestov

lib/Sema/TypeCheckType.cpp

@@ -562,7 +562,7 @@ Type TypeChecker::applyUnboundGenericArguments(
       type = ArchetypeBuilder::mapTypeOutOfContext(TAD, TAD->getUnderlyingType());
     }
 
-    type = type.subst(dc->getParentModule(), subs, None);
+    type = type.subst(dc->getParentModule(), subs, SubstFlags::UseErrorType);
 
     // FIXME: return a SubstitutedType to preserve the fact that
     // we resolved a generic TypeAlias, for availability diagnostics.

test/decl/typealias/generic.swift

@@ -224,6 +224,9 @@ func takesSugaredType2(m: GenericClass<Int>.TA<Float>) {
 }
 
 
+//
+// Error paths
+//
 
 extension A {}  // expected-error {{non-nominal type 'A' cannot be extended}}
 extension A<T> {}  // expected-error {{generic type 'A' specialized with too few type parameters (got 1, but expected 2)}}
@@ -232,6 +235,26 @@ extension C<T> {}  // expected-error {{use of undeclared type 'T'}}
 extension C<Int> {}  // expected-error {{constrained extension must be declared on the unspecialized generic type 'MyType' with constraints specified by a 'where' clause}}
 
 
+protocol ErrorQ {
+  associatedtype Y
+}
+protocol ErrorP {
+  associatedtype X: ErrorQ // expected-note {{protocol requires nested type 'X'; do you want to add it?}}
+}
+
+typealias ErrorA<T: ErrorP> = T.X.Y
+
+struct ErrorB : ErrorP { // expected-error {{type 'ErrorB' does not conform to protocol 'ErrorP'}}
+  typealias X = ErrorC // expected-note {{possibly intended match 'ErrorB.X' (aka 'ErrorC') does not conform to 'ErrorQ'}}
+}
+
+struct ErrorC {
+  typealias Y = Int
+}
+
+typealias Y = ErrorA<ErrorB>
+
+
 //
 // Generic typealiases in protocols
 //