diff --git a/lib/Sema/TypeCheckGeneric.cpp b/lib/Sema/TypeCheckGeneric.cpp
index 9feb689a74511..038d02430f3eb 100644
--- a/lib/Sema/TypeCheckGeneric.cpp
+++ b/lib/Sema/TypeCheckGeneric.cpp
@@ -202,6 +202,11 @@ Type CompleteGenericTypeResolver::resolveDependentMemberType(
     if (auto proto =
           concrete->getDeclContext()
             ->getAsProtocolOrProtocolExtensionContext()) {
+      // Fast path: if there are no type parameters in the concrete type, just
+      // return it.
+      if (!concrete->getInterfaceType()->hasTypeParameter())
+        return concrete->getInterfaceType();
+
       tc.validateDecl(proto);
       auto subMap = SubstitutionMap::getProtocolSubstitutions(
                       proto, baseTy, ProtocolConformanceRef(proto));
diff --git a/test/decl/typealias/protocol.swift b/test/decl/typealias/protocol.swift
index 856863a431acc..e625f2366577c 100644
--- a/test/decl/typealias/protocol.swift
+++ b/test/decl/typealias/protocol.swift
@@ -245,3 +245,10 @@ struct CandyEdible : Edible {
 // Edible.Snack is witnessed by 'typealias Snack' inside the
 // constrained extension of CandyWrapper above
 extension CandyBar : Edible {}
+
+protocol P9 {
+  typealias A = Int
+}
+
+func testT9a<T: P9, U>(_: T, _: U) where T.A == U { }
+func testT9b<T: P9>(_: T) where T.A == Float { } // expected-error{{'T.A' cannot be equal to both 'Float' and 'P9.A' (aka 'Int')}}
diff --git a/validation-test/compiler_crashers/28779-parent-parent-is-nominaltype-parent-is-boundgenerictype-parent-is-unboundgeneric.swift b/validation-test/compiler_crashers_fixed/28779-parent-parent-is-nominaltype-parent-is-boundgenerictype-parent-is-unboundgeneric.swift
similarity index 88%
rename from validation-test/compiler_crashers/28779-parent-parent-is-nominaltype-parent-is-boundgenerictype-parent-is-unboundgeneric.swift
rename to validation-test/compiler_crashers_fixed/28779-parent-parent-is-nominaltype-parent-is-boundgenerictype-parent-is-unboundgeneric.swift
index 631ea5a7a3059..e507d78c2e4f9 100644
--- a/validation-test/compiler_crashers/28779-parent-parent-is-nominaltype-parent-is-boundgenerictype-parent-is-unboundgeneric.swift
+++ b/validation-test/compiler_crashers_fixed/28779-parent-parent-is-nominaltype-parent-is-boundgenerictype-parent-is-unboundgeneric.swift
@@ -6,5 +6,5 @@
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
 // REQUIRES: asserts
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+// RUN: not %target-swift-frontend %s -emit-ir
 protocol P{func a:Self.a.a{}class a{class a