[GSB] Don't add invalid concrete requirements.

When a concrete requirement is invalid due to the concrete type
lacking a conformance to a particular, required protocol, don't emit
that incorrect requirement---it causes invalid states further down the
line.

Fixes SR-5014 / rdar://problem/32402482.

While here, fix a comment that Huon noticed trailed off into oblivion.

Author: DougGregor

include/swift/AST/GenericSignatureBuilder.h

@@ -182,6 +182,9 @@ class GenericSignatureBuilder {
     /// the concrete type.
     unsigned recursiveConcreteType : 1;
 
+    /// Whether we have an invalid concrete type.
+    unsigned invalidConcreteType : 1;
+
     /// Whether we have detected recursion during the substitution of
     /// the superclass type.
     unsigned recursiveSuperclassType : 1;

lib/AST/GenericSignatureBuilder.cpp

@@ -1350,6 +1350,7 @@ GenericSignatureBuilder::resolveConcreteConformance(PotentialArchetype *pa,
                      concrete, proto->getName());
     }
 
+    paEquivClass->invalidConcreteType = true;
     return nullptr;
   }
 
@@ -1754,7 +1755,8 @@ static void concretizeNestedTypeFromConcreteParent(
     }
   }
 
-  // Error condition: parent did not conform to this protocol, so they
+  // Error condition: parent did not conform to this protocol, so there is no
+  // way to resolve the nested type via concrete conformance.
   if (!parentConcreteSource) return;
 
   auto source = parentConcreteSource->viaParent(builder, assocType);
@@ -2448,7 +2450,8 @@ void GenericSignatureBuilder::PotentialArchetype::dump(llvm::raw_ostream &Out,
 
 #pragma mark Equivalence classes
 EquivalenceClass::EquivalenceClass(PotentialArchetype *representative)
-  : recursiveConcreteType(false), recursiveSuperclassType(false)
+  : recursiveConcreteType(false), invalidConcreteType(false),
+    recursiveSuperclassType(false)
 {
   members.push_back(representative);
 }
@@ -3371,6 +3374,7 @@ GenericSignatureBuilder::addSameTypeRequirementBetweenArchetypes(
                                    SameTypeConflictCheckedLater());
     } else {
       equivClass->concreteType = equivClass2->concreteType;
+      equivClass->invalidConcreteType = equivClass2->invalidConcreteType;
     }
 
     equivClass->concreteTypeConstraints.insert(
@@ -5267,8 +5271,9 @@ void GenericSignatureBuilder::enumerateRequirements(llvm::function_ref<
             ? knownAnchor->concreteTypeSource
             : RequirementSource::forAbstract(archetype);
 
-        // Drop recursive concrete-type constraints.
-        if (equivClass->recursiveConcreteType)
+        // Drop recursive and invalid concrete-type constraints.
+        if (equivClass->recursiveConcreteType ||
+            equivClass->invalidConcreteType)
           continue;
 
         f(RequirementKind::SameType, archetype, concreteType, source);

test/Constraints/same_types.swift

@@ -77,7 +77,7 @@ func test4<T: Barrable>(_ t: T) -> Y where T.Bar == Y {
 
 func fail3<T: Barrable>(_ t: T) -> X
   where T.Bar == X { // expected-error {{'X' does not conform to required protocol 'Fooable'}}
-  return t.bar
+  return t.bar // expected-error{{cannot convert return expression of type 'T.Bar' }}
 }
 
 func test5<T: Barrable>(_ t: T) -> X where T.Bar.Foo == X {

validation-test/compiler_crashers_2/0101-sr5014.swift renamed to validation-test/compiler_crashers_2_fixed/0101-sr5014.swift

@@ -1,6 +1,4 @@
-// RUN: not --crash %target-swift-frontend -emit-ir -primary-file %s
-
-// REQUIRES: asserts
+// RUN: not %target-swift-frontend -emit-ir -primary-file %s
 
 struct Version {
 }