Sema: Avoid an assertion when diagnosing failures in some cases with invalid associated types

The solver's simplifyType() can produce a DependentMemberType with a
concrete or archetype base, if that associated type could not be
resolved during associated type inference.

mapTypeOutOfContext() could assert when given a type that violates
invariants in this manner. Instead, let's directly call subst() here,
providing a conformance lookup function (which will probably end up
returning ErrorTypes).

Fixes https://bugs.swift.org/browse/SR-14672 / rdar://problem/78626943.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -874,11 +874,16 @@ static Optional<RequirementMatch> findMissingGenericRequirementForSolutionFix(
   type = solution.simplifyType(type);
   missingType = solution.simplifyType(missingType);
 
-  missingType = missingType->mapTypeOutOfContext();
-  if (missingType->hasTypeParameter())
-    if (auto env = conformance->getGenericEnvironment())
-      if (auto assocType = env->mapTypeIntoContext(missingType))
-        missingType = assocType;
+  if (auto *env = conformance->getGenericEnvironment()) {
+    // We use subst() with LookUpConformanceInModule here, because
+    // associated type inference failures mean that we can end up
+    // here with a DependentMemberType with an ArchetypeType base.
+    missingType = missingType.subst(
+      [&](SubstitutableType *type) -> Type {
+        return env->mapTypeIntoContext(type->mapTypeOutOfContext());
+      },
+      LookUpConformanceInModule(conformance->getDeclContext()->getParentModule()));
+  }
 
   auto missingRequirementMatch = [&](Type type) -> RequirementMatch {
     Requirement requirement(requirementKind, type, missingType);

validation-test/compiler_crashers_2_fixed/sr14672.swift

@@ -0,0 +1,23 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+public protocol PublicContent {
+    associatedtype Model
+    init(_ model: Model)
+}
+
+public protocol PublicConvertible {
+    associatedtype Public
+    func toPublic() -> Public
+}
+
+extension PublicConvertible where Public: PublicContent, Public.Model == Self {
+    public func toPublic() -> Public {
+        Public(self)
+    }
+}
+
+extension Array: PublicConvertible where Element: PublicConvertible {
+    public func toPublic() -> [Element.Public] {
+        map { $0.toPublic() }
+    }
+}