[CSDiag] Fix crash related to failures in contextual type requirements

lib/Sema/CSDiag.cpp

@@ -1289,7 +1289,8 @@ diagnoseUnresolvedDotExprTypeRequirementFailure(ConstraintSystem &cs,
   // If we actually resolved the member to use, use it.
   auto loc = cs.getConstraintLocator(UDE, ConstraintLocator::Member);
   auto *member = cs.findResolvedMemberRef(loc);
-  if (!member)
+  // If the problem is contextual it's diagnosed elsewhere.
+  if (!member || !member->getAsGenericContext())
     return false;
 
   auto req = member->getAsGenericContext()

lib/Sema/CSDiagnostics.h

@@ -160,10 +160,23 @@ class RequirementFailure : public FailureDiagnostic {
   const ApplyExpr *Apply = nullptr;
 
 public:
-  RequirementFailure(Expr *expr, ConstraintSystem &cs,
+  RequirementFailure(ConstraintSystem &cs, Expr *expr, RequirementKind kind,
                      ConstraintLocator *locator)
       : FailureDiagnostic(expr, cs, locator), AffectedDecl(getDeclRef()) {
+    assert(locator);
     assert(AffectedDecl);
+
+    auto path = locator->getPath();
+    assert(!path.empty());
+
+    auto &last = path.back();
+    assert(last.getKind() == ConstraintLocator::TypeParameterRequirement);
+    assert(static_cast<RequirementKind>(last.getValue2()) == kind);
+
+    // It's possible sometimes not to have no base expression.
+    if (!expr)
+      return;
+
     auto *anchor = getAnchor();
     expr->forEachChildExpr([&](Expr *subExpr) -> Expr * {
       auto *AE = dyn_cast<ApplyExpr>(subExpr);
@@ -246,7 +259,7 @@ class MissingConformanceFailure final : public RequirementFailure {
   MissingConformanceFailure(Expr *expr, ConstraintSystem &cs,
                             ConstraintLocator *locator,
                             std::pair<Type, ProtocolDecl *> conformance)
-      : RequirementFailure(expr, cs, locator),
+      : RequirementFailure(cs, expr, RequirementKind::Conformance, locator),
         NonConformingType(conformance.first), Protocol(conformance.second) {}
 
   bool diagnoseAsError() override;
@@ -294,7 +307,8 @@ class SameTypeRequirementFailure final : public RequirementFailure {
 public:
   SameTypeRequirementFailure(Expr *expr, ConstraintSystem &cs, Type lhs,
                              Type rhs, ConstraintLocator *locator)
-      : RequirementFailure(expr, cs, locator), LHS(lhs), RHS(rhs) {}
+      : RequirementFailure(cs, expr, RequirementKind::SameType, locator),
+        LHS(lhs), RHS(rhs) {}
 
   Type getLHS() const override { return LHS; }
   Type getRHS() const override { return RHS; }
@@ -332,7 +346,8 @@ class SuperclassRequirementFailure final : public RequirementFailure {
 public:
   SuperclassRequirementFailure(Expr *expr, ConstraintSystem &cs, Type lhs,
                                Type rhs, ConstraintLocator *locator)
-      : RequirementFailure(expr, cs, locator), LHS(lhs), RHS(rhs) {}
+      : RequirementFailure(cs, expr, RequirementKind::Superclass, locator),
+        LHS(lhs), RHS(rhs) {}
 
   Type getLHS() const override { return LHS; }
   Type getRHS() const override { return RHS; }

test/Constraints/rdar45511837.swift

@@ -0,0 +1,23 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify %s
+// REQUIRES: objc_interop
+
+import Foundation
+
+protocol A: RawRepresentable {}
+
+extension A {
+  static func +(lhs: RawValue, rhs: Self) -> Self {
+    fatalError()
+  }
+}
+
+class Foo<Bar: NSObject> {
+  var foobar: Bar {
+    fatalError()
+  }
+
+  lazy var foo: () -> Void = {
+    // TODO: improve diagnostic message
+    _ = self.foobar + nil // expected-error {{'Foo<Bar>' requires that 'Bar' inherit from 'NSObject'}}
+  }
+}