[CS] Assert that we don't end up with unsolved constraints

Make sure we don't end up in a situation where we
have unsolved constraints left over and consider
the system fully solved.

This requires tweaking the type matching code for
dependent members such that a concrete base is
considered a failure rather than being left
unsolved. This should only happen when not in
diagnostic mode, as otherwise we use a hole.

Author: hamishknight

lib/Sema/CSSimplify.cpp

@@ -4296,6 +4296,15 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
       llvm_unreachable("type variables should have already been handled by now");
 
     case TypeKind::DependentMember: {
+      // If one of the dependent member types has no type variables,
+      // this comparison is effectively illformed, because dependent
+      // member couldn't be simplified down to the actual type, and
+      // we wouldn't be able to solve this constraint, so let's just fail.
+      // This should only happen outside of diagnostic mode, as otherwise the
+      // member is replaced by a hole in simplifyType.
+      if (!desugar1->hasTypeVariable() || !desugar2->hasTypeVariable())
+        return getTypeMatchFailure(locator);
+
       // Nothing we can solve yet, since we need to wait until
       // type variables will get resolved.
       return formUnsolvedResult();

lib/Sema/CSStep.cpp

@@ -354,6 +354,25 @@ StepResult ComponentStep::take(bool prevFailed) {
     return finalize(/*isSuccess=*/false);
   }
 
+  // If we don't have any disjunction or type variable choices left, we're done
+  // solving. Make sure we don't have any unsolved constraints left over, using
+  // report_fatal_error to make sure we trap in release builds instead of
+  // potentially miscompiling.
+  if (!CS.ActiveConstraints.empty()) {
+    CS.print(llvm::errs());
+    llvm::report_fatal_error("Active constraints left over?");
+  }
+  if (!CS.solverState->allowsFreeTypeVariables()) {
+    if (!CS.InactiveConstraints.empty()) {
+      CS.print(llvm::errs());
+      llvm::report_fatal_error("Inactive constraints left over?");
+    }
+    if (CS.hasFreeTypeVariables()) {
+      CS.print(llvm::errs());
+      llvm::report_fatal_error("Free type variables left over?");
+    }
+  }
+
   // If this solution is worse than the best solution we've seen so far,
   // skip it.
   if (CS.worseThanBestSolution())