Sema: Ignore inactive type variables in addTypeVariableConstraintsToWorkList()

Fixes a regression from commit 0c128e5.

The old depthFirstSearch() walked all adjacencies via the constraint graph,
and thus it would visit type variables that are currently inactive because
we're solving a conjunction element.

This was inconsistent with the new union-find which only formed the
connected components from the currently active type variables; adjacencies
involving inactive type variables are no longer considered.

Fix the inconsistency by changing gatherConstraints(), which used from
addTypeVariableConstraintsToWorkList(), to also skip inactive type
variables.

Fixes rdar://problem/143340082.

Author: slavapestov

lib/Sema/CSSolver.cpp

@@ -1946,6 +1946,17 @@ bool ConjunctionElement::attempt(ConstraintSystem &cs) const {
     llvm::SmallPtrSet<TypeVariableType *, 4> referencedVars;
     findReferencedVariables(cs, referencedVars);
 
+    if (cs.isDebugMode()) {
+      auto indent = cs.solverState->getCurrentIndent();
+      auto &log = llvm::errs().indent(indent);
+      log << "(Element type variables in scope: ";
+      interleave(
+        referencedVars,
+        [&](TypeVariableType *typeVar) { log << "$T" << typeVar->getID(); },
+        [&] { log << ", "; });
+      log << ")\n";
+    }
+
     for (auto *typeVar : referencedVars)
       cs.addTypeVariable(typeVar);
   }

lib/Sema/ConstraintGraph.cpp

@@ -565,6 +565,11 @@ static void depthFirstSearch(
     llvm::function_ref<void(Constraint *)> visitConstraint,
     llvm::SmallPtrSet<TypeVariableType *, 4> &typeVars,
     llvm::SmallPtrSet<Constraint *, 8> &visitedConstraints) {
+  // If we're not looking at this type variable right now because we're
+  // solving a conjunction element, don't consider its adjacencies.
+  if (!cg.getConstraintSystem().isActiveTypeVariable(typeVar))
+    return;
+
   // Visit this node. If we've already seen it, bail out.
   if (!typeVars.insert(typeVar).second)
     return;

test/Constraints/rdar143340082.swift

@@ -0,0 +1,13 @@
+// RUN: %target-typecheck-verify-swift
+
+struct Test {
+    var v: String
+    var i: Int
+}
+
+do {
+    let _ = Array(1 ... 20).map { i in
+        _ = 0
+        return Test(v: "\(i * 1000)", i: 1)
+    }
+}