[Constraint solver] Add debug dumps + a test for common result types.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -4826,6 +4826,15 @@ ConstraintSystem::simplifyApplicableFnConstraint(
   if (auto typeVar = desugar2->getAs<TypeVariableType>()) {
     auto choices = getUnboundBindOverloads(typeVar);
     if (Type resultType = findCommonResultType(choices)) {
+      ASTContext &ctx = getASTContext();
+      if (ctx.LangOpts.DebugConstraintSolver) {
+        auto &log = ctx.TypeCheckerDebug->getStream();
+        log.indent(solverState ? solverState->depth * 2 + 2 : 0)
+          << "(common result type for $T" << typeVar->getID() << " is "
+          << resultType.getString()
+          << ")\n";
+      }
+
       addConstraint(ConstraintKind::Bind, func1->getResult(), resultType,
                     locator);
     }

test/Constraints/common_type.swift

@@ -0,0 +1,39 @@
+// RUN: %target-typecheck-verify-swift -debug-constraints 2>%t.err
+// RUN: %FileCheck %s < %t.err
+
+struct X {
+  func g(_: Int) -> Int { return 0 }
+  func g(_: Double) -> Int { return 0 }
+
+  subscript(_: Int) -> String { return "" }
+  subscript(_: Double) -> String { return "" }
+}
+
+struct Y {
+  func g(_: Int) -> Double { return 0 }
+  func g(_: Double) -> Double { return 0 }
+
+  subscript(_: Int) -> Substring { return "" }
+  subscript(_: Double) -> Substring { return "" }
+}
+
+func f(_: Int) -> X { return X() }
+func f(_: Double) -> Y { return Y() }
+
+func testCallCommonType() {
+  // CHECK: overload set choice binding $T{{[0-9]+}} := (Int) -> X
+  // CHECK-NEXT: (common result type for $T{{[0-9]+}} is Int)
+  // CHECK: (overload set choice binding $T{{[0-9]+}} := (Double) -> Y)
+  // CHECK-NEXT: (common result type for $T{{[0-9]+}} is Double)
+  _ = f(0).g(0)
+}
+
+func testSubscriptCommonType() {
+  // FIXME: This will work once we have more filtering of subscripts.
+  // CHECK: subscript_expr
+  // CHECK: overload set choice binding $T{{[0-9]+}} := (Int) -> X
+  // CHECK-NOT: (common result type for $T{{[0-9]+}} is String)
+  // CHECK: (overload set choice binding $T{{[0-9]+}} := (Double) -> Y)
+  // CHECK-NOT: (common result type for $T{{[0-9]+}} is Substring)
+  _ = f(0)[0]
+}