[DSL] Allow function builders to opt in to "if" statements.

If a function builder contains a buildIf function, then "if" statements
will be supported by passing an optional of the "then" branch.
"if" statements with an "else" statement are unsupported at present.

include/swift/AST/KnownIdentifiers.def

@@ -32,6 +32,7 @@ IDENTIFIER(ArrayLiteralElement)
 IDENTIFIER(atIndexedSubscript)
 IDENTIFIER_(bridgeToObjectiveC)
 IDENTIFIER(buildBlock)
+IDENTIFIER(buildIf)
 IDENTIFIER(buildDo)
 IDENTIFIER(Change)
 IDENTIFIER_WITH_NAME(code_, "_code")

lib/AST/Expr.cpp

@@ -117,6 +117,8 @@ namespace {
       return getStartLocImpl(E);
     }
     template <class T> static SourceRange getSourceRange(const T *E) {
+      if (E->getStartLoc().isInvalid() != E->getEndLoc().isInvalid())
+        return SourceRange();
       return { E->getStartLoc(), E->getEndLoc() };
     }
   };

lib/Sema/ConstraintSystem.cpp

@@ -2871,7 +2871,51 @@ namespace {
 
     CONTROL_FLOW_STMT(Yield)
     CONTROL_FLOW_STMT(Defer)
-    CONTROL_FLOW_STMT(If)
+
+    static Expr *getTrivialBooleanCondition(StmtCondition condition) {
+      if (condition.size() != 1)
+        return nullptr;
+
+      return condition.front().getBooleanOrNull();
+    }
+
+    Expr *visitIfStmt(IfStmt *ifStmt) {
+      if (!builderSupports(ctx.Id_buildIf) ||
+          ifStmt->getElseStmt() ||
+          !getTrivialBooleanCondition(ifStmt->getCond())) {
+        this->controlFlowStmt = ifStmt;
+        return nullptr;
+      }
+
+      auto thenArg = visit(ifStmt->getThenStmt());
+      if (!thenArg)
+        return nullptr;
+
+      if (!wantExpr)
+        return nullptr;
+
+      auto optionalDecl = ctx.getOptionalDecl();
+      auto optionalType = optionalDecl->getDeclaredType();
+
+      auto optionalTypeExpr = TypeExpr::createImplicit(optionalType, ctx);
+      auto someRef = new (ctx) UnresolvedDotExpr(
+          optionalTypeExpr, SourceLoc(), ctx.getIdentifier("some"),
+          DeclNameLoc(), /*implicit=*/true);
+      auto someCall = CallExpr::createImplicit(ctx, someRef, thenArg, { });
+
+      optionalTypeExpr = TypeExpr::createImplicit(optionalType, ctx);
+      auto noneRef = new (ctx) UnresolvedDotExpr(
+          optionalTypeExpr, ifStmt->getEndLoc(), ctx.getIdentifier("none"),
+          DeclNameLoc(ifStmt->getEndLoc()), /*implicit=*/true);
+
+      auto ifExpr = new (ctx) IfExpr(
+          getTrivialBooleanCondition(ifStmt->getCond()),
+          SourceLoc(), someCall,
+          SourceLoc(), noneRef);
+      ifExpr->setImplicit();
+      return buildCallIfWanted(ctx.Id_buildIf, ifExpr);
+    }
+
     CONTROL_FLOW_STMT(Guard)
     CONTROL_FLOW_STMT(While)
     CONTROL_FLOW_STMT(DoCatch)

test/Constraints/function_builder.swift

@@ -24,15 +24,16 @@ struct TupleBuilder {
   }
 
   static func buildDo<T>(_ value: T) -> T { return value }
+  static func buildIf<T>(_ value: T?) -> T? { return value }
 }
 
-func tuplify<T>(@TupleBuilder body: () -> T) {
-  print(body())
+func tuplify<T>(_ cond: Bool, @TupleBuilder body: (Bool) -> T) {
+  print(body(cond))
 }
 
-// CHECK: (17, 3.14159, "Hello, DSL", (["nested", "do"], 6))
+// CHECK: (17, 3.14159, "Hello, DSL", (["nested", "do"], 6), Optional((2.71828, ["if", "stmt"])))
 let name = "dsl"
-tuplify {
+tuplify(true) {
   17
   3.14159
   "Hello, \(name.map { $0.uppercased() }.joined())"
@@ -40,4 +41,17 @@ tuplify {
     ["nested", "do"]
     1 + 2 + 3
   }
+  if $0 {
+    2.71828
+    ["if", "stmt"]
+  }
+}
+
+// CHECK: ("Empty optional", nil)
+tuplify(false) {
+  "Empty optional"
+  if $0 {
+    2.71828
+    ["if", "stmt"]
+  }
 }