Sema: Ban single-element tuple expressions

Fixes <rdar://problem/30384023>, <rdar://problem/41474370>,
and <https://bugs.swift.org/browse/SR-8109>.

Author: slavapestov

lib/Sema/MiscDiagnostics.cpp

@@ -203,10 +203,6 @@ static void diagSyntacticUseRestrictions(TypeChecker &TC, const Expr *E,
       while (auto Conv = dyn_cast<ImplicitConversionExpr>(Base))
         Base = Conv->getSubExpr();
 
-      // Record call arguments.
-      if (auto Call = dyn_cast<CallExpr>(Base))
-        CallArgs.insert(Call->getArg());
-
       if (auto *DRE = dyn_cast<DeclRefExpr>(Base)) {
         // Verify metatype uses.
         if (isa<TypeDecl>(DRE->getDecl())) {
@@ -235,7 +231,14 @@ static void diagSyntacticUseRestrictions(TypeChecker &TC, const Expr *E,
       if (isa<TypeExpr>(Base))
         checkUseOfMetaTypeName(Base);
 
+      if (auto *TSE = dyn_cast<TupleShuffleExpr>(E)) {
+        if (CallArgs.count(TSE))
+          CallArgs.insert(TSE->getSubExpr());
+      }
+
       if (auto *SE = dyn_cast<SubscriptExpr>(E)) {
+        CallArgs.insert(SE->getIndex());
+
         // Implicit InOutExpr's are allowed in the base of a subscript expr.
         if (auto *IOE = dyn_cast<InOutExpr>(SE->getBase()))
           if (IOE->isImplicit())
@@ -248,6 +251,13 @@ static void diagSyntacticUseRestrictions(TypeChecker &TC, const Expr *E,
         });
       }
 
+      if (auto *KPE = dyn_cast<KeyPathExpr>(E)) {
+        for (auto Comp : KPE->getComponents()) {
+          if (auto *Arg = Comp.getIndexExpr())
+            CallArgs.insert(Arg);
+        }
+      }
+
       if (auto *AE = dyn_cast<CollectionExpr>(E)) {
         visitCollectionElements(AE, [&](unsigned argIndex, Expr *arg) {
           arg = lookThroughArgument(arg);
@@ -266,6 +276,9 @@ static void diagSyntacticUseRestrictions(TypeChecker &TC, const Expr *E,
       // Check function calls, looking through implicit conversions on the
       // function and inspecting the arguments directly.
       if (auto *Call = dyn_cast<ApplyExpr>(E)) {
+        // Record call arguments.
+        CallArgs.insert(Call->getArg());
+
         // Warn about surprising implicit optional promotions.
         checkOptionalPromotions(Call);
 
@@ -381,6 +394,18 @@ static void diagSyntacticUseRestrictions(TypeChecker &TC, const Expr *E,
         }
       }
 
+      // Diagnose single-element tuple expressions.
+      if (auto *tupleExpr = dyn_cast<TupleExpr>(E)) {
+        if (!CallArgs.count(tupleExpr)) {
+          if (tupleExpr->getNumElements() == 1) {
+            TC.diagnose(tupleExpr->getElementNameLoc(0),
+                        diag::tuple_single_element)
+              .fixItRemoveChars(tupleExpr->getElementNameLoc(0),
+                                tupleExpr->getElement(0)->getStartLoc());
+          }
+        }
+      }
+
       return { true, E };
     }
 

test/Constraints/tuple.swift

@@ -56,7 +56,7 @@ f5((1,1))
 // Tuples with existentials
 var any : Any = ()
 any = (1, 2)
-any = (label: 4)
+any = (label: 4) // expected-error {{cannot create a single-element tuple with an element label}}
 
 // Scalars don't have .0/.1/etc
 i = j.0 // expected-error{{value of type 'Int' has no member '0'}}
@@ -252,3 +252,11 @@ func f(b: Bool) -> (a: Int, b: String)? {
   let y = ""
   return b ? (x, y) : nil
 }
+
+// Single element tuple expressions
+func singleElementTuple() {
+  let _ = (label: 123) // expected-error {{cannot create a single-element tuple with an element label}} {{12-19=}}
+  let _ = (label: 123).label // expected-error {{cannot create a single-element tuple with an element label}} {{12-19=}}
+  let _ = ((label: 123)) // expected-error {{cannot create a single-element tuple with an element label}} {{13-20=}}
+  let _ = ((label: 123)).label // expected-error {{cannot create a single-element tuple with an element label}} {{13-20=}}
+}

test/decl/enum/enumtest.swift

@@ -164,9 +164,9 @@ func test5(_ myorigin: CGPoint) {
   // Dot syntax.
   _ = x2.origin.x
   _ = x1.size.area()
-  _ = (r : x1.size).r.area()
+  _ = (r : x1.size).r.area() // expected-error {{cannot create a single-element tuple with an element label}}
   _ = x1.size.area()
-  _ = (r : x1.size).r.area()
+  _ = (r : x1.size).r.area() // expected-error {{cannot create a single-element tuple with an element label}}
 
   _ = x1.area