[Sema][Diagnostics] Generalize undesirable type warning to include arrays of empty tuples

https://bugs.swift.org/browse/SR-11511

Author: jder

lib/Sema/TypeCheckPattern.cpp

@@ -978,12 +978,10 @@ bool TypeChecker::coercePatternToType(Pattern *&P, TypeResolution resolution,
     TypeChecker::checkForForbiddenPrefix(Context, var->getBaseName());
 
     // If we are inferring a variable to have type AnyObject.Type,
-    // "()", an uninhabited type, or optional thereof, emit a diagnostic.
-    // In the first 2 cases, the coder probably forgot a cast and expected a
-    // concrete type.  In the later case, they probably didn't mean to bind to
-    // a variable, or there is some other bug.  We always tell them that they
-    // can silence the warning with an explicit type annotation
-    // (and provide a fixit) as a note.
+    // "()", "[()]", an uninhabited type, or optional thereof, emit a diagnostic.
+    // They are probably missing a cast or didn't mean to bind to a variable.
+    // We always tell them that they can silence the warning with an
+    // explicit type annotation (and provide a fixit) as a note.
     Type diagTy = type->lookThroughAllOptionalTypes();
     bool isOptional = !type->getOptionalObjectType().isNull();
     if (!diagTy) diagTy = type;
@@ -1010,6 +1008,9 @@ bool TypeChecker::coercePatternToType(Pattern *&P, TypeResolution resolution,
         assert((diagTy->is<EnumType>() || diagTy->is<BoundGenericEnumType>()) &&
           "unknown structurally uninhabited type");
       }
+    } else if (auto *BST = diagTy->getAs<BoundGenericStructType>()) {
+      if (BST->getDecl() == Context.getArrayDecl())
+          shouldRequireType = BST->getGenericArgs()[0]->isEqual(Context.TheEmptyTupleType);
     }
 
     if (shouldRequireType &&
@@ -1019,7 +1020,7 @@ bool TypeChecker::coercePatternToType(Pattern *&P, TypeResolution resolution,
       diags.diagnose(NP->getLoc(), diag, NP->getDecl()->getName(), type,
                      NP->getDecl()->isLet());
       diags.diagnose(NP->getLoc(), diag::add_explicit_type_annotation_to_silence)
-          .fixItInsertAfter(var->getNameLoc(), ": " + type->getString());
+          .fixItInsertAfter(var->getNameLoc(), ": " + type->getWithoutParens()->getString());
     }
 
     return false;

test/SourceKit/Sema/placeholders.swift.placeholders.response

@@ -27,5 +27,29 @@
         key.length: 9
       }
     ]
+  },
+  {
+    key.line: 8,
+    key.column: 5,
+    key.filepath: placeholders.swift,
+    key.severity: source.diagnostic.severity.warning,
+    key.description: "constant 'myArray' inferred to have type '[()]', which may be unexpected",
+    key.diagnostic_stage: source.diagnostic.stage.swift.sema,
+    key.diagnostics: [
+      {
+        key.line: 8,
+        key.column: 5,
+        key.filepath: placeholders.swift,
+        key.severity: source.diagnostic.severity.note,
+        key.description: "add an explicit type annotation to silence this warning",
+        key.fixits: [
+          {
+            key.offset: 236,
+            key.length: 0,
+            key.sourcetext: ": [()]"
+          }
+        ]
+      }
+    ]
   }
 ]

test/decl/var/variables.swift

@@ -60,6 +60,16 @@ func testAnyObjectOptional() -> AnyObject? {
   return x
 }
 
+// SR-11511 Warning for inferring an array of empty tuples
+var arrayOfEmptyTuples = [""].map { print($0) } // expected-warning {{variable 'arrayOfEmptyTuples' inferred to have type '[()]'}} \
+                                                // expected-note {{add an explicit type annotation to silence this warning}} {{23-23=: [()]}}
+
+var maybeEmpty = Optional(arrayOfEmptyTuples) // expected-warning {{variable 'maybeEmpty' inferred to have type '[()]?'}} \
+                                              // expected-note {{add an explicit type annotation to silence this warning}} {{15-15=: [()]?}}
+
+var shouldWarnWithoutSugar = (arrayOfEmptyTuples as Array<()>) // expected-warning {{variable 'shouldWarnWithoutSugar' inferred to have type 'Array<()>'}} \
+                                 // expected-note {{add an explicit type annotation to silence this warning}} {{27-27=: Array<()>}}
+
 class SomeClass {}
 
 // <rdar://problem/16877304> weak let's should be rejected