[Parse] Error if closure has an unnamed parameter

We accepted unnamed closure parameters if the type was an array literal, dictionary literal, tuple or function (because the `[` or `(` starting the type was sufficient to disambiguate the type from the parameter’s name). This was never an accepted syntax and we should disallow it.

Author: ahoppen

include/swift/AST/DiagnosticsParse.def

@@ -1029,8 +1029,6 @@ ERROR(parameter_operator_keyword_argument,none,
 
 ERROR(parameter_unnamed,none,
       "unnamed parameters must be written with the empty name '_'", ())
-WARNING(parameter_unnamed_warn,none,
-        "unnamed parameters must be written with the empty name '_'", ())
 
 ERROR(parameter_curry_syntax_removed,none,
       "cannot have more than one parameter list", ())

lib/Parse/ParsePattern.cpp

@@ -404,26 +404,9 @@ Parser::parseParameterClause(SourceLoc &leftParenLoc,
           // Mark current parameter type as invalid so it is possible
           // to diagnose it as destructuring of the closure parameter list.
           param.isPotentiallyDestructured = true;
-          if (!isClosure) {
-            // Unnamed parameters must be written as "_: Type".
-            diagnose(typeStartLoc, diag::parameter_unnamed)
-                .fixItInsert(typeStartLoc, "_: ");
-          } else {
-            // Unnamed parameters were accidentally possibly accepted after
-            // SE-110 depending on the kind of declaration.  We now need to
-            // warn about the misuse of this syntax and offer to
-            // fix it.
-            // An exception to this rule is when the type is declared with type sugar
-            // Reference: https://github.com/apple/swift/issues/54133
-            if (isa<OptionalTypeRepr>(param.Type)
-                || isa<ImplicitlyUnwrappedOptionalTypeRepr>(param.Type)) {
-                diagnose(typeStartLoc, diag::parameter_unnamed)
-                    .fixItInsert(typeStartLoc, "_: ");
-            } else {
-                diagnose(typeStartLoc, diag::parameter_unnamed_warn)
-                    .fixItInsert(typeStartLoc, "_: ");
-            }
-          }
+          // Unnamed parameters must be written as "_: Type".
+          diagnose(typeStartLoc, diag::parameter_unnamed)
+              .fixItInsert(typeStartLoc, "_: ");
         }
       } else {
         // Otherwise, we're not sure what is going on, but this doesn't smell

test/Constraints/closures.swift

@@ -1026,7 +1026,7 @@ func rdar_59741308() {
 func r60074136() {
   func takesClosure(_ closure: ((Int) -> Void) -> Void) {}
 
-  takesClosure { ((Int) -> Void) -> Void in // expected-warning {{unnamed parameters must be written with the empty name '_'}}
+  takesClosure { ((Int) -> Void) -> Void in // expected-error {{unnamed parameters must be written with the empty name '_'}}
   }
 }
 

test/Constraints/tuple_arguments.swift

@@ -1490,7 +1490,7 @@ do {
   let tuple = (1, (2, 3))
   [tuple].map { (x, (y, z)) -> Int in x + y + z } // expected-note 2 {{'x' declared here}}
   // expected-error@-1 {{closure tuple parameter does not support destructuring}} {{21-27=arg1}} {{39-39=let (y, z) = arg1; }}
-  // expected-warning@-2 {{unnamed parameters must be written with the empty name '_'}}  {{21-21=_: }}
+  // expected-error@-2 {{unnamed parameters must be written with the empty name '_'}}  {{21-21=_: }}
   // expected-error@-3 {{cannot find 'y' in scope; did you mean 'x'?}}
   // expected-error@-4 {{cannot find 'z' in scope; did you mean 'x'?}}
 }
@@ -1501,7 +1501,7 @@ r31892961_1.forEach { (k, v) in print(k + v) }
 
 let r31892961_2 = [1, 2, 3]
 // expected-error@+2 {{closure tuple parameter does not support destructuring}} {{48-60=arg0}} {{+1:3-3=\n  let (index, val) = arg0\n  }}
-// expected-warning@+1 {{unnamed parameters must be written with the empty name '_'}} {{48-48=_: }}
+// expected-error@+1 {{unnamed parameters must be written with the empty name '_'}} {{48-48=_: }}
 let _: [Int] = r31892961_2.enumerated().map { ((index, val)) in
   val + 1
   // expected-error@-1 {{cannot find 'val' in scope}}
@@ -1518,13 +1518,13 @@ _ = [r31892961_4].map { x, y in x + y }
 let r31892961_5 = (x: 1, (y: 2, (w: 3, z: 4)))
 [r31892961_5].map { (x: Int, (y: Int, (w: Int, z: Int))) in x + y } // expected-note {{'x' declared here}}
 // expected-error@-1 {{closure tuple parameter does not support destructuring}} {{30-56=arg1}} {{61-61=let (y, (w, z)) = arg1; }}
-// expected-warning@-2 {{unnamed parameters must be written with the empty name '_'}} {{30-30=_: }}
+// expected-error@-2 {{unnamed parameters must be written with the empty name '_'}} {{30-30=_: }}
 // expected-error@-3{{cannot find 'y' in scope; did you mean 'x'?}}
 
 let r31892961_6 = (x: 1, (y: 2, z: 4))
 [r31892961_6].map { (x: Int, (y: Int, z: Int)) in x + y } // expected-note {{'x' declared here}}
 // expected-error@-1 {{closure tuple parameter does not support destructuring}} {{30-46=arg1}} {{51-51=let (y, z) = arg1; }}
-// expected-warning@-2 {{unnamed parameters must be written with the empty name '_'}} {{30-30=_: }}
+// expected-error@-2 {{unnamed parameters must be written with the empty name '_'}} {{30-30=_: }}
 // expected-error@-3{{cannot find 'y' in scope; did you mean 'x'?}}
 
 // rdar://problem/32214649 -- these regressed in Swift 4 mode

test/decl/func/functions.swift

@@ -192,8 +192,8 @@ func bogusDestructuring() {
     func registerCallback(_ callback: @escaping (Bar?) -> Void) {}
   }
 
-  Foo().registerCallback { ([Bar]) in } // expected-warning {{unnamed parameters must be written with the empty name '_'}} {{29-29=_: }}
-  Foo().registerCallback { ([String: Bar]) in }// expected-warning {{unnamed parameters must be written with the empty name '_'}} {{29-29=_: }}
+  Foo().registerCallback { ([Bar]) in } // expected-error {{unnamed parameters must be written with the empty name '_'}} {{29-29=_: }}
+  Foo().registerCallback { ([String: Bar]) in }// expected-error {{unnamed parameters must be written with the empty name '_'}} {{29-29=_: }}
   Foo().registerCallback { (Bar?) in } // expected-error {{unnamed parameters must be written with the empty name '_'}}
 
 }