[Diagnostics] Adjust wording of type_cannot_conform error message.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -1661,8 +1661,9 @@ ERROR(use_of_equal_instead_of_equality,none,
       "use of '=' in a boolean context, did you mean '=='?", ())
 
 ERROR(type_cannot_conform, none,
-      "%0 type %1 cannot conform to %2; only struct/enum/class "
-      "types can conform to protocols", (StringRef, Type, Type))
+      "%select{|protocol }0type %1 cannot conform to %2; "
+      "only struct/enum/class types can conform to protocols",
+      (bool, Type, Type))
 ERROR(protocol_does_not_conform_static,none,
       "%0 cannot be used as a type conforming to protocol %1 because %1 "
       "has static requirements",

lib/Sema/CSDiagnostics.cpp

@@ -576,21 +576,8 @@ bool MissingConformanceFailure::diagnoseAsError() {
     return true;
   }
 
-  // Diagnose types that cannot conform to protocols.
-  Optional<StringRef> prohibitedTypeKind = None;
-  if (nonConformingType->is<AnyFunctionType>())
-    prohibitedTypeKind = "function";
-  else if (nonConformingType->is<TupleType>())
-    prohibitedTypeKind = "tuple";
-  else if (nonConformingType->isExistentialType())
-    prohibitedTypeKind = "protocol";
-  else if (nonConformingType->is<AnyMetatypeType>())
-    prohibitedTypeKind = "metatype";
-
-  if (prohibitedTypeKind.hasValue()) {
-    emitDiagnostic(anchor->getLoc(), diag::type_cannot_conform,
-                   *prohibitedTypeKind, nonConformingType, protocolType);
-    return true;
+  if (diagnoseTypeCannotConform(anchor, nonConformingType, protocolType)) {
+      return true;
   }
 
   if (atParameterPos) {
@@ -608,6 +595,21 @@ bool MissingConformanceFailure::diagnoseAsError() {
   return RequirementFailure::diagnoseAsError();
 }
 
+bool MissingConformanceFailure::diagnoseTypeCannotConform(Expr *anchor,
+    Type nonConformingType, Type protocolType) const {
+  if (nonConformingType->is<AnyFunctionType>() ||
+      nonConformingType->is<TupleType>() ||
+      nonConformingType->isExistentialType() ||
+      nonConformingType->is<AnyMetatypeType>()) {
+    emitDiagnostic(anchor->getLoc(), diag::type_cannot_conform,
+                   nonConformingType->isExistentialType(), nonConformingType,
+                   protocolType);
+    return true;
+  }
+
+  return false;
+}
+
 Optional<Diag<Type, Type>> GenericArgumentsMismatchFailure::getDiagnosticFor(
     ContextualTypePurpose context) {
   switch (context) {

lib/Sema/CSDiagnostics.h

@@ -356,6 +356,10 @@ class MissingConformanceFailure final : public RequirementFailure {
   DiagAsNote getDiagnosticAsNote() const override {
     return diag::candidate_types_conformance_requirement;
   }
+
+private:
+  bool diagnoseTypeCannotConform(Expr *anchor, Type nonConformingType,
+                                 Type protocolType) const;
 };
 
 /// Diagnose failures related to same-type generic requirements, e.g.

lib/Sema/TypeCheckProtocol.cpp

@@ -3970,7 +3970,7 @@ static void diagnoseConformanceFailure(Type T,
       TypeChecker::containsProtocol(T, Proto, DC, None)) {
 
     if (!T->isObjCExistentialType()) {
-      diags.diagnose(ComplainLoc, diag::type_cannot_conform, "protocol",
+      diags.diagnose(ComplainLoc, diag::type_cannot_conform, true,
                      T, Proto->getDeclaredType());
       return;
     }

test/Constraints/diagnostics.swift

@@ -47,9 +47,9 @@ f0(i, i,
 
 
 // Cannot conform to protocols.
-f5(f4)  // expected-error {{function type '(Int) -> Int' cannot conform to 'P2'; only struct/enum/class types can conform to protocols}}
-f5((1, "hello"))  // expected-error {{tuple type '(Int, String)' cannot conform to 'P2'; only struct/enum/class types can conform to protocols}}
-f5(Int.self) // expected-error {{metatype type 'Int.Type' cannot conform to 'P2'; only struct/enum/class types can conform to protocols}}
+f5(f4)  // expected-error {{type '(Int) -> Int' cannot conform to 'P2'; only struct/enum/class types can conform to protocols}}
+f5((1, "hello"))  // expected-error {{type '(Int, String)' cannot conform to 'P2'; only struct/enum/class types can conform to protocols}}
+f5(Int.self) // expected-error {{type 'Int.Type' cannot conform to 'P2'; only struct/enum/class types can conform to protocols}}
 
 // Tuple element not convertible.
 f0(i,
@@ -99,7 +99,7 @@ func f8<T:P2>(_ n: T, _ f: @escaping (T) -> T) {}
 f8(3, f4) // expected-error {{argument type 'Int' does not conform to expected type 'P2'}}
 typealias Tup = (Int, Double)
 func f9(_ x: Tup) -> Tup { return x }
-f8((1,2.0), f9) // expected-error {{tuple type 'Tup' (aka '(Int, Double)') cannot conform to 'P2'; only struct/enum/class types can conform to protocols}}
+f8((1,2.0), f9) // expected-error {{type 'Tup' (aka '(Int, Double)') cannot conform to 'P2'; only struct/enum/class types can conform to protocols}}
 
 // <rdar://problem/19658691> QoI: Incorrect diagnostic for calling nonexistent members on literals
 1.doesntExist(0)  // expected-error {{value of type 'Int' has no member 'doesntExist'}}

test/Constraints/function_builder_diags.swift

@@ -166,7 +166,7 @@ struct Label<L> : P where L : P { // expected-note {{'L' declared as parameter t
 }
 
 func test_51167632() -> some P {
-  AnyP(G { // expected-error {{metatype type 'Label<Any>.Type' cannot conform to 'P'; only struct/enum/class types can conform to protocols}}
+  AnyP(G { // expected-error {{type 'Label<Any>.Type' cannot conform to 'P'; only struct/enum/class types can conform to protocols}}
     Text("hello")
     Label  // expected-error {{generic parameter 'L' could not be inferred}}
     // expected-note@-1 {{explicitly specify the generic arguments to fix this issue}} {{10-10=<<#L: P#>>}}

test/type/opaque.swift

@@ -377,7 +377,7 @@ protocol P_51641323 {
 func rdar_51641323() {
   struct Foo: P_51641323 {
     var foo: some P_51641323 { {} }
-    // expected-error@-1 {{function type '() -> ()' cannot conform to 'P_51641323'; only struct/enum/class types can conform to protocols}}
+    // expected-error@-1 {{type '() -> ()' cannot conform to 'P_51641323'; only struct/enum/class types can conform to protocols}}
   }
 }
 

validation-test/Sema/type_checker_crashers_fixed/rdar27830834.swift

@@ -1,4 +1,4 @@
 // RUN: %target-swift-frontend %s -typecheck -verify
 
 var d = [String:String]()
-_ = "\(d.map{ [$0 : $0] })" // expected-error {{tuple type '(key: String, value: String)' cannot conform to 'Hashable'; only struct/enum/class types can conform to protocols}}
+_ = "\(d.map{ [$0 : $0] })" // expected-error {{type '(key: String, value: String)' cannot conform to 'Hashable'; only struct/enum/class types can conform to protocols}}

validation-test/compiler_crashers_2_fixed/0196-rdar48937223.swift

@@ -5,7 +5,7 @@ protocol P {}
 func fn<T, U: P>(_ arg1: T, arg2: (T) -> U) {}
 
 func test(str: String) {
-  fn(str) { arg in // expected-error {{tuple type '()' cannot conform to 'P'; only struct/enum/class types can conform to protocols}}
+  fn(str) { arg in // expected-error {{type '()' cannot conform to 'P'; only struct/enum/class types can conform to protocols}}
     <#FOO#> // expected-error {{editor placeholder in source file}}
   }
 }