[ConstraintSystem] Downgrade some invalid specialization uses to a warning until Swift 6 language mode

include/swift/AST/DiagnosticsSema.def

@@ -4606,8 +4606,8 @@ NOTE(duplicated_key_declared_here, none,
      "duplicate key declared here", ())
 
 // Generic specializations
-ERROR(cannot_explicitly_specialize_generic_function,none,
-      "cannot explicitly specialize a generic function", ())
+ERROR(cannot_explicitly_specialize_function,none,
+      "cannot explicitly specialize %kind0", (ValueDecl *))
 ERROR(not_a_generic_type,none,
       "cannot specialize non-generic type %0", (Type))
 ERROR(not_a_generic_macro,none,

include/swift/Sema/CSFix.h

@@ -465,8 +465,8 @@ enum class FixKind : uint8_t {
   /// Ignore an attempt to specialize a non-generic type.
   AllowConcreteTypeSpecialization,
 
-  /// Ignore an attempt to specialize a generic function.
-  AllowGenericFunctionSpecialization,
+  /// Ignore an attempt to specialize a (generic) function reference.
+  AllowFunctionSpecialization,
 
   /// Ignore an out-of-place \c then statement.
   IgnoreOutOfPlaceThenStmt,
@@ -3750,17 +3750,19 @@ class AllowConcreteTypeSpecialization final : public ConstraintFix {
   }
 };
 
-class AllowGenericFunctionSpecialization final : public ConstraintFix {
+class AllowFunctionSpecialization final : public ConstraintFix {
   ValueDecl *Decl;
 
-  AllowGenericFunctionSpecialization(ConstraintSystem &cs, ValueDecl *decl,
-                                     ConstraintLocator *locator)
-      : ConstraintFix(cs, FixKind::AllowGenericFunctionSpecialization, locator),
+  AllowFunctionSpecialization(ConstraintSystem &cs, ValueDecl *decl,
+                              ConstraintLocator *locator,
+                              FixBehavior fixBehavior)
+      : ConstraintFix(cs, FixKind::AllowFunctionSpecialization, locator,
+                      fixBehavior),
         Decl(decl) {}
 
 public:
   std::string getName() const override {
-    return "allow generic function specialization";
+    return "allow (generic) function specialization";
   }
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
@@ -3769,11 +3771,11 @@ class AllowGenericFunctionSpecialization final : public ConstraintFix {
     return diagnose(*commonFixes.front().first);
   }
 
-  static AllowGenericFunctionSpecialization *
+  static AllowFunctionSpecialization *
   create(ConstraintSystem &cs, ValueDecl *decl, ConstraintLocator *locator);
 
   static bool classof(const ConstraintFix *fix) {
-    return fix->getKind() == FixKind::AllowGenericFunctionSpecialization;
+    return fix->getKind() == FixKind::AllowFunctionSpecialization;
   }
 };
 

lib/Sema/CSDiagnostics.cpp

@@ -9365,8 +9365,8 @@ bool ConcreteTypeSpecialization::diagnoseAsError() {
   return true;
 }
 
-bool GenericFunctionSpecialization::diagnoseAsError() {
-  emitDiagnostic(diag::cannot_explicitly_specialize_generic_function);
+bool InvalidFunctionSpecialization::diagnoseAsError() {
+  emitDiagnostic(diag::cannot_explicitly_specialize_function, Decl);
   emitDiagnosticAt(Decl, diag::decl_declared_here, Decl);
   return true;
 }

lib/Sema/CSDiagnostics.h

@@ -3133,13 +3133,15 @@ class ConcreteTypeSpecialization final : public FailureDiagnostic {
   bool diagnoseAsError() override;
 };
 
-class GenericFunctionSpecialization final : public FailureDiagnostic {
+/// Diagnose attempts to specialize (generic) function references.
+class InvalidFunctionSpecialization final : public FailureDiagnostic {
   ValueDecl *Decl;
 
 public:
-  GenericFunctionSpecialization(const Solution &solution, ValueDecl *decl,
-                                ConstraintLocator *locator)
-      : FailureDiagnostic(solution, locator), Decl(decl) {}
+  InvalidFunctionSpecialization(const Solution &solution, ValueDecl *decl,
+                                ConstraintLocator *locator,
+                                FixBehavior fixBehavior)
+      : FailureDiagnostic(solution, locator, fixBehavior), Decl(decl) {}
 
   bool diagnoseAsError() override;
 };

lib/Sema/CSFix.cpp

@@ -2618,16 +2618,21 @@ AllowConcreteTypeSpecialization *AllowConcreteTypeSpecialization::create(
       cs, concreteTy, decl, locator, fixBehavior);
 }
 
-bool AllowGenericFunctionSpecialization::diagnose(const Solution &solution,
-                                                  bool asNote) const {
-  GenericFunctionSpecialization failure(solution, Decl, getLocator());
+bool AllowFunctionSpecialization::diagnose(const Solution &solution,
+                                           bool asNote) const {
+  InvalidFunctionSpecialization failure(solution, Decl, getLocator(),
+                                        fixBehavior);
   return failure.diagnose(asNote);
 }
 
-AllowGenericFunctionSpecialization *AllowGenericFunctionSpecialization::create(
-    ConstraintSystem &cs, ValueDecl *decl, ConstraintLocator *locator) {
+AllowFunctionSpecialization *
+AllowFunctionSpecialization::create(ConstraintSystem &cs, ValueDecl *decl,
+                                    ConstraintLocator *locator) {
+  auto fixBehavior = cs.getASTContext().isSwiftVersionAtLeast(6)
+                         ? FixBehavior::Error
+                         : FixBehavior::DowngradeToWarning;
   return new (cs.getAllocator())
-      AllowGenericFunctionSpecialization(cs, decl, locator);
+      AllowFunctionSpecialization(cs, decl, locator, fixBehavior);
 }
 
 bool IgnoreOutOfPlaceThenStmt::diagnose(const Solution &solution,

lib/Sema/CSSimplify.cpp

@@ -14069,6 +14069,13 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
 
   auto genericParams = getGenericParams(decl);
   if (!decl->getAsGenericContext() || !genericParams) {
+    if (isa<AbstractFunctionDecl>(decl)) {
+      return recordFix(AllowFunctionSpecialization::create(
+                 *this, decl, getConstraintLocator(locator)))
+                 ? SolutionKind::Error
+                 : SolutionKind::Solved;
+    }
+
     // Allow concrete macros to have specializations with just a warning.
     return recordFix(AllowConcreteTypeSpecialization::create(
                *this, type1, decl, getConstraintLocator(locator),
@@ -14111,10 +14118,7 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
   // FIXME: We could support explicit function specialization.
   if (openedGenericParams.empty() ||
       (isa<AbstractFunctionDecl>(decl) && !hasParameterPack)) {
-    if (!shouldAttemptFixes())
-      return SolutionKind::Error;
-
-    return recordFix(AllowGenericFunctionSpecialization::create(
+    return recordFix(AllowFunctionSpecialization::create(
                *this, decl, getConstraintLocator(locator)))
                ? SolutionKind::Error
                : SolutionKind::Solved;
@@ -15246,7 +15250,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::AllowAssociatedValueMismatch:
   case FixKind::GenericArgumentsMismatch:
   case FixKind::AllowConcreteTypeSpecialization:
-  case FixKind::AllowGenericFunctionSpecialization:
+  case FixKind::AllowFunctionSpecialization:
   case FixKind::IgnoreGenericSpecializationArityMismatch:
   case FixKind::IgnoreKeyPathSubscriptIndexMismatch:
   case FixKind::AllowMemberRefOnExistential: {

test/Parse/generic_disambiguation.swift

@@ -1,4 +1,5 @@
-// RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -verify-additional-prefix swift5-
+// RUN: %target-typecheck-verify-swift -swift-version 6 -verify-additional-prefix swift6-
 
 struct A<B> { // expected-note{{generic type 'A' declared here}}
   init(x:Int) {}
@@ -40,7 +41,9 @@ _ = (a < b, c > d)
 _ = a>(b)
 _ = a > (b)
 
-generic<Int>(0) // expected-error{{cannot explicitly specialize a generic function}}
+generic<Int>(0)
+// expected-swift5-warning@-1{{cannot explicitly specialize global function 'generic'}}
+// expected-swift6-error@-2 {{cannot explicitly specialize global function 'generic'}}
 
 A<B>.c()
 A<A<B>>.c()

test/Sema/generic_specialization.swift

@@ -1,7 +1,8 @@
-// RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -verify-additional-prefix swift5-
+// RUN: %target-typecheck-verify-swift -swift-version 6 -verify-additional-prefix swift6-
 
 extension Int {
-  func foo() -> Int {}
+  func foo() -> Int {} // expected-note 2 {{'foo()' declared here}}
   var bar: Int {
     get {}
   }
@@ -14,28 +15,53 @@ extension Int {
 
 // https://github.com/swiftlang/swift/issues/74857
 func test(i: Int) {
-  let _ = i.foo<Int>() // expected-error {{cannot specialize non-generic type '() -> Int'}}
+  let _ = i.foo<Int>()
+  // expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'foo()'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize instance method 'foo()'}}
 
-  let _ = i.gen<Int>() // expected-error {{cannot explicitly specialize a generic function}}
-  // expected-error@-1 {{generic parameter 'T' could not be inferred}}
+  let _ = i.gen<Int>()
+  // expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'gen()'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize instance method 'gen()'}}
+  // expected-error@-3 {{generic parameter 'T' could not be inferred}}
 
-  let _ = 0.foo<Int>() // expected-error {{cannot specialize non-generic type '() -> Int'}}
+  let _ = 0.foo<Int>()
+  // expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'foo()'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize instance method 'foo()'}}
 
-  let _ = i.gen<Int> // expected-error {{cannot explicitly specialize a generic function}}
-  // expected-error@-1 {{generic parameter 'T' could not be inferred}}
-  let _ = i.bar<Int> // expected-error {{cannot specialize non-generic type 'Int'}}
-  let _ = 0.bar<Int> // expected-error {{cannot specialize non-generic type 'Int'}}
+  let _ = i.gen<Int>
+  // expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'gen()'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize instance method 'gen()'}}
+  // expected-error@-3 {{generic parameter 'T' could not be inferred}}
+  let _ = i.bar<Int>
+  // expected-swift5-error@-1 {{cannot specialize non-generic type 'Int'}}
+  // expected-swift6-error@-2 {{cannot specialize non-generic type 'Int'}}
+  let _ = 0.bar<Int>
+  // expected-swift5-error@-1 {{cannot specialize non-generic type 'Int'}}
+  // expected-swift6-error@-2 {{cannot specialize non-generic type 'Int'}}
 }
 
 extension Bool {
   func foo<T>() -> T {} // expected-note {{'foo()' declared here}}
 }
 
-let _: () -> Bool = false.foo<Int> // expected-error {{cannot explicitly specialize a generic function}}
+let _: () -> Bool = false.foo<Int>
+// expected-swift5-warning@-1 {{cannot explicitly specialize instance method 'foo()'}}
+// expected-swift6-error@-2 {{cannot explicitly specialize instance method 'foo()'}}
 
 func foo(_ x: Int) {
   _ = {
-    _ = x<String> // expected-error {{cannot specialize non-generic type 'Int'}}
+    _ = x<String>
+    // expected-swift5-error@-1 {{cannot specialize non-generic type 'Int'}}
+    // expected-swift6-error@-2 {{cannot specialize non-generic type 'Int'}}
   }
 }
 
+do {
+  struct Test<T> {
+    init(_: (T) -> Void) {} // expected-note {{'init(_:)' declared here}}
+  }
+
+  _ = Test.init<Int>({ (_: Int) -> Void in })
+  // expected-swift5-warning@-1 {{cannot explicitly specialize initializer 'init(_:)'}}
+  // expected-swift6-error@-2 {{cannot explicitly specialize initializer 'init(_:)'}}
+}