Sema: Reject generic arguments applied to 'Self'

Previously these were simply ignored.

Fixes https://bugs.swift.org/browse/SR-14731 / rdar://problem/79051797.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -3649,6 +3649,10 @@ ERROR(not_a_generic_definition,none,
       "cannot specialize a non-generic definition", ())
 ERROR(not_a_generic_type,none,
       "cannot specialize non-generic type %0", (Type))
+ERROR(cannot_specialize_self,none,
+      "cannot specialize 'Self'", ())
+NOTE(specialize_explicit_type_instead,none,
+     "did you mean to explicitly reference %0 instead?", (Type))
 ERROR(type_parameter_count_mismatch,none,
       "generic type %0 specialized with %select{too many|too few}3 type "
       "parameters (got %2, but expected %1)",

lib/Sema/TypeCheckType.cpp

@@ -1338,6 +1338,25 @@ static SelfTypeKind getSelfTypeKind(DeclContext *dc,
   }
 }
 
+static void diagnoseGenericArgumentsOnSelf(TypeResolution resolution,
+                                           ComponentIdentTypeRepr *comp,
+                                           DeclContext *typeDC) {
+  ASTContext &ctx = resolution.getASTContext();
+  auto &diags = ctx.Diags;
+
+  auto *selfNominal = typeDC->getSelfNominalTypeDecl();
+  auto declaredType = selfNominal->getDeclaredType();
+
+  diags.diagnose(comp->getNameLoc(), diag::cannot_specialize_self);
+
+  if (selfNominal->isGeneric() && !isa<ProtocolDecl>(selfNominal)) {
+    diags.diagnose(comp->getNameLoc(), diag::specialize_explicit_type_instead,
+                   declaredType)
+        .fixItReplace(comp->getNameLoc().getSourceRange(),
+                      declaredType.getString());
+  }
+}
+
 /// Resolve the given identifier type representation as an unqualified type,
 /// returning the type it references.
 ///
@@ -1448,6 +1467,13 @@ static Type resolveTopLevelIdentTypeComponent(TypeResolution resolution,
 
       // Check if we can reference 'Self' here, and if so, what kind of Self it is.
       auto selfTypeKind = getSelfTypeKind(DC, options);
+
+      // We don't allow generic arguments on 'Self'.
+      if (selfTypeKind != SelfTypeKind::InvalidSelf &&
+          isa<GenericIdentTypeRepr>(comp)) {
+        diagnoseGenericArgumentsOnSelf(resolution, comp, typeDC);
+      }
+
       switch (selfTypeKind) {
       case SelfTypeKind::StaticSelf:
         return selfType;

test/type/self.swift

@@ -360,3 +360,20 @@ do {
     }
   }
 }
+
+// https://bugs.swift.org/browse/SR-14731
+struct Generic<T> {
+  func foo() -> Self<Int> {}
+  // expected-error@-1 {{cannot specialize 'Self'}}
+  // expected-note@-2 {{did you mean to explicitly reference 'Generic' instead?}}{{17-21=Generic}}
+}
+
+struct NonGeneric {
+  func foo() -> Self<Int> {}
+  // expected-error@-1 {{cannot specialize 'Self'}}
+}
+
+protocol P {
+  func foo() -> Self<Int>
+  // expected-error@-1 {{cannot specialize non-generic type 'Self'}}
+}