Sema: Diagnose types nested inside closures in generic context

Neither of the following is supported:

func g<T>(_: T) {
  _ = {
    struct S {}
  }
}

struct G<T> {
  let fn = {
    struct S {}
  }
}

Even though nested generic types are supported, the second example
is more like a type inside a function than a type inside a type,
because 'S' has no parent type.

Technically this is source-breaking but since neither SILGen nor
IRGen knew how to generate code for these I doubt anything worked.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -1178,6 +1178,9 @@ ERROR(pattern_binds_no_variables,none,
 ERROR(unsupported_type_nested_in_generic_function,none,
       "type %0 cannot be nested in generic function %1",
       (Identifier, Identifier))
+ERROR(unsupported_type_nested_in_generic_closure,none,
+      "type %0 cannot be nested in closure in generic context",
+      (Identifier))
 ERROR(unsupported_type_nested_in_protocol,none,
       "type %0 cannot be nested in protocol %1",
       (Identifier, Identifier))

lib/Sema/TypeCheckDecl.cpp

@@ -3974,6 +3974,10 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
                       diag::unsupported_type_nested_in_generic_function,
                       NTD->getName(),
                       AFD->getName());
+        } else {
+          TC.diagnose(NTD->getLoc(),
+                      diag::unsupported_type_nested_in_generic_closure,
+                      NTD->getName());
         }
       }
     }

test/decl/nested/type_in_function.swift

@@ -21,6 +21,14 @@ func outerGenericFunction<T>(_ t: T) {
     func nonGenericMethod(_ t: T, u: U) {}
     func genericMethod<V>(_ t: T, u: U) -> V where V : Racoon, V.Stripes == T {}
   }
+
+  _ = {
+    struct ConcreteInClosure { // expected-error{{type 'ConcreteInClosure' cannot be nested in closure in generic context}}
+    }
+
+    struct GenericInClosure<U> { // expected-error{{type 'GenericInClosure' cannot be nested in closure in generic context}}
+    }
+  }
 }
 
 class OuterNonGenericClass {