Ban IUOs in more places.

We were allowing them in parens in some contexts, but shouldn't have.

Also added tests for function types and tuple return types to ensure
we're not allowing them in these places.

Author: rudkx

lib/Sema/TypeCheckType.cpp

@@ -2845,6 +2845,9 @@ Type TypeResolver::resolveTupleType(TupleTypeRepr *repr,
   // ImmediateFunctionInput marker and install a FunctionInput one instead.
   auto elementOptions = options;
   if (repr->isParenType()) {
+    // We also want to disallow IUO within even a paren.
+    elementOptions -= TR_AllowIUO;
+
     // If we have a single ParenType, don't clear the context bits; we
     // still want to parse the type contained therein as if it were in
     // parameter position, meaning function types are not @escaping by

test/Sema/diag_erroneous_iuo.swift

@@ -137,3 +137,19 @@ let _: Generic<Int!, // expected-error {{implicitly unwrapped optionals are only
 
 func vararg(_ first: Int, more: Int!...) { // expected-error {{implicitly unwrapped optionals are only allowed at top level and as function results}}
 }
+
+func iuoInTuple() -> (Int!) { // expected-error {{implicitly unwrapped optionals are only allowed at top level and as function results}}
+  return 1
+}
+
+func iuoInTuple2() -> (Float, Int!) { // expected-error {{implicitly unwrapped optionals are only allowed at top level and as function results}}
+  return 1
+}
+
+func takesFunc(_ fn: (Int!) -> Int) -> Int { // expected-error {{implicitly unwrapped optionals are only allowed at top level and as function results}}
+  return fn(0)
+}
+
+func returnsFunc() -> (Int!) -> Int { // expected-error {{implicitly unwrapped optionals are only allowed at top level and as function results}}
+  return { $0 }
+}