[Macros] Tighten restriction on non-expression macros not having return types

There is no reason to special-case `Void` to permit it. Rather, make
this a syntactic rule. Thanks to Alex Hoppen for the suggestion.

Author: DougGregor

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2072,11 +2072,10 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     }
     }
 
-    // If the macro has a (non-Void) result type, it must have the freestanding
+    // If the macro has a result type, it must have the freestanding
     // expression role. Other roles cannot have result types.
     if (auto resultTypeRepr = MD->getResultTypeRepr()) {
-      if (!MD->getMacroRoles().contains(MacroRole::Expression) &&
-          !MD->getResultInterfaceType()->isEqual(Ctx.getVoidType())) {
+      if (!MD->getMacroRoles().contains(MacroRole::Expression)) {
         auto resultType = MD->getResultInterfaceType();
         Ctx.Diags.diagnose(
             MD->arrowLoc, diag::macro_result_type_cannot_be_used, resultType)

test/Macros/attached_macros_diags.swift

@@ -6,17 +6,17 @@
 // expected-warning@-1{{external macro implementation type 'MyMacros.Macro1' could not be found for macro 'm1()'}}
 // expected-note@-2{{'m1()' declared here}}
 
-@attached(accessor) macro m2(_: Int) -> Void = #externalMacro(module: "MyMacros", type: "Macro2")
+@attached(accessor) macro m2(_: Int) = #externalMacro(module: "MyMacros", type: "Macro2")
 // expected-warning@-1{{external macro implementation type 'MyMacros.Macro2' could not be found for macro 'm2'}}
 // expected-note@-2{{candidate has partially matching parameter list (Int)}}
 // expected-note@-3{{candidate expects value of type 'Int' for parameter #1 (got 'String')}}
 
-@attached(accessor) macro m2(_: Double) -> Void = #externalMacro(module: "MyMacros", type: "Macro2")
+@attached(accessor) macro m2(_: Double) = #externalMacro(module: "MyMacros", type: "Macro2")
 // expected-warning@-1{{external macro implementation type 'MyMacros.Macro2' could not be found for macro 'm2'}}
 // expected-note@-2{{candidate has partially matching parameter list (Double)}}
 // expected-note@-3{{candidate expects value of type 'Double' for parameter #1 (got 'String')}}
 
-@attached(accessor) macro m3(message: String) -> Void = #externalMacro(module: "MyMacros", type: "Macro3")
+@attached(accessor) macro m3(message: String) = #externalMacro(module: "MyMacros", type: "Macro3")
 // expected-warning@-1{{external macro implementation type 'MyMacros.Macro3' could not be found for macro 'm3(message:)'}}
 
 @freestanding(expression) macro stringify<T>(_ value: T) -> (T, String) = #externalMacro(module: "MyMacros", type: "StringifyMacro")

test/Macros/macros_diagnostics.swift

@@ -211,6 +211,9 @@ struct SomeType {
 
 @freestanding(declaration) macro nonExpressionReturnsVoid<T>(_: T) -> Void = #externalMacro(module: "A", type: "B")
 // expected-warning@-1{{external macro implementation type}}
+// expected-error@-2{{only a freestanding expression macro can produce a result of type 'Void'}}
+// expected-note@-3{{make this macro a freestanding expression macro}}{{1-1=@freestanding(expression)\n}}
+// expected-note@-4{{remove the result type if the macro does not produce a value}}{{68-76=}}
 
 
 @freestanding(expression)

test/Macros/parsing.swift

@@ -51,7 +51,7 @@ macro am1()
   named, // expected-error{{introduced name kind 'named' requires a single argument '(name)'}}
   arbitrary(a) // expected-error{{introduced name kind 'arbitrary' must not have an argument}}
 )
-macro am2() -> Void
+macro am2()
 // expected-error@-1{{macro 'am2()' requires a definition}}
 
 #m1 + 1