[Macros] Only freestanding expression macros can have a non-Void result type

Fixes rdar://108871352.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -7101,6 +7101,14 @@ ERROR(macro_in_nested,none,
 ERROR(macro_without_role,none,
       "macro %0 must declare its applicable roles via '@freestanding' or @attached'",
        (DeclName))
+ERROR(macro_result_type_cannot_be_used,none,
+      "only a freestanding expression macro can produce a result of type %0",
+      (Type))
+NOTE(macro_remove_result_type,none,
+      "remove the result type if the macro does not produce a value",
+      ())
+NOTE(macro_make_freestanding_expression,none,
+      "make this macro a freestanding expression macro", ())
 ERROR(macro_expansion_missing_pound,none,
       "expansion of macro %0 requires leading '#'", (DeclName))
 ERROR(macro_expansion_missing_arguments,none,

lib/AST/Decl.cpp

@@ -3357,12 +3357,18 @@ TypeRepr *ValueDecl::getResultTypeRepr() const {
     returnRepr = FD->getResultTypeRepr();
   } else if (auto *SD = dyn_cast<SubscriptDecl>(this)) {
     returnRepr = SD->getElementTypeRepr();
+  } else if (auto *MD = dyn_cast<MacroDecl>(this)) {
+    returnRepr = MD->resultType.getTypeRepr();
   }
 
   return returnRepr;
 }
 
 TypeRepr *ValueDecl::getOpaqueResultTypeRepr() const {
+  // FIXME: Macros don't allow opaque result types yet.
+  if (isa<MacroDecl>(this))
+    return nullptr;
+
   auto *returnRepr = this->getResultTypeRepr();
 
   auto *dc = getDeclContext();

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2060,6 +2060,23 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       break;
     }
     }
+
+    // If the macro has a (non-Void) 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())) {
+        auto resultType = MD->getResultInterfaceType();
+        Ctx.Diags.diagnose(
+            MD->arrowLoc, diag::macro_result_type_cannot_be_used, resultType)
+          .highlight(resultTypeRepr->getSourceRange());
+        Ctx.Diags.diagnose(MD->arrowLoc, diag::macro_make_freestanding_expression)
+          .fixItInsert(MD->getAttributeInsertionLoc(false),
+                       "@freestanding(expression)\n");
+        Ctx.Diags.diagnose(MD->arrowLoc, diag::macro_remove_result_type)
+          .fixItRemove(SourceRange(MD->arrowLoc, resultTypeRepr->getEndLoc()));
+      }
+    }
   }
 
   void visitMacroExpansionDecl(MacroExpansionDecl *MED) {

test/Macros/macros_diagnostics.swift

@@ -46,7 +46,7 @@ internal struct X { } // expected-note{{type declared here}}
 // expected-warning@-1{{external macro implementation type}}
 
 struct ZZZ {
-  macro m5() -> Int = #externalMacro(module: "BuiltinMacros", type: "Blah")
+  macro m5() = #externalMacro(module: "BuiltinMacros", type: "Blah")
   // expected-error@-1{{macro 'm5()' can only be declared at file scope}}
   // expected-error@-2{{macro 'm5()' must declare its applicable roles}}
   // expected-warning@-3{{external macro implementation type}}
@@ -200,3 +200,14 @@ struct SomeType {
   // expected-error@-2{{use of protocol 'Hashable' as a type must be written 'any Hashable'}}
   // expected-error@-3{{external macro implementation type}}
 }
+
+
+
+@freestanding(declaration) macro nonExpressionReturnsInt<T>(_: T) -> Int = #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 'Int'}}
+// 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}}{{67-74=}}
+
+@freestanding(declaration) macro nonExpressionReturnsVoid<T>(_: T) -> Void = #externalMacro(module: "A", type: "B")
+// expected-warning@-1{{external macro implementation type}}