[Sema] Only resolve PlaceholderType result for FuncDecl with body

We allow resolving PlaceholderType here to allow MiscDiagnostics to
suggest replacing it with the inferred result type, but we won't run
that logic for non-FuncDecls and FuncDecls without bodies. Make sure
we reject placeholders in those cases.

Author: hamishknight

lib/Sema/TypeCheckDecl.cpp

@@ -2121,10 +2121,17 @@ ResultTypeRequest::evaluate(Evaluator &evaluator, ValueDecl *decl) const {
   if (decl->preconcurrency())
     options |= TypeResolutionFlags::Preconcurrency;
 
+  // Placeholders are only currently allowed for FuncDecls with bodies, which
+  // we diagnose in ReturnTypePlaceholderReplacer.
+  HandlePlaceholderTypeReprFn placeholderOpener;
+  if (auto *FD = dyn_cast<FuncDecl>(decl)) {
+    if (FD->hasBody() && !FD->isBodySkipped())
+      placeholderOpener = PlaceholderType::get;
+  }
   auto *const dc = decl->getInnermostDeclContext();
   return TypeResolution::forInterface(dc, options,
                                       /*unboundTyOpener*/ nullptr,
-                                      PlaceholderType::get,
+                                      placeholderOpener,
                                       /*packElementOpener*/ nullptr)
       .resolveType(resultTyRepr);
 }

test/Sema/placeholder_type.swift

@@ -291,3 +291,42 @@ do {
     _ = X(content: <#T##() -> _#>) // expected-error {{editor placeholder in source file}}
   }
 }
+
+// Make sure we reject placeholders here.
+protocol TestPlaceholderRequirement {
+  func foo(_:_) // expected-error {{type placeholder may not appear in top-level parameter}}
+  func bar() -> _ // expected-error {{type placeholder not allowed here}}
+  func baz() -> [_] // expected-error {{type placeholder not allowed here}}
+  func qux(_: [_]) // expected-error {{type placeholder may not appear in top-level parameter}}
+
+  // FIXME: Shouldn't diagnose twice
+  subscript(_: _) -> Void { get } // expected-error 2{{type placeholder may not appear in top-level parameter}}
+  subscript() -> _ { get } // expected-error {{type placeholder not allowed here}}
+}
+
+func testPlaceholderFn1(_:_) {} // expected-error {{type placeholder may not appear in top-level parameter}}
+func testPlaceholderFn2() -> _ {} // expected-error {{type placeholder may not appear in function return type}}
+
+var testPlaceholderComputed1: _ { 0 } // expected-error {{type placeholder not allowed here}}
+var testPlaceholderComputed2: [_] { [0] } // expected-error {{type placeholder not allowed here}}
+
+struct TestPlaceholderSubscript {
+  subscript(_: _) -> Void { () } // expected-error 2{{type placeholder may not appear in top-level parameter}}
+  subscript(_: [_]) -> Void { () } // expected-error 2{{type placeholder may not appear in top-level parameter}}
+  subscript() -> _ { () } // expected-error {{type placeholder not allowed here}}
+  subscript() -> [_] { [0] } // expected-error {{type placeholder not allowed here}}
+}
+
+enum TestPlaceholderInEnumElement {
+  case a(_) // expected-error {{type placeholder may not appear in top-level parameter}}
+  case b([_]) // expected-error {{type placeholder may not appear in top-level parameter}}
+}
+
+@freestanding(expression) macro testPlaceholderMacro(_ x: _) -> String = #file
+// expected-error@-1 {{type placeholder may not appear in top-level parameter}}
+
+@freestanding(expression) macro testPlaceholderMacro(_ x: [_]) -> String = #file
+// expected-error@-1 {{type placeholder may not appear in top-level parameter}}
+
+@freestanding(expression) macro testPlaceholderMacro() -> _ = #file
+// expected-error@-1 {{type placeholder not allowed here}}

validation-test/compiler_crashers_2/ce57354c94ae5aad.swift renamed to validation-test/compiler_crashers_2_fixed/ce57354c94ae5aad.swift

@@ -1,5 +1,5 @@
 // {"kind":"emit-silgen","signature":"swift::CanTypeVisitor<swift::Lowering::TypeConverter::computeLoweredRValueType(swift::TypeExpansionContext, swift::Lowering::AbstractionPattern, swift::CanType)::LoweredRValueTypeVisitor, swift::CanType>::visit(swift::CanType)"}
-// RUN: not --crash %target-swift-frontend -emit-silgen %s
+// RUN: not %target-swift-frontend -emit-silgen %s
 protocol a {
   func b() -> _
 }

validation-test/compiler_crashers_2/edb1d98948183f4c.swift renamed to validation-test/compiler_crashers_2_fixed/edb1d98948183f4c.swift

@@ -1,5 +1,5 @@
 // {"kind":"emit-silgen","signature":"emitRawApply(swift::Lowering::SILGenFunction&, swift::SILLocation, swift::Lowering::ManagedValue, swift::SubstitutionMap, llvm::ArrayRef<swift::Lowering::ManagedValue>, swift::CanTypeWrapper<swift::SILFunctionType>, swift::optionset::OptionSet<swift::ApplyFlags, unsigned char>, llvm::ArrayRef<swift::SILValue>, swift::SILValue, llvm::SmallVectorImpl<swift::SILValue>&, swift::Lowering::ExecutorBreadcrumb)"}
-// RUN: not --crash %target-swift-frontend -emit-silgen %s
+// RUN: not %target-swift-frontend -emit-silgen %s
 protocol a {
   func b() -> [(_) -> Self]
 }