[Function Builders] Teach diagnostics about function builder body

result types in order to properly diagnose requirement failures on
a builder-transformed type.

Author: hborla

lib/Sema/BuilderTransform.cpp

@@ -1583,8 +1583,12 @@ ConstraintSystem::matchFunctionBuilder(
   // If builder is applied to the closure expression then
   // `closure body` to `closure result` matching should
   // use special locator.
-  if (auto *closure = fn.getAbstractClosureExpr())
+  if (auto *closure = fn.getAbstractClosureExpr()) {
     locator = getConstraintLocator(closure, ConstraintLocator::ClosureResult);
+  } else {
+    locator = getConstraintLocator(locator.getAnchor(),
+                                   ConstraintLocator::FunctionBuilderBodyResult);
+  }
 
   // Bind the body result type to the type of the transformed expression.
   addConstraint(bodyResultConstraintKind, transformedType, bodyResultType,

lib/Sema/CSDiagnostics.cpp

@@ -176,6 +176,13 @@ Type FailureDiagnostic::restoreGenericParameters(
   });
 }
 
+SourceLoc RequirementFailure::getLoc() const {
+  if (getLocator()->isForFunctionBuilderBodyResult())
+    return getConstraintSystem().DC->getAsDecl()->getLoc();
+
+  return FailureDiagnostic::getLoc();
+}
+
 Type RequirementFailure::getOwnerType() const {
   auto anchor = getRawAnchor();
 
@@ -250,6 +257,14 @@ ValueDecl *RequirementFailure::getDeclRef() const {
     return type->getAnyGeneric();
   };
 
+  // If the locator is for a function builder body result type, the requirement
+  // came from the function's return type.
+  if (getLocator()->isForFunctionBuilderBodyResult()) {
+    auto *func = dyn_cast<FuncDecl>(getConstraintSystem().DC->getAsDecl());
+    assert(getSolution().functionBuilderTransformed.count(func));
+    return getAffectedDeclFromType(func->getResultInterfaceType());
+  }
+
   if (isFromContextualType())
     return getAffectedDeclFromType(getContextualType(getRawAnchor()));
 

lib/Sema/CSDiagnostics.h

@@ -269,6 +269,8 @@ class RequirementFailure : public FailureDiagnostic {
       Apply = dyn_cast<ApplyExpr>(parentExpr);
   }
 
+  SourceLoc getLoc() const override;
+
   unsigned getRequirementIndex() const {
     auto reqElt =
         getLocator()->castLastElementTo<LocatorPathElt::AnyRequirement>();

lib/Sema/ConstraintLocator.cpp

@@ -90,6 +90,7 @@ unsigned LocatorPathElt::getNewSummaryFlags() const {
   case ConstraintLocator::ClosureResult:
   case ConstraintLocator::ClosureBody:
   case ConstraintLocator::ConstructorMember:
+  case ConstraintLocator::FunctionBuilderBodyResult:
   case ConstraintLocator::InstanceType:
   case ConstraintLocator::AutoclosureResult:
   case ConstraintLocator::OptionalPayload:
@@ -247,6 +248,11 @@ bool ConstraintLocator::isForOptionalTry() const {
   return directlyAt<OptionalTryExpr>();
 }
 
+bool ConstraintLocator::isForFunctionBuilderBodyResult() const {
+  auto elt = getFirstElementAs<LocatorPathElt::FunctionBuilderBodyResult>();
+  return elt.hasValue();
+}
+
 GenericTypeParamType *ConstraintLocator::getGenericParameter() const {
   // Check whether we have a path that terminates at a generic parameter.
   return isForGenericParameter() ?
@@ -345,6 +351,10 @@ void ConstraintLocator::dump(SourceManager *sm, raw_ostream &out) const {
       out << "function result";
       break;
 
+    case FunctionBuilderBodyResult:
+      out << "function builder body result";
+      break;
+
     case SequenceElementType:
       out << "sequence element type";
       break;

lib/Sema/ConstraintLocator.h

@@ -375,6 +375,9 @@ class ConstraintLocator : public llvm::FoldingSetNode {
   /// Determine whether this locator points to the `try?` expression.
   bool isForOptionalTry() const;
 
+  /// Determine whether this locator is for a function builder body result type.
+  bool isForFunctionBuilderBodyResult() const;
+
   /// Determine whether this locator points directly to a given expression.
   template <class E> bool directlyAt() const {
     auto *anchor = getAnchor();

lib/Sema/ConstraintLocatorPathElts.def

@@ -75,6 +75,9 @@ SIMPLE_LOCATOR_PATH_ELT(FunctionArgument)
 /// The result type of a function.
 SIMPLE_LOCATOR_PATH_ELT(FunctionResult)
 
+/// The result type of a function builder body.
+SIMPLE_LOCATOR_PATH_ELT(FunctionBuilderBodyResult)
+
 /// A generic argument.
 /// FIXME: Add support for named generic arguments?
 CUSTOM_LOCATOR_PATH_ELT(GenericArgument)

test/Constraints/function_builder_diags.swift

@@ -572,3 +572,16 @@ wrapperifyInfer(true) { x in // expected-error{{unable to infer type of a closur
   intValue + x
 }
 
+struct DoesNotConform {}
+
+struct MyView {
+  @TupleBuilder var value: some P { // expected-error {{return type of property 'value' requires that 'DoesNotConform' conform to 'P'}}
+    // expected-note@-1 {{opaque return type declared here}}
+    DoesNotConform()
+  }
+
+  @TupleBuilder func test() -> some P { // expected-error {{return type of instance method 'test()' requires that 'DoesNotConform' conform to 'P'}}
+    // expected-note@-1 {{opaque return type declared here}}
+    DoesNotConform()
+  }
+}