[CSSimplify] Propagate contextual result type into result builder tra…

lib/Sema/CSSimplify.cpp

@@ -3532,11 +3532,29 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
     }
   }
 
+  auto resultMatchKind = subKind;
+  // Performance optimization: Propagate fully or partially resolved contextual
+  // type down into the body of result builder transformed closure by eagerly
+  // binding intermediate body result type to the contextual one. This helps to
+  // determine when closure body could be solved early.
+  //
+  // TODO: This could be extended to cover all multi-statement closures.
+  //
+  // See \c BindingSet::favoredOverConjunction for more details.
+  if (resultMatchKind >= ConstraintKind::Subtype &&
+      !func2->getResult()->isTypeVariableOrMember()) {
+    if (auto *closure = getAsExpr<ClosureExpr>(locator.trySimplifyToExpr())) {
+      if (!closure->hasExplicitResultType() &&
+          getAppliedResultBuilderTransform(closure)) {
+        resultMatchKind = ConstraintKind::Equal;
+      }
+    }
+  }
+
   // Result type can be covariant (or equal).
-  return matchTypes(func1->getResult(), func2->getResult(), subKind,
-                     subflags,
-                     locator.withPathElement(
-                       ConstraintLocator::FunctionResult));
+  return matchTypes(func1->getResult(), func2->getResult(), resultMatchKind,
+                    subflags,
+                    locator.withPathElement(ConstraintLocator::FunctionResult));
 }
 
 ConstraintSystem::TypeMatchResult

test/Constraints/result_builder.swift

@@ -1383,3 +1383,22 @@ testOptionalIfElseSequences()
 // CHECK-NEXT: second(Optional(main.B()), nil)
 // CHECK-NEXT: second(nil, Optional(main.Either<main.C, main.D>.first(main.C())))
 // CHECK-NEXT: second(nil, Optional(main.Either<main.C, main.D>.second(main.D())))
+
+// rdar://106364495 - ambiguous use of `buildFinalResult`
+func testBuildFinalResultDependentOnContextualType() {
+  @resultBuilder
+  struct MyBuilder {
+    static func buildBlock(_ v: Int) -> Int { v }
+    static func buildFinalResult(_ v: Int) -> Int { v }
+    static func buildFinalResult(_ v: Int) -> String { "" }
+  }
+
+  func test(@MyBuilder _ fn: () -> Int?) { print(fn()) }
+
+  test {
+    42
+  }
+}
+
+testBuildFinalResultDependentOnContextualType()
+// CHECK: Optional(42)