RangeInfo: unbox ifstmt to get the agreed-upon return type from selected statements. rdar://31691723 (#9380)

Author: nkcsgexi

lib/IDE/SwiftSourceDocInfo.cpp

@@ -383,6 +383,9 @@ struct RangeResolver::Implementation {
 
   /// Collect the type that an ASTNode should be evaluated to.
   Type resolveNodeType(ASTNode N, RangeKind Kind) {
+    auto VoidTy = Ctx.getVoidDecl()->getDeclaredInterfaceType();
+    if (N.isNull())
+        return VoidTy;
     switch(Kind) {
     case RangeKind::Invalid:
     case RangeKind::SingleDecl:
@@ -407,9 +410,24 @@ struct RangeResolver::Implementation {
                                    RangeKind::SingleStatement);
           }
         }
+
+        // Unbox the if statement to find its type.
+        if (auto *IS = dyn_cast<IfStmt>(N.get<Stmt*>())) {
+          auto ThenTy = resolveNodeType(IS->getThenStmt(),
+                                        RangeKind::SingleStatement);
+          auto ElseTy = resolveNodeType(IS->getElseStmt(),
+                                        RangeKind::SingleStatement);
+
+          // If two branches agree on the return type, return that type.
+          if (ThenTy->isEqual(ElseTy))
+            return ThenTy;
+
+          // Otherwise, return the error type.
+          return Ctx.TheErrorType;
+        }
       }
       // For other statements, the type should be void.
-      return Ctx.getVoidDecl()->getDeclaredInterfaceType();
+      return VoidTy;
     }
     }
   }

test/IDE/range_info_branches.swift

@@ -0,0 +1,17 @@
+func foo(_ a: Bool) -> Int{
+  if a {
+    return 1
+  } else {
+  }
+  if a {
+    return 0
+  } else {
+    return 1
+  }
+}
+
+// RUN: %target-swift-ide-test -range -pos=2:1 -end-pos 5:4 -source-filename %s | %FileCheck %s -check-prefix=CHECK-ERR
+// RUN: %target-swift-ide-test -range -pos=6:1 -end-pos 10:4 -source-filename %s | %FileCheck %s -check-prefix=CHECK-INT
+
+// CHECK-ERR: <Type><<error type>></Type>
+// CHECK-INT: <Type>Int</Type>