Skip invalid completion check immediately after newline (#55061)

Co-authored-by: Isabel Duan <isabelduan@microsoft.com>

src/services/completions.ts

@@ -4666,6 +4666,10 @@ function getCompletionData(
         return undefined;
     }
 
+    function isInDifferentLineThanContextToken(contextToken: Node, position: number): boolean {
+        return sourceFile.getLineEndOfPosition(contextToken.getEnd()) < position;
+    }
+
     /**
      * @returns true if we are certain that the currently edited location must define a new location; false otherwise.
      */
@@ -4733,7 +4737,7 @@ function getCompletionData(
             case SyntaxKind.SetKeyword:
                 return !isFromObjectTypeDeclaration(contextToken);
 
-            case SyntaxKind.Identifier:
+            case SyntaxKind.Identifier: {
                 if (containingNodeKind === SyntaxKind.ImportSpecifier &&
                     contextToken === (parent as ImportSpecifier).name &&
                     (contextToken as Identifier).text === "type"
@@ -4741,7 +4745,16 @@ function getCompletionData(
                     // import { type | }
                     return false;
                 }
+                const ancestorVariableDeclaration = findAncestor(
+                    contextToken.parent, isVariableDeclaration);
+                if (ancestorVariableDeclaration
+                    && isInDifferentLineThanContextToken(contextToken, position)) {
+                    // let a
+                    // |
+                    return false;
+                }
                 break;
+            }
 
             case SyntaxKind.ClassKeyword:
             case SyntaxKind.EnumKeyword:

tests/cases/fourslash/completionAfterNewline.ts

@@ -0,0 +1,21 @@
+/// <reference path="fourslash.ts" />
+
+// issue: https://github.com/microsoft/TypeScript/issues/54729
+// Tests that `isCompletionListBlocker` returns true at position 1, and returns false after a newline.
+
+
+////let foo /*1*/
+/////*2*/
+/////*3*/
+
+verify.completions(
+  { marker: "1", exact: undefined },
+  {
+    marker: ["2", "3"],
+    exact: completion.globalsPlus([
+      {
+        name: "foo",
+      },
+    ]),
+  }
+);

tests/cases/fourslash/completionAfterNewline2.ts

@@ -0,0 +1,19 @@
+/// <reference path="fourslash.ts" />
+
+// issue: https://github.com/microsoft/TypeScript/issues/54729
+// Tests that `isCompletionListBlocker` returns true at position 1, and returns false after a newline.
+
+////let foo = 5 as const /*1*/
+/////*2*/
+
+verify.completions(
+  { marker: "1", exact: undefined },
+  {
+    marker: "2",
+    exact: completion.globalsPlus([
+      {
+        name: "foo",
+      },
+    ]),
+  }
+);