Merge pull request #13806 from Microsoft/Fix13789

Get string literal completions from the other operand of `==` operators

Author: mhegazy

src/services/completions.ts

@@ -173,6 +173,18 @@ namespace ts.Completions {
             //      var y = require("/*completion position*/");
             return getStringLiteralCompletionEntriesFromModuleNames(<StringLiteral>node, compilerOptions, host, typeChecker);
         }
+        else if (isEqualityExpression(node.parent)) {
+            // Get completions from the type of the other operand
+            // i.e. switch (a) {
+            //         case '/*completion position*/'
+            //      }
+            return getStringLiteralCompletionEntriesFromType(typeChecker.getTypeAtLocation(node.parent.left === node ? node.parent.right : node.parent.left), typeChecker);
+        }
+        else if (isCaseOrDefaultClause(node.parent)) {
+            // Get completions from the type of the switch expression
+            // i.e. x === '/*completion position'
+            return getStringLiteralCompletionEntriesFromType(typeChecker.getTypeAtLocation((<SwitchStatement>node.parent.parent.parent).expression), typeChecker);
+        }
         else {
             const argumentInfo = SignatureHelp.getImmediatelyContainingArgumentInfo(node, position, sourceFile);
             if (argumentInfo) {
@@ -184,7 +196,7 @@ namespace ts.Completions {
 
             // Get completion for string literal from string literal type
             // i.e. var x: "hi" | "hello" = "/*completion position*/"
-            return getStringLiteralCompletionEntriesFromContextualType(<StringLiteral>node, typeChecker);
+            return getStringLiteralCompletionEntriesFromType(typeChecker.getContextualType(<StringLiteral>node), typeChecker);
         }
     }
 
@@ -228,8 +240,7 @@ namespace ts.Completions {
         return undefined;
     }
 
-    function getStringLiteralCompletionEntriesFromContextualType(node: StringLiteral, typeChecker: TypeChecker): CompletionInfo | undefined {
-        const type = typeChecker.getContextualType(node);
+    function getStringLiteralCompletionEntriesFromType(type: Type, typeChecker: TypeChecker): CompletionInfo | undefined {
         if (type) {
             const entries: CompletionEntry[] = [];
             addStringLiteralCompletionsFromType(type, entries, typeChecker);
@@ -1756,4 +1767,15 @@ namespace ts.Completions {
         catch (e) {}
         return undefined;
     }
+
+    function isEqualityExpression(node: Node): node is BinaryExpression {
+        return isBinaryExpression(node) && isEqualityOperatorKind(node.operatorToken.kind);
+    }
+
+    function isEqualityOperatorKind(kind: SyntaxKind) {
+        return kind == SyntaxKind.EqualsEqualsToken ||
+            kind === SyntaxKind.ExclamationEqualsToken ||
+            kind === SyntaxKind.EqualsEqualsEqualsToken ||
+            kind === SyntaxKind.ExclamationEqualsEqualsToken;
+    }
 }

tests/cases/fourslash/completionForStringLiteral10.ts

@@ -0,0 +1,12 @@
+/// <reference path='fourslash.ts'/>
+
+////type As = 'arf' | 'abacus' | 'abaddon';
+////let a: As;
+////if ('/**/' != a
+
+goTo.marker();
+verify.completionListContains("arf");
+verify.completionListContains("abacus");
+verify.completionListContains("abaddon");
+verify.completionListCount(3);
+

tests/cases/fourslash/completionForStringLiteral11.ts

@@ -0,0 +1,14 @@
+/// <reference path='fourslash.ts'/>
+
+////type As = 'arf' | 'abacus' | 'abaddon';
+////let a: As;
+////switch (a) {
+////    case '/**/
+////}
+
+goTo.marker();
+verify.completionListContains("arf");
+verify.completionListContains("abacus");
+verify.completionListContains("abaddon");
+verify.completionListCount(3);
+

tests/cases/fourslash/completionForStringLiteral8.ts

@@ -0,0 +1,12 @@
+/// <reference path='fourslash.ts'/>
+
+////type As = 'arf' | 'abacus' | 'abaddon';
+////let a: As;
+////if (a === '/**/
+
+goTo.marker();
+verify.completionListContains("arf");
+verify.completionListContains("abacus");
+verify.completionListContains("abaddon");
+verify.completionListCount(3);
+