Fix completion preselection in ternary conditional expressions (#2211)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: jakebailey <[email protected]>

Author: Copilot

internal/fourslash/tests/manual/completionInTernaryConditional_test.go

@@ -0,0 +1,102 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	. "github.com/microsoft/typescript-go/internal/fourslash/tests/util"
+	"github.com/microsoft/typescript-go/internal/lsp/lsproto"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestCompletionInTernaryConditional(t *testing.T) {
+	t.Parallel()
+
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	const content = `export enum Bar { }
+export enum Foo { }
+
+
+function foo(x: Foo) { return x; }
+function bar(z: string, x: Foo) { return x; }
+
+const a = '';
+
+foo(/*1*/);
+bar(a, a == '' ? /*2*/);
+bar(a, a == '' ? /*3*/ : /*4*/);`
+	f, done := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	defer done()
+
+	// Test marker 1 - should have Foo preselected in simple call
+	f.VerifyCompletions(t, "1", &fourslash.CompletionsExpectedList{
+		IsIncomplete: false,
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{
+				&lsproto.CompletionItem{
+					Label:     "Foo",
+					Kind:      PtrTo(lsproto.CompletionItemKindEnum),
+					Preselect: PtrTo(true),
+				},
+			},
+		},
+	})
+
+	// Test marker 2 - should have Foo preselected after ? in incomplete ternary
+	f.VerifyCompletions(t, "2", &fourslash.CompletionsExpectedList{
+		IsIncomplete: false,
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{
+				&lsproto.CompletionItem{
+					Label:     "Foo",
+					Kind:      PtrTo(lsproto.CompletionItemKindEnum),
+					Preselect: PtrTo(true),
+				},
+			},
+		},
+	})
+
+	// Test marker 3 - should have Foo preselected after ? in ternary with colon
+	f.VerifyCompletions(t, "3", &fourslash.CompletionsExpectedList{
+		IsIncomplete: false,
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{
+				&lsproto.CompletionItem{
+					Label:     "Foo",
+					Kind:      PtrTo(lsproto.CompletionItemKindEnum),
+					Preselect: PtrTo(true),
+				},
+			},
+		},
+	})
+
+	// Test marker 4 - should have Foo preselected after : in ternary
+	f.VerifyCompletions(t, "4", &fourslash.CompletionsExpectedList{
+		IsIncomplete: false,
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{
+				&lsproto.CompletionItem{
+					Label:     "Foo",
+					Kind:      PtrTo(lsproto.CompletionItemKindEnum),
+					Preselect: PtrTo(true),
+				},
+			},
+		},
+	})
+}

internal/ls/completions.go

@@ -2923,6 +2923,21 @@ func isStaticProperty(symbol *ast.Symbol) bool {
 		ast.IsClassLike(symbol.ValueDeclaration.Parent)
 }
 
+// getContextualTypeForConditionalExpression handles completion within a conditional expression
+// (ternary operator) by using the parent expression to find the contextual type.
+func getContextualTypeForConditionalExpression(conditionalExpr *ast.Node, position int, file *ast.SourceFile, typeChecker *checker.Checker) *checker.Type {
+	argInfo := getArgumentInfoForCompletions(conditionalExpr, position, file, typeChecker)
+	if argInfo != nil {
+		return typeChecker.GetContextualTypeForArgumentAtIndex(argInfo.invocation, argInfo.argumentIndex)
+	}
+	// Fall through to regular contextual type logic if not in an argument
+	contextualType := typeChecker.GetContextualType(conditionalExpr, checker.ContextFlagsCompletions)
+	if contextualType != nil {
+		return contextualType
+	}
+	return typeChecker.GetContextualType(conditionalExpr, checker.ContextFlagsNone)
+}
+
 func getContextualType(previousToken *ast.Node, position int, file *ast.SourceFile, typeChecker *checker.Checker) *checker.Type {
 	parent := previousToken.Parent
 	switch previousToken.Kind {
@@ -2952,6 +2967,23 @@ func getContextualType(previousToken *ast.Node, position int, file *ast.SourceFi
 			return typeChecker.GetContextualTypeForJsxAttribute(parent.Parent)
 		}
 		return nil
+	case ast.KindQuestionToken:
+		// When completing after `?` in a ternary conditional (e.g., `foo(a ? /*here*/)`),
+		// we need to look at the parent conditional expression to find the contextual type.
+		if ast.IsConditionalExpression(parent) {
+			return getContextualTypeForConditionalExpression(parent, position, file, typeChecker)
+		}
+		return nil
+	case ast.KindColonToken:
+		// When completing after `:` in a ternary conditional (e.g., `foo(a ? b : /*here*/)`),
+		// we need to look at the parent conditional expression to find the contextual type.
+		// Only handle this if parent is ConditionalExpression, otherwise fall through to default
+		// (colons are used in other contexts like object literals, type annotations, etc.)
+		if ast.IsConditionalExpression(parent) {
+			return getContextualTypeForConditionalExpression(parent, position, file, typeChecker)
+		}
+		// Fall through to default for other colon contexts (object literals, etc.)
+		fallthrough
 	default:
 		argInfo := getArgumentInfoForCompletions(previousToken, position, file, typeChecker)
 		if argInfo != nil {