Fix completions crash and enable contextual type completions in array literals (#2258)

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

Author: Copilot

internal/checker/services.go

@@ -671,6 +671,18 @@ func (c *Checker) GetContextualDeclarationsForObjectLiteralElement(objectLiteral
 	return result
 }
 
+// GetContextualTypeForArrayElement returns the contextual type for an element at the given index
+// in an array with the given contextual type.
+func (c *Checker) GetContextualTypeForArrayElement(contextualArrayType *Type, elementIndex int) *Type {
+	if contextualArrayType == nil {
+		return nil
+	}
+	// Pass -1 for length, firstSpreadIndex, and lastSpreadIndex since we don't have
+	// access to the actual array literal. This falls back to getting the iterated type
+	// or checking numeric properties, which is appropriate for completion contexts.
+	return c.getContextualTypeForElementExpression(contextualArrayType, elementIndex, -1, -1, -1)
+}
+
 var knownGenericTypeNames = map[string]struct{}{
 	"Array":            {},
 	"ArrayLike":        {},

internal/fourslash/tests/manual/completionsInArrayLiteralWithContextualType_test.go

@@ -0,0 +1,69 @@
+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/testutil"
+)
+
+// Test that string literal completions are suggested in tuple contexts
+// even without typing a quote character.
+func TestCompletionsInArrayLiteralWithContextualType(t *testing.T) {
+	t.Parallel()
+
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+
+	// Test 1: Completions after `[` in a tuple should suggest string literals
+	const content1 = `let y: ["foo" | "bar", string] = [/*a*/];`
+	f1, done1 := fourslash.NewFourslash(t, nil /*capabilities*/, content1)
+	defer done1()
+	f1.VerifyCompletions(t, "a", &fourslash.CompletionsExpectedList{
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{
+				"\"foo\"",
+				"\"bar\"",
+			},
+		},
+	})
+
+	// Test 2: Completions after `,` in a tuple should provide contextual type for second element
+	const content2 = `let z: ["a", "b" | "c"] = ["a", /*b*/];`
+	f2, done2 := fourslash.NewFourslash(t, nil /*capabilities*/, content2)
+	defer done2()
+	f2.VerifyCompletions(t, "b", &fourslash.CompletionsExpectedList{
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{
+				"\"b\"",
+				"\"c\"",
+			},
+		},
+	})
+
+	// Test 3: Verify that properties named "-1" are NOT suggested in array literals
+	// This was a bug in the old implementation where passing -1 as an index would
+	// check for a property named "-1" and suggest its value
+	const content3 = `let x: { "-1": "hello" } = [/*c*/];`
+	f3, done3 := fourslash.NewFourslash(t, nil /*capabilities*/, content3)
+	defer done3()
+	f3.VerifyCompletions(t, "c", &fourslash.CompletionsExpectedList{
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Excludes: []string{
+				"\"hello\"",
+			},
+		},
+	})
+}

internal/fourslash/tests/manual/completionsInMapConstructorNoCrash_test.go

@@ -0,0 +1,49 @@
+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/testutil"
+)
+
+// Test for issue: Completions crash in call to `new Map(...)`.
+// When requesting completions inside a Map constructor's array literal,
+// IndexOfNode returns -1 causing a panic in getContextualTypeForElementExpression
+// when trying to access array elements without a bounds check.
+func TestCompletionsInMapConstructorNoCrash(t *testing.T) {
+	t.Parallel()
+
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+
+	// Test completion at position /*a*/ - before the string literal
+	const content1 = `const m = new Map([
+    [/*a*/'0', ['0', false]],
+]);`
+	f1, done1 := fourslash.NewFourslash(t, nil /*capabilities*/, content1)
+	defer done1()
+	// Just verify that completions don't crash - accept any completion list
+	f1.VerifyCompletions(t, "a", &fourslash.CompletionsExpectedList{
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{},
+	})
+
+	// Test completion at position /*b*/ - after the array literal
+	const content2 = `const m = new Map([
+    ['0', ['0', false]]/*b*/,
+]);`
+	f2, done2 := fourslash.NewFourslash(t, nil /*capabilities*/, content2)
+	defer done2()
+	// Just verify that completions don't crash - accept any completion list
+	f2.VerifyCompletions(t, "b", &fourslash.CompletionsExpectedList{
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{},
+	})
+}

internal/ls/completions.go

@@ -2967,6 +2967,37 @@ func getContextualType(previousToken *ast.Node, position int, file *ast.SourceFi
 			return typeChecker.GetContextualTypeForJsxAttribute(parent.Parent)
 		}
 		return nil
+	case ast.KindOpenBracketToken:
+		// When completing after `[` in an array literal (e.g., `[/*here*/]`),
+		// we should provide contextual type for the first element
+		if ast.IsArrayLiteralExpression(parent) {
+			contextualArrayType := typeChecker.GetContextualType(parent, checker.ContextFlagsNone)
+			if contextualArrayType != nil {
+				// Get the type for the first element (index 0)
+				return typeChecker.GetContextualTypeForArrayElement(contextualArrayType, 0)
+			}
+		}
+		return nil
+	case ast.KindCommaToken:
+		// When completing after `,` in an array literal (e.g., `[x, /*here*/]`),
+		// we should provide contextual type for the element after the comma
+		if ast.IsArrayLiteralExpression(parent) {
+			contextualArrayType := typeChecker.GetContextualType(parent, checker.ContextFlagsNone)
+			if contextualArrayType != nil {
+				// Count how many elements come before the cursor position
+				arrayLiteral := parent.AsArrayLiteralExpression()
+				elementIndex := 0
+				for _, elem := range arrayLiteral.Elements.Nodes {
+					if elem.Pos() < position {
+						elementIndex++
+					} else {
+						break
+					}
+				}
+				return typeChecker.GetContextualTypeForArrayElement(contextualArrayType, elementIndex)
+			}
+		}
+		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.