Fix hang in signature help due to loop in findPrecedingMatchingToken

tsc/internal/fourslash/tests/signatureHelpUnclosedParen_test.go

@@ -0,0 +1,19 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/TypeScript/tsc/internal/fourslash"
+	"github.com/microsoft/TypeScript/tsc/internal/testutil"
+)
+
+func TestSignatureHelpUnclosedTypeArgumentHang(t *testing.T) {
+	t.Parallel()
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	const content = `<()(/**/`
+	f, done := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	defer done()
+	f.GoToMarker(t, "")
+
+	f.VerifyNoSignatureHelp(t)
+}

tsc/internal/ls/utilities.go

@@ -1161,12 +1161,13 @@ func findPrecedingMatchingToken(token *ast.Node, matchingTokenKind ast.Kind, sou
 	matchingTokenText := scanner.TokenToString(matchingTokenKind)
 	// Text-scan based fast path - can be bamboozled by comments and other trivia, but often provides
 	// a good, fast approximation without too much extra work in the cases where it fails.
-	bestGuessIndex := strings.LastIndex(sourceFile.Text(), matchingTokenText)
+	precedingText := sourceFile.Text()[:token.Pos()]
+	bestGuessIndex := strings.LastIndex(precedingText, matchingTokenText)
 	if bestGuessIndex == -1 {
 		return nil // if the token text doesn't appear in the file, there can't be a match - super fast bail
 	}
 	// we can only use the textual result directly if we didn't have to count any close tokens within the range
-	if strings.LastIndex(sourceFile.Text(), closeTokenText) < bestGuessIndex {
+	if strings.LastIndex(precedingText, closeTokenText) < bestGuessIndex {
 		nodeAtGuess := astnav.FindPrecedingToken(sourceFile, bestGuessIndex+1)
 		if nodeAtGuess != nil && nodeAtGuess.Kind == matchingTokenKind {
 			return nodeAtGuess