Fix inferFromUsage on index signatures (#25934)

* Fix inferFromUsage on index signatures

1. Check the argumentExpression to determine the index signature type.
Previously, the code mistakenly checked the type of the element access
itself, which never returned a good type.
2. If inference for the index signature type fails, substitute anyType.
Previously, the code would create a bad index signature with an
undefined type.
3. Add tests. Previously, there were no tests.

* Fixing (1) means that number index signatures are now created.
Previously, only string index signatures would be created.
* Fixing (2) means that index signatures will be inferred from single
usage like `return a[0]`. Previously, the refactoring would fail,
perhaps because of a check when stringifying the index signature (I
haven't tracked down why.)

* Update fourslash test with improved inference

Author: sandersn

src/services/codefixes/inferFromUsage.ts

@@ -501,7 +501,7 @@ namespace ts.codefix {
                 return;
             }
             else {
-                const indexType = checker.getTypeAtLocation(parent);
+                const indexType = checker.getTypeAtLocation(parent.argumentExpression);
                 const indexUsageContext = {};
                 inferTypeFromContext(parent, checker, indexUsageContext);
                 if (indexType.flags & TypeFlags.NumberLike) {
@@ -562,11 +562,11 @@ namespace ts.codefix {
                 }
 
                 if (usageContext.numberIndexContext) {
-                    numberIndexInfo = checker.createIndexInfo(getTypeFromUsageContext(usageContext.numberIndexContext, checker)!, /*isReadonly*/ false); // TODO: GH#18217
+                    numberIndexInfo = checker.createIndexInfo(getTypeFromUsageContext(usageContext.numberIndexContext, checker) || checker.getAnyType(), /*isReadonly*/ false);
                 }
 
                 if (usageContext.stringIndexContext) {
-                    stringIndexInfo = checker.createIndexInfo(getTypeFromUsageContext(usageContext.stringIndexContext, checker)!, /*isReadonly*/ false);
+                    stringIndexInfo = checker.createIndexInfo(getTypeFromUsageContext(usageContext.stringIndexContext, checker) || checker.getAnyType(), /*isReadonly*/ false);
                 }
 
                 return checker.createAnonymousType(/*symbol*/ undefined!, members, callSignatures, constructSignatures, stringIndexInfo, numberIndexInfo); // TODO: GH#18217

tests/cases/fourslash/codeFixInferFromUsageNumberIndexSignature.ts

@@ -0,0 +1,8 @@
+/// <reference path='fourslash.ts' />
+
+// @noImplicitAny: true
+////function f([|a |]) {
+////    return a[0] + 1;
+////}
+
+verify.rangeAfterCodeFix("a: { [x: number]: number; }",/*includeWhiteSpace*/ undefined, /*errorCode*/ undefined, 0);

tests/cases/fourslash/codeFixInferFromUsageStringIndexSignature.ts

@@ -0,0 +1,8 @@
+/// <reference path='fourslash.ts' />
+
+// @noImplicitAny: true
+////function f([|a |]) {
+////    return a['hi'];
+////}
+
+verify.rangeAfterCodeFix("a: { [x: string]: any; }",/*includeWhiteSpace*/ undefined, /*errorCode*/ undefined, 0);

tests/cases/fourslash/typeToStringCrashInCodeFix.ts

@@ -1,6 +1,6 @@
 /// <reference path="fourslash.ts" />
 
 // @noImplicitAny: true
-//// function f(y, z = { p: y[
+//// function f([|y |], z = { p: y[
 
-verify.not.codeFixAvailable();
+verify.rangeAfterCodeFix("y: { [x: string]: any; }",/*includeWhiteSpace*/ undefined, /*errorCode*/ undefined, 0);