fix(60375): Parameter inlay hint is incorrect when function has a this type (#60378)

Author: a-tarasyuk

Diff for: src/services/inlayHints.ts

@@ -103,6 +103,7 @@ import {
     NodeArray,
     NodeBuilderFlags,
     ParameterDeclaration,
+    parameterIsThisKeyword,
     PrefixUnaryExpression,
     PropertyDeclaration,
     QuotePreference,
@@ -437,24 +438,26 @@ export function provideInlayHints(context: InlayHintsContext): InlayHint[] {
             return;
         }
 
-        for (let i = 0; i < node.parameters.length && i < signature.parameters.length; ++i) {
-            const param = node.parameters[i];
-            if (!isHintableDeclaration(param)) {
-                continue;
+        let pos = 0;
+        for (const param of node.parameters) {
+            if (isHintableDeclaration(param)) {
+                addParameterTypeHint(param, parameterIsThisKeyword(param) ? signature.thisParameter : signature.parameters[pos]);
             }
-
-            const effectiveTypeAnnotation = getEffectiveTypeAnnotationNode(param);
-            if (effectiveTypeAnnotation) {
+            if (parameterIsThisKeyword(param)) {
                 continue;
             }
+            pos++;
+        }
+    }
 
-            const typeHints = getParameterDeclarationTypeHints(signature.parameters[i]);
-            if (!typeHints) {
-                continue;
-            }
+    function addParameterTypeHint(node: ParameterDeclaration, symbol: Symbol | undefined) {
+        const effectiveTypeAnnotation = getEffectiveTypeAnnotationNode(node);
+        if (effectiveTypeAnnotation || symbol === undefined) return;
 
-            addTypeHints(typeHints, param.questionToken ? param.questionToken.end : param.name.end);
-        }
+        const typeHints = getParameterDeclarationTypeHints(symbol);
+        if (typeHints === undefined) return;
+
+        addTypeHints(typeHints, node.questionToken ? node.questionToken.end : node.name.end);
     }
 
     function getParameterDeclarationTypeHints(symbol: Symbol) {

Diff for: tests/baselines/reference/inlayHintsThisParameter.baseline

@@ -0,0 +1,45 @@
+// === Inlay Hints ===
+fn(function (this, a, b) { });
+                 ^
+{
+  "text": ": any",
+  "position": 126,
+  "kind": "Type",
+  "whitespaceBefore": true
+}
+
+fn(function (this, a, b) { });
+                    ^
+{
+  "text": ": number",
+  "position": 129,
+  "kind": "Type",
+  "whitespaceBefore": true
+}
+
+fn(function (this, a, b) { });
+                       ^
+{
+  "text": ": string",
+  "position": 132,
+  "kind": "Type",
+  "whitespaceBefore": true
+}
+
+fn(function (this: I, a, b) { });
+                       ^
+{
+  "text": ": number",
+  "position": 163,
+  "kind": "Type",
+  "whitespaceBefore": true
+}
+
+fn(function (this: I, a, b) { });
+                          ^
+{
+  "text": ": string",
+  "position": 166,
+  "kind": "Type",
+  "whitespaceBefore": true
+}

Diff for: tests/cases/fourslash/inlayHintsThisParameter.ts

@@ -0,0 +1,17 @@
+/// <reference path="fourslash.ts" />
+
+////interface I {
+////    a: number;
+////}
+////
+////declare function fn(
+////    callback: (a: number, b: string) => void
+////): void;
+////
+////
+////fn(function (this, a, b) { });
+////fn(function (this: I, a, b) { });
+
+verify.baselineInlayHints(undefined, {
+    includeInlayFunctionParameterTypeHints: true,
+});