Resolve function-this in parameter initialisers when explicitly provided

src/compiler/checker.ts

@@ -8392,7 +8392,10 @@ namespace ts {
             if (needToCaptureLexicalThis) {
                 captureLexicalThis(node, container);
             }
-            if (isFunctionLike(container) && !isInParameterInitializerBeforeContainingFunction(node)) {
+            if (isFunctionLike(container) &&
+                (!isInParameterInitializerBeforeContainingFunction(node) || getFunctionLikeThisParameter(container))) {
+                // Note: a parameter initializer should refer to class-this unless function-this is explicitly annotated.
+
                 // If this is a function in a JS file, it might be a class method. Check if it's the RHS
                 // of a x.prototype.y = function [name]() { .... }
                 if (container.kind === SyntaxKind.FunctionExpression &&
@@ -18473,6 +18476,14 @@ namespace ts {
             }
         }
 
+        function getFunctionLikeThisParameter(func: FunctionLikeDeclaration) {
+            if (func.parameters.length &&
+                func.parameters[0].name.kind === SyntaxKind.Identifier &&
+                (<Identifier>func.parameters[0].name).originalKeywordKind === SyntaxKind.ThisKeyword) {
+                return func.parameters[0];
+            }
+        }
+
         function checkGrammarForNonSymbolComputedProperty(node: DeclarationName, message: DiagnosticMessage) {
             if (isDynamicName(node)) {
                 return grammarErrorOnNode(node, message);

tests/baselines/reference/inferParameterWithMethodCallInitializer.js

@@ -10,6 +10,14 @@ class Example {
         return a;
     }
 }
+function weird(this: Example, a = this.getNumber()) {
+    return a;
+}
+class Weird {
+    doSomething(this: Example, a = this.getNumber()) {
+        return a;
+    }
+}
 
 
 //// [inferParameterWithMethodCallInitializer.js]
@@ -28,3 +36,16 @@ var Example = (function () {
     };
     return Example;
 }());
+function weird(a) {
+    if (a === void 0) { a = this.getNumber(); }
+    return a;
+}
+var Weird = (function () {
+    function Weird() {
+    }
+    Weird.prototype.doSomething = function (a) {
+        if (a === void 0) { a = this.getNumber(); }
+        return a;
+    };
+    return Weird;
+}());

tests/baselines/reference/inferParameterWithMethodCallInitializer.symbols

@@ -24,4 +24,32 @@ class Example {
 >a : Symbol(a, Decl(inferParameterWithMethodCallInitializer.ts, 7, 16))
     }
 }
+function weird(this: Example, a = this.getNumber()) {
+>weird : Symbol(weird, Decl(inferParameterWithMethodCallInitializer.ts, 10, 1))
+>this : Symbol(this, Decl(inferParameterWithMethodCallInitializer.ts, 11, 15))
+>Example : Symbol(Example, Decl(inferParameterWithMethodCallInitializer.ts, 2, 1))
+>a : Symbol(a, Decl(inferParameterWithMethodCallInitializer.ts, 11, 29))
+>this.getNumber : Symbol(Example.getNumber, Decl(inferParameterWithMethodCallInitializer.ts, 3, 15))
+>this : Symbol(Example, Decl(inferParameterWithMethodCallInitializer.ts, 2, 1))
+>getNumber : Symbol(Example.getNumber, Decl(inferParameterWithMethodCallInitializer.ts, 3, 15))
+
+    return a;
+>a : Symbol(a, Decl(inferParameterWithMethodCallInitializer.ts, 11, 29))
+}
+class Weird {
+>Weird : Symbol(Weird, Decl(inferParameterWithMethodCallInitializer.ts, 13, 1))
+
+    doSomething(this: Example, a = this.getNumber()) {
+>doSomething : Symbol(Weird.doSomething, Decl(inferParameterWithMethodCallInitializer.ts, 14, 13))
+>this : Symbol(this, Decl(inferParameterWithMethodCallInitializer.ts, 15, 16))
+>Example : Symbol(Example, Decl(inferParameterWithMethodCallInitializer.ts, 2, 1))
+>a : Symbol(a, Decl(inferParameterWithMethodCallInitializer.ts, 15, 30))
+>this.getNumber : Symbol(Example.getNumber, Decl(inferParameterWithMethodCallInitializer.ts, 3, 15))
+>this : Symbol(Example, Decl(inferParameterWithMethodCallInitializer.ts, 2, 1))
+>getNumber : Symbol(Example.getNumber, Decl(inferParameterWithMethodCallInitializer.ts, 3, 15))
+
+        return a;
+>a : Symbol(a, Decl(inferParameterWithMethodCallInitializer.ts, 15, 30))
+    }
+}
 

tests/baselines/reference/inferParameterWithMethodCallInitializer.types

@@ -27,4 +27,34 @@ class Example {
 >a : number
     }
 }
+function weird(this: Example, a = this.getNumber()) {
+>weird : (this: Example, a?: number) => number
+>this : Example
+>Example : Example
+>a : number
+>this.getNumber() : number
+>this.getNumber : () => number
+>this : Example
+>getNumber : () => number
+
+    return a;
+>a : number
+}
+class Weird {
+>Weird : Weird
+
+    doSomething(this: Example, a = this.getNumber()) {
+>doSomething : (this: Example, a?: number) => number
+>this : Example
+>Example : Example
+>a : number
+>this.getNumber() : number
+>this.getNumber : () => number
+>this : Example
+>getNumber : () => number
+
+        return a;
+>a : number
+    }
+}
 

tests/cases/compiler/inferParameterWithMethodCallInitializer.ts

@@ -10,3 +10,11 @@ class Example {
         return a;
     }
 }
+function weird(this: Example, a = this.getNumber()) {
+    return a;
+}
+class Weird {
+    doSomething(this: Example, a = this.getNumber()) {
+        return a;
+    }
+}