In JS, fix contextual type of this-property assignments

src/compiler/checker.ts

@@ -16127,6 +16127,16 @@ namespace ts {
                         return true;
                     }
                 case SpecialPropertyAssignmentKind.ThisProperty:
+                    if (!binaryExpression.symbol ||
+                        binaryExpression.symbol.valueDeclaration && !!getJSDocTypeTag(binaryExpression.symbol.valueDeclaration)) {
+                        return true;
+                    }
+                    const thisAccess = binaryExpression.left as PropertyAccessExpression;
+                    if (!isObjectLiteralMethod(getThisContainer(thisAccess.expression, /*includeArrowFunctions*/ false))) {
+                        return false;
+                    }
+                    const thisType = checkThisExpression(thisAccess.expression);
+                    return thisType && !!getPropertyOfType(thisType, thisAccess.name.escapedText);
                 case SpecialPropertyAssignmentKind.ModuleExports:
                     return !binaryExpression.symbol || binaryExpression.symbol.valueDeclaration && !!getJSDocTypeTag(binaryExpression.symbol.valueDeclaration);
                 default:

tests/baselines/reference/typeFromContextualThisType.symbols

@@ -0,0 +1,40 @@
+=== tests/cases/conformance/salsa/bug25926.js ===
+/** @type {{ a(): void; b?(n: number): number; }} */
+const o1 = {
+>o1 : Symbol(o1, Decl(bug25926.js, 1, 5))
+
+    a() {
+>a : Symbol(a, Decl(bug25926.js, 1, 12))
+
+        this.b = n => n;
+>this.b : Symbol(b, Decl(bug25926.js, 0, 23))
+>this : Symbol(__type, Decl(bug25926.js, 0, 11))
+>b : Symbol(b, Decl(bug25926.js, 2, 9))
+>n : Symbol(n, Decl(bug25926.js, 3, 16))
+>n : Symbol(n, Decl(bug25926.js, 3, 16))
+    }
+};
+
+/** @type {{ d(): void; e?(n: number): number; f?(n: number): number; g?: number }} */
+const o2 = {
+>o2 : Symbol(o2, Decl(bug25926.js, 8, 5))
+
+    d() {
+>d : Symbol(d, Decl(bug25926.js, 8, 12))
+
+        this.e = this.f = m => this.g || m;
+>this.e : Symbol(e, Decl(bug25926.js, 7, 23))
+>this : Symbol(__type, Decl(bug25926.js, 7, 11))
+>e : Symbol(e, Decl(bug25926.js, 9, 9))
+>this.f : Symbol(f, Decl(bug25926.js, 7, 46))
+>this : Symbol(__type, Decl(bug25926.js, 7, 11))
+>f : Symbol(f, Decl(bug25926.js, 10, 16))
+>m : Symbol(m, Decl(bug25926.js, 10, 25))
+>this.g : Symbol(g, Decl(bug25926.js, 7, 69))
+>this : Symbol(__type, Decl(bug25926.js, 7, 11))
+>g : Symbol(g, Decl(bug25926.js, 7, 69))
+>m : Symbol(m, Decl(bug25926.js, 10, 25))
+    }
+};
+
+

tests/baselines/reference/typeFromContextualThisType.types

@@ -0,0 +1,48 @@
+=== tests/cases/conformance/salsa/bug25926.js ===
+/** @type {{ a(): void; b?(n: number): number; }} */
+const o1 = {
+>o1 : { a(): void; }
+>{    a() {        this.b = n => n;    }} : { a(): void; }
+
+    a() {
+>a : () => void
+
+        this.b = n => n;
+>this.b = n => n : (n: number) => number
+>this.b : ((n: number) => number) | undefined
+>this : { a(): void; }
+>b : ((n: number) => number) | undefined
+>n => n : (n: number) => number
+>n : number
+>n : number
+    }
+};
+
+/** @type {{ d(): void; e?(n: number): number; f?(n: number): number; g?: number }} */
+const o2 = {
+>o2 : { d(): void; g?: number | undefined; }
+>{    d() {        this.e = this.f = m => this.g || m;    }} : { d(): void; }
+
+    d() {
+>d : () => void
+
+        this.e = this.f = m => this.g || m;
+>this.e = this.f = m => this.g || m : (m: number) => number
+>this.e : ((n: number) => number) | undefined
+>this : { d(): void; g?: number | undefined; }
+>e : ((n: number) => number) | undefined
+>this.f = m => this.g || m : (m: number) => number
+>this.f : ((n: number) => number) | undefined
+>this : { d(): void; g?: number | undefined; }
+>f : ((n: number) => number) | undefined
+>m => this.g || m : (m: number) => number
+>m : number
+>this.g || m : number
+>this.g : number | undefined
+>this : { d(): void; g?: number | undefined; }
+>g : number | undefined
+>m : number
+    }
+};
+
+

tests/cases/conformance/salsa/typeFromContextualThisType.ts

@@ -0,0 +1,20 @@
+// @noEmit: true
+// @allowJs: true
+// @checkJs: true
+// @strict: true
+// @Filename: bug25926.js
+
+/** @type {{ a(): void; b?(n: number): number; }} */
+const o1 = {
+    a() {
+        this.b = n => n;
+    }
+};
+
+/** @type {{ d(): void; e?(n: number): number; f?(n: number): number; g?: number }} */
+const o2 = {
+    d() {
+        this.e = this.f = m => this.g || m;
+    }
+};
+