Make sure to instantiate merged type parameters

src/compiler/checker.ts

@@ -5464,7 +5464,7 @@ namespace ts {
                         const declaration = <DeclarationWithTypeParameters>node;
                         if (declaration.typeParameters) {
                             for (const d of declaration.typeParameters) {
-                                if (contains(mappedTypes, getDeclaredTypeOfTypeParameter(d.symbol))) {
+                                if (contains(mappedTypes, getDeclaredTypeOfTypeParameter(getMergedSymbol(d.symbol)))) {
                                     return true;
                                 }
                             }

tests/baselines/reference/instantiateCrossFileMerge.js

@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/instantiateCrossFileMerge.ts] ////
+
+//// [first.ts]
+declare class P<R> {
+    constructor(callback: (resolve: (value: R) => void) => void);
+}
+
+//// [second.ts]
+interface P<R> { }
+new P<string>(r => { r('foo') });
+
+
+//// [first.js]
+//// [second.js]
+new P(function (r) { r('foo'); });

tests/baselines/reference/instantiateCrossFileMerge.symbols

@@ -0,0 +1,22 @@
+=== tests/cases/compiler/first.ts ===
+declare class P<R> {
+>P : Symbol(P, Decl(first.ts, 0, 0), Decl(second.ts, 0, 0))
+>R : Symbol(R, Decl(first.ts, 0, 16), Decl(second.ts, 0, 12))
+
+    constructor(callback: (resolve: (value: R) => void) => void);
+>callback : Symbol(callback, Decl(first.ts, 1, 16))
+>resolve : Symbol(resolve, Decl(first.ts, 1, 27))
+>value : Symbol(value, Decl(first.ts, 1, 37))
+>R : Symbol(R, Decl(first.ts, 0, 16), Decl(second.ts, 0, 12))
+}
+
+=== tests/cases/compiler/second.ts ===
+interface P<R> { }
+>P : Symbol(P, Decl(first.ts, 0, 0), Decl(second.ts, 0, 0))
+>R : Symbol(R, Decl(first.ts, 0, 16), Decl(second.ts, 0, 12))
+
+new P<string>(r => { r('foo') });
+>P : Symbol(P, Decl(first.ts, 0, 0), Decl(second.ts, 0, 0))
+>r : Symbol(r, Decl(second.ts, 1, 14))
+>r : Symbol(r, Decl(second.ts, 1, 14))
+

tests/baselines/reference/instantiateCrossFileMerge.types

@@ -0,0 +1,26 @@
+=== tests/cases/compiler/first.ts ===
+declare class P<R> {
+>P : P<R>
+>R : R
+
+    constructor(callback: (resolve: (value: R) => void) => void);
+>callback : (resolve: (value: R) => void) => void
+>resolve : (value: R) => void
+>value : R
+>R : R
+}
+
+=== tests/cases/compiler/second.ts ===
+interface P<R> { }
+>P : P<R>
+>R : R
+
+new P<string>(r => { r('foo') });
+>new P<string>(r => { r('foo') }) : P<string>
+>P : typeof P
+>r => { r('foo') } : (r: (value: string) => void) => void
+>r : (value: string) => void
+>r('foo') : void
+>r : (value: string) => void
+>'foo' : string
+

tests/cases/compiler/instantiateCrossFileMerge.ts

@@ -0,0 +1,8 @@
+// @filename: first.ts
+declare class P<R> {
+    constructor(callback: (resolve: (value: R) => void) => void);
+}
+
+// @filename: second.ts
+interface P<R> { }
+new P<string>(r => { r('foo') });