Fix checkJs behavior for .mjs and .cjs files next to declarations

tsc/internal/tsoptions/tsconfigparsing.go

@@ -1890,12 +1890,6 @@ func hasFileWithHigherPriorityExtension(file string, extensions [][]string, hasF
 			return false
 		}
 		if hasFile(tspath.ChangeExtension(file, ext)) {
-			if ext == tspath.ExtensionDts && (tspath.FileExtensionIs(file, tspath.ExtensionJs) || tspath.FileExtensionIs(file, tspath.ExtensionJsx)) {
-				// LEGACY BEHAVIOR: An off-by-one bug somewhere in the extension priority system for wildcard module loading allowed declaration
-				// files to be loaded alongside their js(x) counterparts. We regard this as generally undesirable, but retain the behavior to
-				// prevent breakage.
-				continue
-			}
 			return true
 		}
 	}

tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.symbols

@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/checkJsExtensionPriority.ts] ////
+
+=== /a.d.ts ===
+export declare const n: number;
+>n : Symbol(n, Decl(a.d.ts, 0, 20))
+
+=== /b.d.mts ===
+export declare const n: number;
+>n : Symbol(n, Decl(b.d.mts, 0, 20))
+
+=== /c.d.cts ===
+export declare const n: number;
+>n : Symbol(n, Decl(c.d.cts, 0, 20))
+
+

tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority.types

@@ -0,0 +1,15 @@
+//// [tests/cases/compiler/checkJsExtensionPriority.ts] ////
+
+=== /a.d.ts ===
+export declare const n: number;
+>n : number
+
+=== /b.d.mts ===
+export declare const n: number;
+>n : number
+
+=== /c.d.cts ===
+export declare const n: number;
+>n : number
+
+

tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.symbols

@@ -0,0 +1,14 @@
+//// [tests/cases/compiler/checkJsExtensionPriority_includeOrder.ts] ////
+
+=== /a.d.ts ===
+export declare const n: number;
+>n : Symbol(n, Decl(a.d.ts, 0, 20))
+
+=== /b.d.mts ===
+export declare const n: number;
+>n : Symbol(n, Decl(b.d.mts, 0, 20))
+
+=== /c.d.cts ===
+export declare const n: number;
+>n : Symbol(n, Decl(c.d.cts, 0, 20))
+

tsc/testdata/baselines/reference/compiler/checkJsExtensionPriority_includeOrder.types

@@ -0,0 +1,14 @@
+//// [tests/cases/compiler/checkJsExtensionPriority_includeOrder.ts] ////
+
+=== /a.d.ts ===
+export declare const n: number;
+>n : number
+
+=== /b.d.mts ===
+export declare const n: number;
+>n : number
+
+=== /c.d.cts ===
+export declare const n: number;
+>n : number
+

tsc/testdata/tests/cases/compiler/checkJsExtensionPriority.ts

@@ -0,0 +1,42 @@
+// @Filename: /tsconfig.json
+{
+    "compilerOptions": {
+        "target": "esnext",
+        "module": "nodenext",
+        "moduleResolution": "nodenext",
+        "allowJs": true,
+        "checkJs": true,
+        "strict": true,
+        "noEmit": true
+    }
+}
+
+// @Filename: /package.json
+{
+    "type": "module"
+}
+
+// @Filename: /a.js
+export const n = 1;
+const bad = null;
+bad.a.b.c();
+
+// @Filename: /a.d.ts
+export declare const n: number;
+
+// @Filename: /b.mjs
+export const n = 1;
+const bad = null;
+bad.a.b.c();
+
+// @Filename: /b.d.mts
+export declare const n: number;
+
+// @Filename: /c.cjs
+export const n = 1;
+const bad = null;
+bad.a.b.c();
+
+// @Filename: /c.d.cts
+export declare const n: number;
+

tsc/testdata/tests/cases/compiler/checkJsExtensionPriority_includeOrder.ts

@@ -0,0 +1,46 @@
+// @Filename: /tsconfig.json
+{
+    "compilerOptions": {
+        "target": "esnext",
+        "module": "nodenext",
+        "moduleResolution": "nodenext",
+        "allowJs": true,
+        "checkJs": true,
+        "strict": true,
+        "noEmit": true
+    },
+    "include": [
+        "**/*.js",
+        "**/*.d.ts",
+        "**/*.mjs",
+        "**/*.d.mts",
+        "**/*.cjs",
+        "**/*.d.cts"
+    ]
+}
+
+// @Filename: /package.json
+{
+    "type": "module"
+}
+
+// @Filename: /a.js
+const bad = null;
+bad.a();
+
+// @Filename: /a.d.ts
+export declare const n: number;
+
+// @Filename: /b.mjs
+const bad = null;
+bad.b();
+
+// @Filename: /b.d.mts
+export declare const n: number;
+
+// @Filename: /c.cjs
+const bad = null;
+bad.c();
+
+// @Filename: /c.d.cts
+export declare const n: number;