Deprecate module keyword for namespace declarations

src/compiler/checker.ts

@@ -341,6 +341,7 @@ import {
     getNamespaceDeclarationNode,
     getNewTargetContainer,
     getNonAugmentationDeclaration,
+    getNonModifierTokenPosOfNode,
     getNormalizedAbsolutePath,
     getObjectFlags,
     getOriginalNode,
@@ -46886,6 +46887,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
             if (isIdentifier(node.name)) {
                 checkCollisionsForDeclarationName(node, node.name);
+                if (!(node.flags & (NodeFlags.Namespace | NodeFlags.GlobalAugmentation))) {
+                    const sourceFile = getSourceFileOfNode(node);
+                    const pos = getNonModifierTokenPosOfNode(node);
+                    const span = getSpanOfTokenAtPosition(sourceFile, pos);
+                    suggestionDiagnostics.add(
+                        createFileDiagnostic(sourceFile, span.start, span.length, Diagnostics.A_namespace_declaration_should_not_be_declared_using_the_module_keyword_Please_use_the_namespace_keyword_instead),
+                    );
+                }
             }
 
             checkExportsOnMergedDeclarations(node);

src/compiler/diagnosticMessages.json

@@ -1813,6 +1813,11 @@
         "category": "Error",
         "code": 1539
     },
+    "A 'namespace' declaration should not be declared using the 'module' keyword. Please use the 'namespace' keyword instead.": {
+        "category": "Suggestion",
+        "code": 1540,
+        "reportsDeprecated": true
+    },
 
     "The types of '{0}' are incompatible between these types.": {
         "category": "Error",

src/compiler/utilities.ts

@@ -1259,6 +1259,16 @@ export function getNonDecoratorTokenPosOfNode(node: Node, sourceFile?: SourceFil
     return skipTrivia((sourceFile || getSourceFileOfNode(node)).text, lastDecorator.end);
 }
 
+/** @internal */
+export function getNonModifierTokenPosOfNode(node: Node, sourceFile?: SourceFileLike): number {
+    const lastModifier = !nodeIsMissing(node) && canHaveModifiers(node) && node.modifiers ? last(node.modifiers) : undefined;
+    if (!lastModifier) {
+        return getTokenPosOfNode(node, sourceFile);
+    }
+
+    return skipTrivia((sourceFile || getSourceFileOfNode(node)).text, lastModifier.end);
+}
+
 /** @internal */
 export function getSourceTextOfNodeFromSourceFile(sourceFile: SourceFile, node: Node, includeTrivia = false): string {
     return getTextOfNodeFromSourceText(sourceFile.text, node, includeTrivia);

tests/cases/fourslash/moduleDeclarationDeprecated_suggestion1.ts

@@ -0,0 +1,30 @@
+///<reference path="fourslash.ts" />
+// @Filename: a.ts
+////[|module|] mod1 { export let x: number }
+////declare [|module|] mod2 { export let x: number }
+////export [|module|] mod3 { export let x: number }
+////export declare [|module|] mod4 { export let x: number }
+////namespace mod5 { export let x: number }
+////declare namespace mod6 { export let x: number }
+////declare module "module-augmentation" {}
+////declare global {}
+////mod1.x = 1;
+////mod2.x = 1;
+////mod5.x = 1;
+////mod6.x = 1;
+
+// @Filename: b.ts
+////module "global-ambient-module" {}
+
+goTo.file("a.ts")
+const diagnostics = test.ranges().map(range => ({
+    code: 1540,
+    message: "A 'namespace' declaration should not be declared using the 'module' keyword. Please use the 'namespace' keyword instead.",
+    reportsDeprecated: true,
+    range,
+}));
+verify.getSuggestionDiagnostics(diagnostics)
+
+goTo.file("b.ts")
+verify.getSuggestionDiagnostics([])
+

tests/cases/fourslash/moduleDeclarationDeprecated_suggestion2.ts

@@ -0,0 +1,6 @@
+///<reference path="fourslash.ts" />
+// @Filename: a.ts
+////declare module
+
+const ranges = test.ranges();
+verify.getSuggestionDiagnostics([])