Deprecate module keyword for namespace declarations

src/compiler/checker.ts

@@ -340,6 +340,7 @@ import {
     getNamespaceDeclarationNode,
     getNewTargetContainer,
     getNonAugmentationDeclaration,
+    getNonModifierTokenPosOfNode,
     getNormalizedAbsolutePath,
     getObjectFlags,
     getOriginalNode,
@@ -46365,6 +46366,14 @@ 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

@@ -1797,6 +1797,11 @@
         "category": "Error",
         "code": 1535
     },
+    "A namespace declaration should not be declared using the module keyword. Please use the namespace keyword instead.": {
+        "category": "Suggestion",
+        "code": 1536,
+        "reportsDeprecated": true
+    },
 
     "The types of '{0}' are incompatible between these types.": {
         "category": "Error",

src/compiler/utilities.ts

@@ -1207,6 +1207,16 @@ 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 +1,41 @@
+///<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 global {}
+////mod1.x = 1;
+////mod2.x = 1;
+////mod5.x = 1;
+////mod6.x = 1;
+
+const ranges = test.ranges();
+verify.getSuggestionDiagnostics([
+    {
+        "code": 1536,
+        "message": "A namespace declaration should not be declared using the module keyword. Please use the namespace keyword instead.",
+        "reportsDeprecated": true,
+        "range": ranges[0]
+    },
+    {
+        "code": 1536,
+        "message": "A namespace declaration should not be declared using the module keyword. Please use the namespace keyword instead.",
+        "reportsDeprecated": true,
+        "range": ranges[1]
+    },
+    {
+        "code": 1536,
+        "message": "A namespace declaration should not be declared using the module keyword. Please use the namespace keyword instead.",
+        "reportsDeprecated": true,
+        "range": ranges[2]
+    },
+    {
+        "code": 1536,
+        "message": "A namespace declaration should not be declared using the module keyword. Please use the namespace keyword instead.",
+        "reportsDeprecated": true,
+        "range": ranges[3]
+    },
+])