Basic groundwork for suggestioning removing non-null assertions.

src/compiler/checker.ts

@@ -16551,13 +16551,18 @@ namespace ts {
             undefinedDiagnostic?: DiagnosticMessage,
             nullOrUndefinedDiagnostic?: DiagnosticMessage,
         ) {
-            return checkNonNullType(
-                checkExpression(node),
+            const originalType = checkExpression(node);
+            const nonNullType = checkNonNullType(
+                originalType,
                 node,
                 nullDiagnostic,
                 undefinedDiagnostic,
                 nullOrUndefinedDiagnostic
             );
+            if (strictNullChecks && originalType === nonNullType) {
+                error(node, Diagnostics.This_non_null_assertion_operator_is_unnecessary_for_type_1, typeToString(originalType));
+            }
+            return nonNullType
         }
 
         function checkNonNullType(

src/compiler/diagnosticMessages.json

@@ -3894,6 +3894,10 @@
         "category": "Suggestion",
         "code": 80004
     },
+    "This non-null assertion operator is unnecessary for type '{1}'": {
+        "category": "Suggestion",
+        "code": 80005
+    },
 
     "Add missing 'super()' call": {
         "category": "Message",
@@ -4170,5 +4174,13 @@
     "Generate 'get' and 'set' accessors": {
         "category": "Message",
         "code": 95046
+    },
+    "Remove unnecessary non-null assertion operator": {
+        "category": "Message",
+        "code": 95047
+    },
+    "Remove all unnecessary non-null assertion operators": {
+        "category": "Message",
+        "code": 95048
     }
 }

src/harness/tsconfig.json

@@ -113,6 +113,7 @@
         "../services/codefixes/fixInvalidImportSyntax.ts",
         "../services/codefixes/fixStrictClassInitialization.ts",
         "../services/codefixes/useDefaultImport.ts",
+        "../services/codefixes/removeUnnecessaryNonNullAssertion.ts",
         "../services/refactors/extractSymbol.ts",
         "../services/refactors/generateGetAccessorAndSetAccessor.ts",
         "../services/sourcemaps.ts",

src/server/tsconfig.json

@@ -109,6 +109,7 @@
         "../services/codefixes/fixInvalidImportSyntax.ts",
         "../services/codefixes/fixStrictClassInitialization.ts",
         "../services/codefixes/useDefaultImport.ts",
+        "../services/codefixes/removeUnnecessaryNonNullAssertion.ts",
         "../services/refactors/extractSymbol.ts",
         "../services/refactors/generateGetAccessorAndSetAccessor.ts",
         "../services/sourcemaps.ts",

src/services/codefixes/removeUnnecessaryNonNullAssertion.ts

@@ -0,0 +1,31 @@
+/* @internal */
+namespace ts.codefix {
+    const fixId = "removeUnnecessaryNonNullAssertion";
+    const errorCodes = [Diagnostics.This_non_null_assertion_operator_is_unnecessary_for_type_1.code];
+    registerCodeFix({
+        errorCodes,
+        getCodeActions(context) {
+            const qualifiedName = getNonNullAssertion(context.sourceFile, context.span.start);
+            const changes = textChanges.ChangeTracker.with(context, t => doChange(t, context.sourceFile, qualifiedName));
+            return [createCodeFixAction(fixId, changes, Diagnostics.Remove_unnecessary_non_null_assertion_operator, fixId, Diagnostics.Remove_all_unnecessary_non_null_assertion_operators)];
+        },
+        fixIds: [fixId],
+        getAllCodeActions: (context) => codeFixAll(context, errorCodes, (changes, diag) => {
+            const q = getNonNullAssertion(diag.file, diag.start);
+            if (q) {
+                doChange(changes, diag.file, q);
+            }
+        }),
+    });
+
+    function getNonNullAssertion(sourceFile: SourceFile, pos: number): NonNullExpression {
+        const qualifiedName = findAncestor(getTokenAtPosition(sourceFile, pos, /*includeJsDocComment*/ true), isNonNullExpression)!;
+        Debug.assert(!!qualifiedName, "Expected position to be owned by a non-null expression.");
+        return qualifiedName;
+    }
+
+    function doChange(changeTracker: textChanges.ChangeTracker, sourceFile: SourceFile, nonNullExpression: NonNullExpression): void {
+        const expr = nonNullExpression.expression;
+        changeTracker.replaceNode(sourceFile, nonNullExpression, expr);
+    }
+}

src/services/tsconfig.json

@@ -106,6 +106,7 @@
         "codefixes/fixInvalidImportSyntax.ts",
         "codefixes/fixStrictClassInitialization.ts",
         "codefixes/useDefaultImport.ts",
+        "codefixes/removeUnnecessaryNonNullAssertion.ts",
         "refactors/extractSymbol.ts",
         "refactors/generateGetAccessorAndSetAccessor.ts",
         "sourcemaps.ts",