no-unnecessary-type-assertion: Don't check ! if --strictNullChecks …

…is not enabled (#3724)

src/rules/noUnnecessaryTypeAssertionRule.ts

@@ -45,7 +45,13 @@ export class Rule extends Lint.Rules.TypedRule {
 
     public applyWithProgram(sourceFile: ts.SourceFile, program: ts.Program): Lint.RuleFailure[] {
         return this.applyWithWalker(
-            new Walker(sourceFile, this.ruleName, this.ruleArguments, program.getTypeChecker()),
+            new Walker(
+                sourceFile,
+                this.ruleName,
+                this.ruleArguments,
+                program.getTypeChecker(),
+                !!program.getCompilerOptions().strictNullChecks,
+            ),
         );
     }
 }
@@ -56,6 +62,7 @@ class Walker extends Lint.AbstractWalker<string[]> {
         ruleName: string,
         options: string[],
         private readonly checker: ts.TypeChecker,
+        private readonly strictNullChecks: boolean,
     ) {
         super(sourceFile, ruleName, options);
     }
@@ -64,7 +71,9 @@ class Walker extends Lint.AbstractWalker<string[]> {
         const cb = (node: ts.Node): void => {
             switch (node.kind) {
                 case ts.SyntaxKind.NonNullExpression:
-                    this.checkNonNullAssertion(node as ts.NonNullExpression);
+                    if (this.strictNullChecks) {
+                        this.checkNonNullAssertion(node as ts.NonNullExpression);
+                    }
                     break;
                 case ts.SyntaxKind.TypeAssertionExpression:
                 case ts.SyntaxKind.AsExpression:

test/rules/no-unnecessary-type-assertion/noStrictNullChecks/test.ts.lint

@@ -0,0 +1,6 @@
+declare const x: string | undefined;
+x!;
+
+declare const y: string;
+y as string;
+~~~~~~~~~~~  [This assertion is unnecessary since it does not change the type of the expression.]

test/rules/no-unnecessary-type-assertion/noStrictNullChecks/tsconfig.json

@@ -0,0 +1 @@
+{}

test/rules/no-unnecessary-type-assertion/noStrictNullChecks/tslint.json

@@ -0,0 +1,5 @@
+{
+  "rules": {
+    "no-unnecessary-type-assertion": true
+  }
+}