Always respect the --strict_mode_input flag.

Previously, it was silently ignored in ES3, ES5, ES6, ES5_STRICT, ES6_STRICT,
and ES6_TYPED language modes.

Similar to my recent change for about the --emit_use_strict flag.

Note that this means that users of CommandLineRunner who want to avoid strict
mode checks will now need to pass --strict_mode_input=false.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=156799045

src/com/google/javascript/jscomp/CommandLineRunner.java

@@ -173,8 +173,7 @@ private static class Flags {
     @Option(
         name = "--strict_mode_input",
         handler = BooleanOptionHandler.class,
-        usage = "Assume input sources are to run in strict mode."
-            + " Ignored for language modes earlier than ECMASCRIPT_2016.")
+        usage = "Assume input sources are to run in strict mode.")
     private boolean strictModeInput = true;
 
     // Turn on (very slow) extra sanity checks for use when modifying the

src/com/google/javascript/jscomp/Compiler.java

@@ -473,7 +473,7 @@ protected void reconcileOptionsWithGuards() {
           options.checkGlobalThisLevel);
     }
 
-    if (expectStrictModeInput()) {
+    if (options.expectStrictModeInput()) {
       options.setWarningLevel(
           DiagnosticGroups.ES5_STRICT,
           CheckLevel.ERROR);
@@ -491,21 +491,6 @@ protected void reconcileOptionsWithGuards() {
     }
   }
 
-  private boolean expectStrictModeInput() {
-    switch (options.getLanguageIn()) {
-      case ECMASCRIPT3:
-      case ECMASCRIPT5:
-      case ECMASCRIPT6:
-        return false;
-      case ECMASCRIPT5_STRICT:
-      case ECMASCRIPT6_STRICT:
-      case ECMASCRIPT6_TYPED:
-        return true;
-      default:
-        return options.isStrictModeInput();
-    }
-  }
-
   /**
    * Initializes the instance state needed for a compile job.
    */
@@ -2681,7 +2666,7 @@ Config getParserConfig(ConfigContext context) {
           Config.LanguageMode configLanguageMode = getParserConfigLanguageMode(
               options.getLanguageIn());
           Config.StrictMode strictMode =
-              expectStrictModeInput() ? Config.StrictMode.STRICT : Config.StrictMode.SLOPPY;
+              options.expectStrictModeInput() ? Config.StrictMode.STRICT : Config.StrictMode.SLOPPY;
           parserConfig = createConfig(configLanguageMode, strictMode);
           // Externs must always be parsed with at least ES5 language mode.
           externsParserConfig =

src/com/google/javascript/jscomp/CompilerOptions.java

@@ -1151,10 +1151,8 @@ public void setWrapGoogModulesForWhitespaceOnly(boolean enable) {
 
   /**
    * Are the input files written for strict mode?
-   *
-   * <p>Ignored for language modes that do not support strict mode.
    */
-  private boolean isStrictModeInput = true;
+  private Optional<Boolean> isStrictModeInput = Optional.absent();
 
   /** Which algorithm to use for locating ES6 and CommonJS modules */
   ModuleLoader.ResolutionMode moduleResolutionMode;
@@ -2721,18 +2719,7 @@ public void setConformanceConfigs(List<ConformanceConfig> configs) {
   }
 
   public boolean shouldEmitUseStrict() {
-    return this.emitUseStrict.or(getDefaultEmitUseStrict());
-  }
-
-  boolean getDefaultEmitUseStrict() {
-    switch (getLanguageOut()) {
-      case ECMASCRIPT3:
-      case ECMASCRIPT5:
-      case ECMASCRIPT6:
-        return false;
-      default:
-        return true;
-    }
+    return this.emitUseStrict.or(getLanguageOut().isDefaultStrict());
   }
 
   public CompilerOptions setEmitUseStrict(boolean emitUseStrict) {
@@ -3031,6 +3018,19 @@ public static enum LanguageMode {
      */
     NO_TRANSPILE;
 
+
+    /** Whether this language mode defaults to strict mode */
+    boolean isDefaultStrict() {
+      switch (this) {
+        case ECMASCRIPT3:
+        case ECMASCRIPT5:
+        case ECMASCRIPT6:
+          return false;
+        default:
+          return true;
+      }
+    }
+
     /** Whether this is ECMAScript 5 or higher. */
     public boolean isEs5OrHigher() {
       Preconditions.checkState(this != NO_TRANSPILE);
@@ -3310,12 +3310,12 @@ boolean isExplicitlyOn() {
     }
   }
 
-  public boolean isStrictModeInput() {
-    return isStrictModeInput;
+  public boolean expectStrictModeInput() {
+    return isStrictModeInput.or(getLanguageIn().isDefaultStrict());
   }
 
   public CompilerOptions setStrictModeInput(boolean isStrictModeInput) {
-    this.isStrictModeInput = isStrictModeInput;
+    this.isStrictModeInput = Optional.of(isStrictModeInput);
     return this;
   }
 

src/com/google/javascript/jscomp/DefaultPassConfig.java

@@ -2357,7 +2357,7 @@ protected CompilerPass create(AbstractCompiler compiler) {
               options.inlineFunctions,
               options.inlineLocalFunctions,
               true,
-              options.assumeStrictThis() || options.isStrictModeInput(),
+              options.assumeStrictThis() || options.expectStrictModeInput(),
               options.assumeClosuresOnlyCaptureReferences,
               options.maxFunctionSizeAfterInlining);
         }

src/com/google/javascript/jscomp/parsing/Config.java

@@ -16,8 +16,6 @@
 
 package com.google.javascript.jscomp.parsing;
 
-import static com.google.common.base.Preconditions.checkArgument;
-
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.javascript.jscomp.parsing.parser.FeatureSet;
@@ -140,7 +138,6 @@ public enum RunMode {
       LanguageMode languageMode,
       boolean parseInlineSourceMaps,
       StrictMode strictMode) {
-    checkArgument(!(languageMode == LanguageMode.ECMASCRIPT3 && strictMode == StrictMode.STRICT));
     this.parseInlineSourceMaps = parseInlineSourceMaps;
     this.annotationNames = buildAnnotationNames(annotationWhitelist);
     this.parseJsDocDocumentation = parseJsDocDocumentation;

src/com/google/javascript/jscomp/parsing/parser/Parser.java

@@ -16,8 +16,6 @@
 
 package com.google.javascript.jscomp.parsing.parser;
 
-import static com.google.common.base.Preconditions.checkArgument;
-
 import com.google.common.collect.ImmutableList;
 import com.google.common.io.BaseEncoding;
 import com.google.errorprone.annotations.FormatMethod;
@@ -234,7 +232,6 @@ public static enum Mode {
     private final boolean warnTrailingCommas;
 
     public Config(Mode mode, boolean isStrictMode) {
-      checkArgument(!(mode == Mode.ES3 && isStrictMode));
       parseTypeSyntax = mode == Mode.TYPESCRIPT;
       atLeast6 = !(mode == Mode.ES3 || mode == Mode.ES5);
       this.isStrictMode = isStrictMode;

test/com/google/javascript/jscomp/CommandLineRunnerTest.java

@@ -1493,11 +1493,25 @@ public void testTwoParseErrors() {
   public void testES3() {
     args.add("--language_in=ECMASCRIPT3");
     args.add("--language_out=ECMASCRIPT3");
+    args.add("--strict_mode_input=false");
     useStringComparison = true;
     test(
         "var x = f.function",
         "var x=f[\"function\"];",
         RhinoErrorReporter.INVALID_ES3_PROP_NAME);
+    testSame("var let;");
+  }
+
+  public void testES3plusStrictModeChecks() {
+    args.add("--language_in=ECMASCRIPT3");
+    args.add("--language_out=ECMASCRIPT3");
+    useStringComparison = true;
+    test(
+        "var x = f.function",
+        "var x=f[\"function\"];",
+        RhinoErrorReporter.INVALID_ES3_PROP_NAME);
+    test("var let", RhinoErrorReporter.PARSE_ERROR);
+    test("function f(x) { delete x; }", StrictModeCheck.DELETE_VARIABLE);
   }
 
   public void testES6TranspiledByDefault() {
@@ -1515,6 +1529,7 @@ public void testES5ChecksInVerbose() {
 
   public void testES5() {
     args.add("--language_in=ECMASCRIPT5");
+    args.add("--strict_mode_input=false");
     test("var x = f.function", "var x = f.function");
     test("var let", "var let");
   }