Merge pull request #25978 from Microsoft/capturedGlobalThis

Better error message for captured global 'this' in noImplicitThis

Author: DanielRosenwasser

src/compiler/checker.ts

@@ -15279,7 +15279,7 @@ namespace ts {
             // Stop at the first arrow function so that we can
             // tell whether 'this' needs to be captured.
             let container = getThisContainer(node, /* includeArrowFunctions */ true);
-            let needToCaptureLexicalThis = false;
+            let capturedByArrowFunction = false;
 
             if (container.kind === SyntaxKind.Constructor) {
                 checkThisBeforeSuper(node, container, Diagnostics.super_must_be_called_before_accessing_this_in_the_constructor_of_a_derived_class);
@@ -15288,9 +15288,7 @@ namespace ts {
             // Now skip arrow functions to get the "real" owner of 'this'.
             if (container.kind === SyntaxKind.ArrowFunction) {
                 container = getThisContainer(container, /* includeArrowFunctions */ false);
-
-                // When targeting es6, arrow function lexically bind "this" so we do not need to do the work of binding "this" in emitted code
-                needToCaptureLexicalThis = (languageVersion < ScriptTarget.ES2015);
+                capturedByArrowFunction = true;
             }
 
             switch (container.kind) {
@@ -15320,14 +15318,19 @@ namespace ts {
                     break;
             }
 
-            if (needToCaptureLexicalThis) {
+            // When targeting es6, mark that we'll need to capture `this` in its lexically bound scope.
+            if (capturedByArrowFunction && languageVersion < ScriptTarget.ES2015) {
                 captureLexicalThis(node, container);
             }
 
             const type = tryGetThisTypeAt(node, container);
             if (!type && noImplicitThis) {
                 // With noImplicitThis, functions may not reference 'this' if it has type 'any'
-                error(node, Diagnostics.this_implicitly_has_type_any_because_it_does_not_have_a_type_annotation);
+                error(
+                    node,
+                    capturedByArrowFunction ?
+                        Diagnostics.this_implicitly_has_type_any_because_it_does_not_have_a_type_annotation :
+                        Diagnostics.The_containing_arrow_function_captures_the_global_value_of_this_which_implicitly_has_type_any);
             }
             return type || anyType;
         }

src/compiler/diagnosticMessages.json

@@ -3921,6 +3921,10 @@
         "category": "Error",
         "code": 7040
     },
+    "The containing arrow function captures the global value of 'this' which implicitly has type 'any'.": {
+        "category": "Error",
+        "code": 7041
+    },
     "You cannot rename this element.": {
         "category": "Error",
         "code": 8000

tests/baselines/reference/noImplicitThisFunctions.errors.txt

@@ -1,4 +1,4 @@
-tests/cases/compiler/noImplicitThisFunctions.ts(13,12): error TS2683: 'this' implicitly has type 'any' because it does not have a type annotation.
+tests/cases/compiler/noImplicitThisFunctions.ts(13,12): error TS7041: The containing arrow function captures the global value of 'this' which implicitly has type 'any'.
 tests/cases/compiler/noImplicitThisFunctions.ts(17,38): error TS2683: 'this' implicitly has type 'any' because it does not have a type annotation.
 
 
@@ -17,7 +17,7 @@ tests/cases/compiler/noImplicitThisFunctions.ts(17,38): error TS2683: 'this' imp
         // error: this is implicitly any
         return this.a + z;
                ~~~~
-!!! error TS2683: 'this' implicitly has type 'any' because it does not have a type annotation.
+!!! error TS7041: The containing arrow function captures the global value of 'this' which implicitly has type 'any'.
     }
 
     // error: `this` is `window`, but is still of type `any`

tests/baselines/reference/thisBinding2.errors.txt

@@ -1,4 +1,4 @@
-tests/cases/compiler/thisBinding2.ts(10,11): error TS2683: 'this' implicitly has type 'any' because it does not have a type annotation.
+tests/cases/compiler/thisBinding2.ts(10,11): error TS7041: The containing arrow function captures the global value of 'this' which implicitly has type 'any'.
 
 
 ==== tests/cases/compiler/thisBinding2.ts (1 errors) ====
@@ -13,7 +13,7 @@ tests/cases/compiler/thisBinding2.ts(10,11): error TS2683: 'this' implicitly has
        var x = 1;
        return this.x;
               ~~~~
-!!! error TS2683: 'this' implicitly has type 'any' because it does not have a type annotation.
+!!! error TS7041: The containing arrow function captures the global value of 'this' which implicitly has type 'any'.
       }();
         }  
     }