fix(compiler): Don't set expression text to synthetic `$implicit` when empty (#40583)

When parsing interpolations, if we encounter an empty interpolation
(`{{}}`), the current code uses a "pretend" value of `$implicit` for the
name as if the interplotion were really `{{$implicit}}`. This is
problematic because the spans are then incorrect downstream since they
are based off of the `$implicit` text.

This commit changes the interpretation of empty interpolations so that
the text is simply an empty string.

Fixes https://github.com/angular/vscode-ng-language-service/issues/1077
Fixes https://github.com/angular/vscode-ng-language-service/issues/1078

PR Close #40583

packages/compiler/src/expression_parser/parser.ts

@@ -243,14 +243,12 @@ export class Parser {
         const fullEnd = exprEnd + interpEnd.length;
 
         const text = input.substring(exprStart, exprEnd);
-        if (text.trim().length > 0) {
+        if (text.trim().length === 0) {
-          expressions.push({text, start: fullStart, end: fullEnd});
-        } else {
           this._reportError(
               'Blank expressions are not allowed in interpolated strings', input,
               `at column ${i} in`, location);
-          expressions.push({text: '$implicit', start: fullStart, end: fullEnd});
         }
+        expressions.push({text, start: fullStart, end: fullEnd});
         offsets.push(exprStart);
 
         i = fullEnd;
@@ -571,7 +569,14 @@ export class _ParseAST {
         this.error(`Unexpected token '${this.next}'`);
       }
     }
-    if (exprs.length == 0) return new EmptyExpr(this.span(start), this.sourceSpan(start));
+    if (exprs.length == 0) {
+      // We have no expressions so create an empty expression that spans the entire input length
+      const artificialStart = this.offset;
+      const artificialEnd = this.offset + this.inputLength;
+      return new EmptyExpr(
+          this.span(artificialStart, artificialEnd),
+          this.sourceSpan(artificialStart, artificialEnd));
+    }
     if (exprs.length == 1) return exprs[0];
     return new Chain(this.span(start), this.sourceSpan(start), exprs);
   }

packages/compiler/test/expression_parser/parser_spec.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {AbsoluteSourceSpan, ASTWithSource, BindingPipe, Interpolation, ParserError, TemplateBinding, VariableBinding} from '@angular/compiler/src/expression_parser/ast';
+import {AbsoluteSourceSpan, ASTWithSource, BindingPipe, EmptyExpr, Interpolation, ParserError, TemplateBinding, VariableBinding} from '@angular/compiler/src/expression_parser/ast';
 import {Lexer} from '@angular/compiler/src/expression_parser/lexer';
 import {IvyParser, Parser, SplitInterpolation} from '@angular/compiler/src/expression_parser/parser';
 import {expect} from '@angular/platform-browser/testing/src/matchers';
@@ -910,6 +910,12 @@ describe('parser', () => {
           'Parser Error: Blank expressions are not allowed in interpolated strings');
     });
 
+    it('should produce an empty expression ast for empty interpolations', () => {
+      const parsed = parseInterpolation('{{}}')!.ast as Interpolation;
+      expect(parsed.expressions.length).toBe(1);
+      expect(parsed.expressions[0]).toBeAnInstanceOf(EmptyExpr);
+    });
+
     it('should parse conditional expression', () => {
       checkInterpolation('{{ a < b ? a : b }}');
     });

packages/language-service/ivy/completions.ts

@@ -220,12 +220,7 @@ export class CompletionBuilder<N extends TmplAstNode|AST> {
 
     const {componentContext, templateContext} = completions;
 
-    let replacementSpan: ts.TextSpan|undefined = undefined;
+    const replacementSpan = makeReplacementSpanFromAst(this.node);
-    // Non-empty nodes get replaced with the completion.
-    if (!(this.node instanceof EmptyExpr || this.node instanceof LiteralPrimitive ||
-          this.node instanceof BoundEvent)) {
-      replacementSpan = makeReplacementSpanFromAst(this.node);
-    }
 
     // Merge TS completion results with results from the template scope.
     let entries: ts.CompletionEntry[] = [];
@@ -631,7 +626,14 @@ function makeReplacementSpanFromParseSourceSpan(span: ParseSourceSpan): ts.TextS
 }
 
 function makeReplacementSpanFromAst(node: PropertyRead|PropertyWrite|MethodCall|SafePropertyRead|
-                                    SafeMethodCall|BindingPipe): ts.TextSpan {
+                                    SafeMethodCall|BindingPipe|EmptyExpr|LiteralPrimitive|
+                                    BoundEvent): ts.TextSpan|undefined {
+  if ((node instanceof EmptyExpr || node instanceof LiteralPrimitive ||
+       node instanceof BoundEvent)) {
+    // empty nodes do not replace any existing text
+    return undefined;
+  }
+
   return {
     start: node.nameSpan.start,
     length: node.nameSpan.end - node.nameSpan.start,