From a027cfa8acbfea3eb3753ef32435f77d3f0a0a23 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 13 Apr 2022 06:16:43 -0700
Subject: [PATCH] More permissive parsing of instantiation expressions (#48659)

* More permissive parsing of expressions with type arguments

* Add test
---
 src/compiler/parser.ts                        | 28 ++-----------------
 .../instantiationExpressionErrors.errors.txt  |  5 ++++
 .../instantiationExpressionErrors.js          | 12 ++++++++
 .../instantiationExpressionErrors.symbols     |  8 ++++++
 .../instantiationExpressionErrors.types       | 10 +++++++
 .../instantiationExpressionErrors.ts          |  5 ++++
 6 files changed, 42 insertions(+), 26 deletions(-)

diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index 2a96a3b7cd3..247381f0251 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -5735,34 +5735,10 @@ namespace ts {
                 case SyntaxKind.OpenParenToken:                 // foo<x>(
                 case SyntaxKind.NoSubstitutionTemplateLiteral:  // foo<T> `...`
                 case SyntaxKind.TemplateHead:                   // foo<T> `...${100}...`
-                // These tokens can't follow in a call expression, nor can they start an
-                // expression. So, consider the type argument list part of an instantiation
-                // expression.
-                // falls through
-                case SyntaxKind.CommaToken:                     // foo<x>,
-                case SyntaxKind.DotToken:                       // foo<x>.
-                case SyntaxKind.QuestionDotToken:               // foo<x>?.
-                case SyntaxKind.CloseParenToken:                // foo<x>)
-                case SyntaxKind.CloseBracketToken:              // foo<x>]
-                case SyntaxKind.ColonToken:                     // foo<x>:
-                case SyntaxKind.SemicolonToken:                 // foo<x>;
-                case SyntaxKind.QuestionToken:                  // foo<x>?
-                case SyntaxKind.EqualsEqualsToken:              // foo<x> ==
-                case SyntaxKind.EqualsEqualsEqualsToken:        // foo<x> ===
-                case SyntaxKind.ExclamationEqualsToken:         // foo<x> !=
-                case SyntaxKind.ExclamationEqualsEqualsToken:   // foo<x> !==
-                case SyntaxKind.AmpersandAmpersandToken:        // foo<x> &&
-                case SyntaxKind.BarBarToken:                    // foo<x> ||
-                case SyntaxKind.QuestionQuestionToken:          // foo<x> ??
-                case SyntaxKind.CaretToken:                     // foo<x> ^
-                case SyntaxKind.AmpersandToken:                 // foo<x> &
-                case SyntaxKind.BarToken:                       // foo<x> |
-                case SyntaxKind.CloseBraceToken:                // foo<x> }
-                case SyntaxKind.EndOfFileToken:                 // foo<x>
                     return true;
             }
-            // Treat anything else as an expression.
-            return false;
+            // Consider something a type argument list only if the following token can't start an expression.
+            return !isStartOfExpression();
         }
 
         function parsePrimaryExpression(): PrimaryExpression {
diff --git a/tests/baselines/reference/instantiationExpressionErrors.errors.txt b/tests/baselines/reference/instantiationExpressionErrors.errors.txt
index d0ded2ca917..5b60983d594 100644
--- a/tests/baselines/reference/instantiationExpressionErrors.errors.txt
+++ b/tests/baselines/reference/instantiationExpressionErrors.errors.txt
@@ -72,4 +72,9 @@ tests/cases/conformance/types/typeParameters/typeArgumentLists/instantiationExpr
     
     const x3 = f<true>;
     true;
+    
+    // Parsed as instantiation expression
+    
+    const x4 = f<true>
+    if (true) {}
     
\ No newline at end of file
diff --git a/tests/baselines/reference/instantiationExpressionErrors.js b/tests/baselines/reference/instantiationExpressionErrors.js
index c0c4d9b5b68..9f6b17f1378 100644
--- a/tests/baselines/reference/instantiationExpressionErrors.js
+++ b/tests/baselines/reference/instantiationExpressionErrors.js
@@ -40,6 +40,11 @@ true;
 
 const x3 = f<true>;
 true;
+
+// Parsed as instantiation expression
+
+const x4 = f<true>
+if (true) {}
 
 
 //// [instantiationExpressionErrors.js]
@@ -70,6 +75,9 @@ var x2 = f < true >
 // Parsed as instantiation expression
 var x3 = (f);
 true;
+// Parsed as instantiation expression
+var x4 = (f);
+if (true) { }
 
 
 //// [instantiationExpressionErrors.d.ts]
@@ -101,3 +109,7 @@ declare const x3: {
     (): true;
     g<U>(): U;
 };
+declare const x4: {
+    (): true;
+    g<U>(): U;
+};
diff --git a/tests/baselines/reference/instantiationExpressionErrors.symbols b/tests/baselines/reference/instantiationExpressionErrors.symbols
index a9e99be8987..dc1e6d5d64b 100644
--- a/tests/baselines/reference/instantiationExpressionErrors.symbols
+++ b/tests/baselines/reference/instantiationExpressionErrors.symbols
@@ -99,3 +99,11 @@ const x3 = f<true>;
 
 true;
 
+// Parsed as instantiation expression
+
+const x4 = f<true>
+>x4 : Symbol(x4, Decl(instantiationExpressionErrors.ts, 44, 5))
+>f : Symbol(f, Decl(instantiationExpressionErrors.ts, 0, 11))
+
+if (true) {}
+
diff --git a/tests/baselines/reference/instantiationExpressionErrors.types b/tests/baselines/reference/instantiationExpressionErrors.types
index 3886d802526..334cb69865e 100644
--- a/tests/baselines/reference/instantiationExpressionErrors.types
+++ b/tests/baselines/reference/instantiationExpressionErrors.types
@@ -122,3 +122,13 @@ const x3 = f<true>;
 true;
 >true : true
 
+// Parsed as instantiation expression
+
+const x4 = f<true>
+>x4 : { (): true; g<U>(): U; }
+>f : { <T>(): T; g<U>(): U; }
+>true : true
+
+if (true) {}
+>true : true
+
diff --git a/tests/cases/conformance/types/typeParameters/typeArgumentLists/instantiationExpressionErrors.ts b/tests/cases/conformance/types/typeParameters/typeArgumentLists/instantiationExpressionErrors.ts
index c34a12f13bb..1a9377ae9c5 100644
--- a/tests/cases/conformance/types/typeParameters/typeArgumentLists/instantiationExpressionErrors.ts
+++ b/tests/cases/conformance/types/typeParameters/typeArgumentLists/instantiationExpressionErrors.ts
@@ -42,3 +42,8 @@ true;
 
 const x3 = f<true>;
 true;
+
+// Parsed as instantiation expression
+
+const x4 = f<true>
+if (true) {}