From b1d9034b384b2e64aea7ab4133acef0fd092104b Mon Sep 17 00:00:00 2001
From: Ron Buckton <ron.buckton@microsoft.com>
Date: Fri, 21 Aug 2020 18:57:21 -0700
Subject: [PATCH] Use 'void' for the contextual type of an unused expression

---
 src/compiler/checker.ts | 25 ++++++++++++++++++++++---
 src/compiler/types.ts   |  2 +-
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index a85281375e6..819cd98f6fd 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -23042,7 +23042,7 @@ namespace ts {
                 if (result) {
                     return result;
                 }
-                if (!(contextFlags! & ContextFlags.SkipBindingPatterns) && isBindingPattern(declaration.name)) { // This is less a contextual type and more an implied shape - in some cases, this may be undesirable
+                if (!(contextFlags! & ContextFlags.SkipBindingPatternsAndUnusedExpressions) && isBindingPattern(declaration.name)) { // This is less a contextual type and more an implied shape - in some cases, this may be undesirable
                     return getTypeFromBindingPattern(declaration.name, /*includePatternInType*/ true, /*reportErrors*/ false);
                 }
             }
@@ -23190,7 +23190,13 @@ namespace ts {
                         getTypeOfExpression(left) : type;
                 case SyntaxKind.AmpersandAmpersandToken:
                 case SyntaxKind.CommaToken:
-                    return node === right ? getContextualType(binaryExpression, contextFlags) : undefined;
+                    if (node === right) {
+                        return getContextualType(binaryExpression, contextFlags);
+                    }
+                    else if (!(contextFlags! & ContextFlags.SkipBindingPatternsAndUnusedExpressions)) {
+                        return voidType;
+                    }
+                    return undefined;
                 default:
                     return undefined;
             }
@@ -23602,6 +23608,19 @@ namespace ts {
                 case SyntaxKind.JsxOpeningElement:
                 case SyntaxKind.JsxSelfClosingElement:
                     return getContextualJsxElementAttributesType(<JsxOpeningLikeElement>parent, contextFlags);
+                case SyntaxKind.ExpressionStatement:
+                case SyntaxKind.VoidExpression:
+                    if (!(contextFlags! & ContextFlags.SkipBindingPatternsAndUnusedExpressions)) {
+                        return voidType;
+                    }
+                    break;
+                case SyntaxKind.ForStatement: {
+                    const forStatement = parent as ForStatement;
+                    if (!(contextFlags! & ContextFlags.SkipBindingPatternsAndUnusedExpressions) && (node === forStatement.initializer || node === forStatement.incrementor)) {
+                        return voidType;
+                    }
+                    break;
+                }
             }
             return undefined;
 
@@ -25919,7 +25938,7 @@ namespace ts {
             // 'let f: (x: string) => number = wrap(s => s.length)', we infer from the declared type of 'f' to the
             // return type of 'wrap'.
             if (node.kind !== SyntaxKind.Decorator) {
-                const contextualType = getContextualType(node, every(signature.typeParameters, p => !!getDefaultFromTypeParameter(p)) ? ContextFlags.SkipBindingPatterns : ContextFlags.None);
+                const contextualType = getContextualType(node, every(signature.typeParameters, p => !!getDefaultFromTypeParameter(p)) ? ContextFlags.SkipBindingPatternsAndUnusedExpressions : ContextFlags.None);
                 if (contextualType) {
                     // We clone the inference context to avoid disturbing a resolution in progress for an
                     // outer call expression. Effectively we just want a snapshot of whatever has been
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 40f43b3ad6b..0fd0345c9c7 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4139,7 +4139,7 @@ namespace ts {
         Signature      = 1 << 0, // Obtaining contextual signature
         NoConstraints  = 1 << 1, // Don't obtain type variable constraints
         Completions    = 1 << 2, // Ignore inference to current node and parent nodes out to the containing call for completions
-        SkipBindingPatterns = 1 << 3, // Ignore contextual types applied by binding patterns
+        SkipBindingPatternsAndUnusedExpressions = 1 << 3, // Ignore contextual types applied by binding patterns and do not use `void` as the contextual type for unused expressions
     }
 
     // NOTE: If modifying this enum, must modify `TypeFormatFlags` too!