From 475e201abff11742ca47d4b595078aad08937202 Mon Sep 17 00:00:00 2001
From: Daniel Rosenwasser <Daniel.Rosenwasser@microsoft.com>
Date: Thu, 8 Oct 2020 18:06:51 -0700
Subject: [PATCH] Add new 'useUnknownInCatchVariables' flag.

---
 src/compiler/checker.ts              | 2 +-
 src/compiler/commandLineParser.ts    | 8 ++++++++
 src/compiler/diagnosticMessages.json | 4 ++++
 src/compiler/types.ts                | 1 +
 4 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9aefcc9f5d3..c81958ed696 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8603,7 +8603,7 @@ namespace ts {
             const declaration = symbol.valueDeclaration;
             if (isCatchClauseVariableDeclarationOrBindingElement(declaration)) {
                 const decl = declaration as VariableDeclaration;
-                if (!decl.type) return anyType;
+                if (!decl.type) return compilerOptions.useUnknownInCatchVariables ? unknownType : anyType;
                 const type = getTypeOfNode(decl.type);
                 // an errorType will make `checkTryStatement` issue an error
                 return isTypeAny(type) || type === unknownType ? type : errorType;
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 51cf7923a10..c8e6353da08 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -668,6 +668,14 @@ namespace ts {
             category: Diagnostics.Additional_Checks,
             description: Diagnostics.Require_undeclared_properties_from_index_signatures_to_use_element_accesses
         },
+        {
+            name: "useUnknownInCatchVariables",
+            type: "boolean",
+            affectsSemanticDiagnostics: true,
+            showInSimplifiedHelpView: false,
+            category: Diagnostics.Additional_Checks,
+            description: Diagnostics.Type_catch_clause_variables_as_unknown_instead_of_any,
+        },
 
         // Module Resolution
         {
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 4cb607631f5..f97ee06efcb 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -4757,6 +4757,10 @@
         "category": "Message",
         "code": 6800
     },
+    "Type catch clause variables as 'unknown' instead of 'any'.": {
+        "category": "Message",
+        "code": 6801
+    },
 
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 7c9038ed3ba..e56fd42cc97 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -5829,6 +5829,7 @@ namespace ts {
         /* @internal */ suppressOutputPathCheck?: boolean;
         target?: ScriptTarget; // TODO: GH#18217 frequently asserted as defined
         traceResolution?: boolean;
+        useUnknownInCatchVariables?: boolean;
         resolveJsonModule?: boolean;
         types?: string[];
         /** Paths used to compute primary types search locations */