Add new 'useUnknownInCatchVariables' flag.

Author: DanielRosenwasser

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;

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
         {

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",

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 */