Better assignability errors when the target is an intersection that gets reduced to 'never'.

Author: DanielRosenwasser

src/compiler/checker.ts

@@ -4102,7 +4102,9 @@ namespace ts {
                     return undefined!; // TODO: GH#18217
                 }
 
-                type = getReducedType(type);
+                if (!(context.flags & NodeBuilderFlags.PreserveVacuousIntersections)) {
+                    type = getReducedType(type);
+                }
 
                 if (type.flags & TypeFlags.Any) {
                     context.approximateLength += 3;
@@ -15528,6 +15530,10 @@ namespace ts {
                             return result;
                         }
                     }
+                    else if (getObjectFlags(originalTarget) & ObjectFlags.IsNeverIntersection) {
+                        const intersectionString = typeToString(originalTarget, /*enclosingDeclaration*/ undefined, TypeFormatFlags.PreserveVacuousIntersections);
+                        errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.The_type_never_was_reduced_from_the_intersection_0_Each_type_of_that_intersection_has_properties_that_conflict_so_values_of_that_type_can_never_exist, intersectionString);
+                    }
                     if (!headMessage && maybeSuppress) {
                         lastSkippedInfo = [source, target];
                         // Used by, eg, missing property checking to replace the top-level message with a more informative one
@@ -24098,6 +24104,10 @@ namespace ts {
                         relatedInfo = suggestion.valueDeclaration && createDiagnosticForNode(suggestion.valueDeclaration, Diagnostics._0_is_declared_here, suggestedName);
                     }
                     else {
+                        if (getObjectFlags(containingType) & ObjectFlags.IsNeverIntersection) {
+                            const intersectionString = typeToString(containingType, /*enclosingDeclaration*/ undefined, TypeFormatFlags.PreserveVacuousIntersections);
+                            errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.The_type_never_was_reduced_from_the_intersection_0_Each_type_of_that_intersection_has_properties_that_conflict_so_values_of_that_type_can_never_exist, intersectionString);
+                        }
                         errorInfo = chainDiagnosticMessages(errorInfo, Diagnostics.Property_0_does_not_exist_on_type_1, declarationNameToString(propNode), typeToString(containingType));
                     }
                 }

src/compiler/diagnosticMessages.json

@@ -5653,5 +5653,9 @@
     "An optional chain cannot contain private identifiers.": {
         "category": "Error",
         "code": 18030
+    },
+    "The type 'never' was reduced from the intersection '{0}'. Each type of that intersection has properties that conflict, so values of that type can never exist.": {
+        "category": "Error",
+        "code": 18031
     }
 }

src/compiler/types.ts

@@ -3687,6 +3687,7 @@ namespace ts {
         OmitParameterModifiers                  = 1 << 13,  // Omit modifiers on parameters
         UseAliasDefinedOutsideCurrentScope      = 1 << 14,  // Allow non-visible aliases
         UseSingleQuotesForStringLiteralType     = 1 << 28,  // Use single quotes for string literal type
+        PreserveVacuousIntersections            = 1 << 29,  // uh oh...
 
         // Error handling
         AllowThisInObjectLiteral                = 1 << 15,
@@ -3730,6 +3731,7 @@ namespace ts {
 
         UseAliasDefinedOutsideCurrentScope      = 1 << 14, // For a `type T = ... ` defined in a different file, write `T` instead of its value, even though `T` can't be accessed in the current scope.
         UseSingleQuotesForStringLiteralType     = 1 << 28, // Use single quotes for string literal type
+        PreserveVacuousIntersections            = 1 << 29, // uh oh
 
         // Error Handling
         AllowUniqueESSymbolType                 = 1 << 20, // This is bit 20 to align with the same bit in `NodeBuilderFlags`
@@ -3749,7 +3751,7 @@ namespace ts {
         NodeBuilderFlagsMask = NoTruncation | WriteArrayAsGenericType | UseStructuralFallback | WriteTypeArgumentsOfSignature |
             UseFullyQualifiedType | SuppressAnyReturnType | MultilineObjectLiterals | WriteClassExpressionAsTypeLiteral |
             UseTypeOfFunction | OmitParameterModifiers | UseAliasDefinedOutsideCurrentScope | AllowUniqueESSymbolType | InTypeAlias |
-            UseSingleQuotesForStringLiteralType,
+            UseSingleQuotesForStringLiteralType | PreserveVacuousIntersections,
     }
 
     export const enum SymbolFormatFlags {