Reduce types before checking if they are never

When types are checked if they are `never` by their `TypeFlags`, they
must first be reduced, because intersection types of object-like types
which reduce to `never` don't have `TypeFlags.Never`. See microsoft#36696 for
details.

When a function is declared to return such type and it doesn't contain
any return statement or it only contains return statements without
expression, the error messages are incorrect as the checker sees its
return type as non-`never`.

This commit takes into account that intersection types potentially
reduces to `never` and improves error messages.

Author: lowr

src/compiler/checker.ts

@@ -32140,7 +32140,8 @@ namespace ts {
             }
 
             const functionFlags = getFunctionFlags(func);
-            const type = returnType && unwrapReturnType(returnType, functionFlags);
+            const unwrappedType = returnType && unwrapReturnType(returnType, functionFlags);
+            const type = unwrappedType && getReducedType(unwrappedType);
 
             // Functions with with an explicitly specified 'void' or 'any' return type don't need any return expressions.
             if (type && maybeTypeOfKind(type, TypeFlags.Any | TypeFlags.Void)) {
@@ -32157,7 +32158,9 @@ namespace ts {
             const errorNode = getEffectiveReturnTypeNode(func) || func;
 
             if (type && type.flags & TypeFlags.Never) {
-                error(errorNode, Diagnostics.A_function_returning_never_cannot_have_a_reachable_end_point);
+                const elaboration = elaborateNeverIntersection(/*errorInfo*/ undefined, unwrappedType);
+                const diagnostic = chainDiagnosticMessages(elaboration, Diagnostics.A_function_returning_never_cannot_have_a_reachable_end_point);
+                diagnostics.add(createDiagnosticForNodeFromMessageChain(errorNode, diagnostic));
             }
             else if (type && !hasExplicitReturn) {
                 // minimal check: function has syntactic return type annotation and no explicit return statements in the body
@@ -38219,7 +38222,7 @@ namespace ts {
             const signature = getSignatureFromDeclaration(container);
             const returnType = getReturnTypeOfSignature(signature);
             const functionFlags = getFunctionFlags(container);
-            if (strictNullChecks || node.expression || returnType.flags & TypeFlags.Never) {
+            if (strictNullChecks || node.expression || returnType.flags & (TypeFlags.Never | TypeFlags.Intersection)) {
                 const exprType = node.expression ? checkExpressionCached(node.expression) : undefinedType;
                 if (container.kind === SyntaxKind.SetAccessor) {
                     if (node.expression) {