Handle merging unknownSymbol (#28453)

* Handle merging unknownSymbol

* mergeSymbol of unknown target returns source, not unknown

Author: Andy

src/compiler/checker.ts

@@ -870,7 +870,11 @@ namespace ts {
                 (source.flags | target.flags) & SymbolFlags.Assignment) {
                 Debug.assert(source !== target);
                 if (!(target.flags & SymbolFlags.Transient)) {
-                    target = cloneSymbol(resolveSymbol(target));
+                    const resolvedTarget = resolveSymbol(target);
+                    if (resolvedTarget === unknownSymbol) {
+                        return source;
+                    }
+                    target = cloneSymbol(resolvedTarget);
                 }
                 // Javascript static-property-assignment declarations always merge, even though they are also values
                 if (source.flags & SymbolFlags.ValueModule && target.flags & SymbolFlags.ValueModule && target.constEnumOnlyModule && !source.constEnumOnlyModule) {

tests/baselines/reference/exportAsNamespaceConflict.errors.txt

@@ -0,0 +1,13 @@
+/a.d.ts(2,10): error TS2708: Cannot use namespace 'N' as a value.
+/a.d.ts(3,1): error TS2303: Circular definition of import alias 'N'.
+
+
+==== /a.d.ts (2 errors) ====
+    declare global { namespace N {} }
+    export = N;
+             ~
+!!! error TS2708: Cannot use namespace 'N' as a value.
+    export as namespace N;
+    ~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2303: Circular definition of import alias 'N'.
+    

tests/baselines/reference/exportAsNamespaceConflict.symbols

@@ -0,0 +1,11 @@
+=== /a.d.ts ===
+declare global { namespace N {} }
+>global : Symbol(global, Decl(a.d.ts, 0, 0))
+>N : Symbol(N, Decl(a.d.ts, 0, 16))
+
+export = N;
+>N : Symbol(N, Decl(a.d.ts, 0, 16))
+
+export as namespace N;
+>N : Symbol(N, Decl(a.d.ts, 1, 11))
+

tests/baselines/reference/exportAsNamespaceConflict.types

@@ -0,0 +1,10 @@
+=== /a.d.ts ===
+declare global { namespace N {} }
+>global : any
+
+export = N;
+>N : any
+
+export as namespace N;
+>N : any
+

tests/cases/compiler/exportAsNamespaceConflict.ts

@@ -0,0 +1,4 @@
+// @Filename: /a.d.ts
+declare global { namespace N {} }
+export = N;
+export as namespace N;