Forbid duplicate static class and/or namespace members

This isn't fully TS compatible, but refactors targeting internal names,
scoping, merging, etc. are needed to become more compatible. For
instance, if namespace members had unique separators in internal names,
then a non-exported namespace member would override a static class
member, assuming the names are the same.

Note that this change doesn't prevent the compiler from attempting to
compile the duplicate global, and hence the previous commit is needed
for this to work fully.

src/program.ts

@@ -5058,15 +5058,31 @@ function tryMerge(older: Element, newer: Element): DeclaredElement | null {
 
 /** Copies the members of `src` to `dest`. */
 function copyMembers(src: Element, dest: Element): void {
+  let program = src.program;
+  assert(program == dest.program);
+
   let srcMembers = src.members;
   if (srcMembers) {
     let destMembers = dest.members;
     if (!destMembers) dest.members = destMembers = new Map();
     // TODO: for (let [memberName, member] of srcMembers) {
     for (let _keys = Map_keys(srcMembers), i = 0, k = _keys.length; i < k; ++i) {
       let memberName = unchecked(_keys[i]);
-      let member = assert(srcMembers.get(memberName));
-      destMembers.set(memberName, member);
+      let srcMember = assert(srcMembers.get(memberName));
+      // This isn't TS compatible in every case, but the logic involved in
+      // merging, scoping, and making internal names is not currently able to
+      // handle duplicates well.
+      if (destMembers.has(memberName)) {
+        let destMember = assert(destMembers.get(memberName));
+        program.errorRelated(
+          DiagnosticCode.Duplicate_identifier_0,
+          srcMember.declaration.name.range, destMember.declaration.name.range,
+          memberName
+        );
+        continue;
+      }
+
+      destMembers.set(memberName, srcMember);
     }
   }
 }

tests/compiler/issues/2793.json

@@ -0,0 +1,13 @@
+{
+    "stderr": [
+        "Duplicate identifier 'bar'.",
+        "in issues/2793.ts(8,14)",
+        "in issues/2793.ts(2,10)",
+        "Duplicate identifier 'baz'.",
+        "in issues/2793.ts(9,7)",
+        "in issues/2793.ts(3,10)",
+        "Duplicate identifier 'qux'.",
+        "in issues/2793.ts(10,14)",
+        "in issues/2793.ts(4,18)"
+    ]
+}

tests/compiler/issues/2793.ts

@@ -0,0 +1,12 @@
+class Foo {
+  static bar: i32 = 2; // errors in TS
+  static baz: i32 = 2; // does not error in TS
+  private static qux: i32 = 2; // errors in TS
+}
+
+namespace Foo {
+  export let bar: i32 = 1;
+  let baz: string = "baz";
+  export let qux: i32 = 1;
+  let hi: i32 = 1;
+}