Prevent type parameter shadowing of builtin types

When a type parameter is introduced which matches any of the simple UIF
names, it causes any other such simple type to be recognized as a type
parameter reference, which results in the compiler aborting.

In the discussion on ponylang#1526, it was decided that all
shadowing by type parameters should be prohibited.

Hence, in this patch, if we're resolving a type param whose id is still
nominal, we check the nearest module to see if that name is already
defined.

src/libponyc/pass/names.c

@@ -178,6 +178,15 @@ static bool names_typeparam(pass_opt_t* opt, ast_t** astp, ast_t* def)
     return false;
   }
 
+  if (ast_id(ast) == TK_NOMINAL) {
+    ast_t* module = ast_nearest(ast, TK_MODULE);
+    if (module && ast_get(module, ast_name(id), 0)) {
+      ast_error(opt->check.errors, def,
+                "Type parameter shadows existing type");
+      return false;
+    }
+  }
+
   // Change to a typeparamref.
   REPLACE(astp,
     NODE(TK_TYPEPARAMREF,

test/libponyc/badpony.cc

@@ -338,3 +338,12 @@ TEST_F(BadPonyTest, MatchUnionOfDifferentCaps)
   TEST_ERRORS_1(src,
     "match type may not be a union of types with different capabilities");
 }
+
+TEST_F(BadPonyTest, ShadowingBuiltinTypeParameter)
+{
+  const char* src =
+      "class A[I8]"
+        "let b: U8 = 0";
+
+  TEST_ERRORS_1(src, "Type parameter shadows existing type");
+}