lang: reject unbindable $T-only generic returns at declaration (audit follow-up)

examples/1165-diagnostics-generic-return-unbound.sx

@@ -0,0 +1,24 @@
+// A `$T`-generic RETURN type with no parameter mentioning `$T` is rejected
+// at the declaration: the fn isn't a template (type params derive from
+// params), and no call site could ever bind the return. All three declare
+// surfaces diagnose: a top-level fn, a struct-body method, and a
+// (non-parameterised) impl method. Each used to PANIC the compiler at LLVM
+// emission via the `.unresolved` tripwire — even when never called.
+
+#import "modules/std.sx";
+
+make :: () -> $T { 0 }
+
+Foo :: struct {
+    x: s64;
+    weird :: (self: *Foo) -> $T { 0 }
+}
+
+Show2 :: protocol { show2 :: () -> string; }
+IntBox :: struct { v: s64; }
+impl Show2 for IntBox {
+    show2 :: (self: *IntBox) -> string { "x" }
+    leak :: (self: *IntBox) -> $T { 0 }
+}
+
+main :: () { print("ok\n"); }

examples/expected/1165-diagnostics-generic-return-unbound.exit

@@ -0,0 +1 @@
+1

examples/expected/1165-diagnostics-generic-return-unbound.stderr

@@ -0,0 +1,17 @@
+error: generic return type '$T' cannot be bound — 'make' has no parameter mentioning '$T', so no call site can infer it
+  --> examples/1165-diagnostics-generic-return-unbound.sx:10:15
+   |
+10 | make :: () -> $T { 0 }
+   |               ^^
+
+error: generic return type '$T' cannot be bound — 'Foo.weird' has no parameter mentioning '$T', so no call site can infer it
+  --> examples/1165-diagnostics-generic-return-unbound.sx:14:30
+   |
+14 |     weird :: (self: *Foo) -> $T { 0 }
+   |                              ^^
+
+error: generic return type '$T' cannot be bound — 'IntBox.leak' has no parameter mentioning '$T', so no call site can infer it
+  --> examples/1165-diagnostics-generic-return-unbound.sx:21:32
+   |
+21 |     leak :: (self: *IntBox) -> $T { 0 }
+   |                                ^^

examples/expected/1165-diagnostics-generic-return-unbound.stdout

@@ -0,0 +1 @@
+