lang: ast, core: fmt: Catch invalid nil signatures

We accidentally had a bad error triggered.

lang/ast/structs.go

@@ -8474,6 +8474,10 @@ func (obj *ExprCall) Infer() (*types.Type, []*interfaces.UnificationInvariant, e
 				return nil, nil, errwrap.Wrapf(err, "func `%s` infer error", exprFunc.Title)
 			}
 			invariants = append(invariants, invars...)
+			if typ == nil { // should get a sig, not a nil!
+				// programming error
+				return nil, nil, fmt.Errorf("func `%s` infer type was nil", exprFunc.Title)
+			}
 
 			// It's important that we copy the type signature here.
 			// See the above comment which explains the reasoning.

lang/core/fmt/printf_func.go

@@ -178,7 +178,7 @@ func (obj *PrintfFunc) FuncInfer(partialType *types.Type, partialValues []types.
 			// Assume x does not contain unification variables!
 			if x.HasUni() {
 				// programming error (did the compiler change?)
-				return nil, nil, errwrap.Wrapf(err, "programming error at arg index %d", i)
+				return nil, nil, fmt.Errorf("programming error at arg index %d", i)
 			}
 			if err := unificationUtil.UnifyCmp(x, formatList[i-1]); err != nil {
 				return nil, nil, errwrap.Wrapf(err, "inconsistent type at arg index %d", i)