Inverse +proj=cass: fix non-convergence on inputs where easting=false…

…_easting or northing=false_northing

Fixes #4385

For a unknown reason, a test in pj_generic_inverse_2d() was skipping
adding the iterative correction terms when the input was such that
easting=false_easting or northing=false_northing, causing the function
to do nothing useful and fail after its maximum allowed number of
iterations was reached. The remove tests might or might not have been
needed... Only future will tell...

src/generic_inverse.cpp

@@ -87,29 +87,23 @@ PJ_LP pj_generic_inverse_2d(PJ_XY xy, PJ *P, PJ_LP lpInitial,
             }
         }
 
-        if (xy.x != 0) {
-            // Limit the amplitude of correction to avoid overshoots due to
-            // bad initial guess
-            const double delta_lam = std::max(
-                std::min(deltaX * deriv_lam_X + deltaY * deriv_lam_Y, 0.3),
-                -0.3);
-            lp.lam -= delta_lam;
-            if (lp.lam < -M_PI)
-                lp.lam = -M_PI;
-            else if (lp.lam > M_PI)
-                lp.lam = M_PI;
-        }
+        // Limit the amplitude of correction to avoid overshoots due to
+        // bad initial guess
+        const double delta_lam = std::max(
+            std::min(deltaX * deriv_lam_X + deltaY * deriv_lam_Y, 0.3), -0.3);
+        lp.lam -= delta_lam;
+        if (lp.lam < -M_PI)
+            lp.lam = -M_PI;
+        else if (lp.lam > M_PI)
+            lp.lam = M_PI;
 
-        if (xy.y != 0) {
-            const double delta_phi = std::max(
-                std::min(deltaX * deriv_phi_X + deltaY * deriv_phi_Y, 0.3),
-                -0.3);
-            lp.phi -= delta_phi;
-            if (lp.phi < -M_HALFPI)
-                lp.phi = -M_HALFPI;
-            else if (lp.phi > M_HALFPI)
-                lp.phi = M_HALFPI;
-        }
+        const double delta_phi = std::max(
+            std::min(deltaX * deriv_phi_X + deltaY * deriv_phi_Y, 0.3), -0.3);
+        lp.phi -= delta_phi;
+        if (lp.phi < -M_HALFPI)
+            lp.phi = -M_HALFPI;
+        else if (lp.phi > M_HALFPI)
+            lp.phi = M_HALFPI;
     }
     proj_context_errno_set(P->ctx,
                            PROJ_ERR_COORD_TRANSFM_OUTSIDE_PROJECTION_DOMAIN);

test/gie/builtins.gie

@@ -920,6 +920,21 @@ accept  179.99433652777776 -16.841456527777776
 expect  16015.28901692     13369.66005367
 roundtrip 1
 
+
+-------------------------------------------------------------------------------
+# Scenario of https://github.com/OSGeo/PROJ/issues/4385
+-------------------------------------------------------------------------------
+operation +proj=cass +lat_0=50.6177 +lon_0=-1.19725 +x_0=500000 +y_0=100000 +ellps=airy +units=m
+
+tolerance 0.1 mm
+direction inverse
+
+accept 300000 100000
+expect -4.022094267169   50.583438725252
+
+accept 500000 100000
+expect -1.19725 50.6177
+
 ===============================================================================
 # Central Conic
 # 	Sph