C: diagnose pointer conversions that add qualifiers below the first level [GH #514]

src/pending_changes/gh-514-c-mode-cv-qual-conv.txt

@@ -0,0 +1,14 @@
+[GH #514]
+C: pointer conversions adding qualifiers below the first level
+
+In C, the front end accepted an implicit conversion between pointer types
+that adds type qualifiers below the first level, such as "int **" to
+"const int *const *", without a diagnostic.  Such a conversion is valid in
+C++, but in C the types pointed to are not compatible, so it violates a
+constraint and requires a diagnostic (C23 6.5.17.2).  A warning is now
+issued; the conversion is still accepted.  For example:
+
+  void f(int **p) {
+    const int *const *q;
+    q = p;  // Now a warning.
+  }

src/types.c

@@ -11682,11 +11682,15 @@ operators), 3.3.15 (?: operator), and 3.3.16.1 (simple assignment).
           /* Allow conversion between pointers where type qualifiers are
              being added at levels other than the first, e.g.,
              "int **" -> "const int * const *".  These are the const-safe
-             cases.  This is an extension in C mode. */
+             cases.  In C mode, this is an extension, and a diagnostic is
+             issued since the types pointed to are not "compatible". */
           okay = TRUE;
           std_conv->nontrivial_conversion = FALSE;
           std_conv->type_qualifiers_added = qualifiers_added;
           std_conv->warning_suggested = warning_suggested;
+          if (C_mode() && warning_suggested == ec_no_error) {
+            std_conv->warning_suggested = default_warning_code;
+          }  /* if */
           qualifiers_checked = TRUE;
         } else if ((!suppress_extensions || any_cfront_mode()) &&
                     same_type_with_added_qualifiers(

tests/tests/regressions/.gh-514-cpp.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c++20 --strict Test_name.c

tests/tests/regressions/.gh-514-positive.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c --c23 --strict Test_name.c

tests/tests/regressions/.gh-514.rto/default.1.1.txt

@@ -0,0 +1,21 @@
+fe_only -DTEST_NUMBER=1 --c --c23 --no_strict_gnu --gnu_version=130300 Test_name.c
+"Test_name.c", line 16: warning: a value of type "int **" cannot be assigned to an entity of type "const int *const *"
+    q = p;
+      ^
+
+"Test_name.c", line 22: warning: a value of type "int **" cannot be used to initialize an entity of type "const int *const *"
+    const int *const *q = p;
+                          ^
+
+"Test_name.c", line 29: warning: argument of type "int **" is incompatible with parameter of type "const int *const *"
+    take(p);
+         ^
+
+"Test_name.c", line 34: warning: return value type does not match the function type
+    return p;
+           ^
+
+"Test_name.c", line 39: warning: a value of type "int ***" cannot be used to initialize an entity of type "const int *const *const *"
+    const int *const *const *q = p;
+                                 ^
+

tests/tests/regressions/.gh-514.rto/default.2.1.txt

@@ -0,0 +1,21 @@
+fe_only -DTEST_NUMBER=2 --c --c23 --strict_warnings Test_name.c
+"Test_name.c", line 16: warning: a value of type "int **" cannot be assigned to an entity of type "const int *const *"
+    q = p;
+      ^
+
+"Test_name.c", line 22: warning: a value of type "int **" cannot be used to initialize an entity of type "const int *const *"
+    const int *const *q = p;
+                          ^
+
+"Test_name.c", line 29: warning: argument of type "int **" is incompatible with parameter of type "const int *const *"
+    take(p);
+         ^
+
+"Test_name.c", line 34: warning: return value type does not match the function type
+    return p;
+           ^
+
+"Test_name.c", line 39: warning: a value of type "int ***" cannot be used to initialize an entity of type "const int *const *const *"
+    const int *const *const *q = p;
+                                 ^
+

tests/tests/regressions/gh-514-cpp.sft.cpp

@@ -0,0 +1,18 @@
+//type:fp
+//options_all:--c++20 --strict
+//remark:C++: impl. ptr conv. w/ qualifiers below first level
+
+// The const-safe conversions that need a diagnostic in C are standard
+// qualification conversions in C++.
+
+void take(const int *const *);
+void take3(const int *const *const *);
+
+void add_deep_const(int **p, int ***p3)
+{
+  const int *const *q = p;
+
+  take(q);
+  take(p);
+  take3(p3);
+}

tests/tests/regressions/gh-514-positive.sft.c

@@ -0,0 +1,17 @@
+//type:fp
+//options_all:--c --c23 --strict
+//remark:C: impl. ptr conv. adding qualifiers at the first level only
+
+void add_first_level_const(int **p)
+{
+  int *const* q = p;
+
+  (void)q;
+}
+
+void explicit_deep_qualification(int **p)
+{
+  const int *const* q = (const int *const* )p;
+
+  (void)q;
+}

tests/tests/regressions/gh-514.sft.c

@@ -0,0 +1,42 @@
+//type:fp
+//options_all:--c --c23
+//options:--no_strict_gnu --gnu_version=130300:--strict_warnings
+//remark:C: impl. ptr conv. w/ qualifiers below first level
+
+// C++ permits these conversions, but in C the types pointed to are not
+// compatible, so each one requires a diagnostic.
+
+void take(const int *const *);
+void take3(const int *const *const *);
+
+void assignment(int **p)
+{
+  const int *const *q;
+
+  q = p;
+  take(q);
+}
+
+void initialization(int **p)
+{
+  const int *const *q = p;
+
+  take(q);
+}
+
+void argument(int **p)
+{
+  take(p);
+}
+
+const int *const *return_value(int **p)
+{
+  return p;
+}
+
+void three_levels(int ***p)
+{
+  const int *const *const *q = p;
+
+  take3(q);
+}