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

src/pending_changes/gh-472-c-deep-qualification-conversion.txt

@@ -0,0 +1,14 @@
+[GH #472]
+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

@@ -11687,6 +11687,11 @@ operators), 3.3.15 (?: operator), and 3.3.16.1 (simple assignment).
           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) {
+            /* Even const-safe deep qualification conversions require a
+               diagnostic in C: the pointed-to types are incompatible. */
+            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/.c-deep-pointer-qualification-positive.rto/default.1.1.txt

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

tests/tests/regressions/.c-deep-pointer-qualification.rto/default.1.1.txt

@@ -0,0 +1,5 @@
+fe_only -DTEST_NUMBER=1 --c --c23 --no_strict_gnu --gnu_version=130300 Test_name.c
+"Test_name.c", line 11: warning: a value of type "int **" cannot be assigned to an entity of type "const int *const *"
+    q = p;
+      ^
+

tests/tests/regressions/.c-deep-pointer-qualification.rto/default.1.matches.txt

@@ -0,0 +1,4 @@
+Matches for "(?is)warning.*cannot be assigned to an entity of type":
+  <-----------------------------------------------------------------------------
+  warning: a value of type "int **" cannot be assigned to an entity of type
+  ----------------------------------------------------------------------------->

tests/tests/regressions/.c-deep-pointer-qualification.rto/default.2.1.txt

@@ -0,0 +1,5 @@
+fe_only -DTEST_NUMBER=2 --c --c23 --strict_warnings Test_name.c
+"Test_name.c", line 11: warning: a value of type "int **" cannot be assigned to an entity of type "const int *const *"
+    q = p;
+      ^
+

tests/tests/regressions/.c-deep-pointer-qualification.rto/default.2.matches.txt

@@ -0,0 +1,4 @@
+Matches for "(?is)warning.*cannot be assigned to an entity of type":
+  <-----------------------------------------------------------------------------
+  warning: a value of type "int **" cannot be assigned to an entity of type
+  ----------------------------------------------------------------------------->

tests/tests/regressions/.cpp-deep-pointer-qualification.rto/default.1.1.txt

@@ -0,0 +1,5 @@
+fe_only -DTEST_NUMBER=1 --c++20 --strict Test_name.c
+"Test_name.c", line 6: warning: variable "q" was set but never used
+    const int *const* q = p;
+                      ^
+

tests/tests/regressions/c-deep-pointer-qualification-positive.sft.c

@@ -0,0 +1,16 @@
+//type:fp
+//options_all:--c --c23 --strict
+
+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/c-deep-pointer-qualification.sft.c

@@ -0,0 +1,13 @@
+//type:fp
+//options_all:--c --c23
+//options:--no_strict_gnu --gnu_version=130300:--strict_warnings
+//match_regex:(?is)warning.*cannot be assigned to an entity of type
+
+void deep_pointer_qualification(int **p)
+{
+  const int *const* q;
+
+  /* C++ permits this conversion, but C requires a diagnostic. */
+  q = p;
+  (void)q;
+}

tests/tests/regressions/cpp-deep-pointer-qualification.sft.cpp

@@ -0,0 +1,9 @@
+//type:fp
+//options_all:--c++20 --strict
+
+void add_deep_const(int **p)
+{
+  const int *const* q = p;
+
+  (void)q;
+}