diff --git a/c/cert/src/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.ql b/c/cert/src/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.ql
index cb742859cc..8dda9012d2 100644
--- a/c/cert/src/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.ql
+++ b/c/cert/src/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.ql
@@ -16,14 +16,11 @@ import cpp
 import codingstandards.c.cert
 import codingstandards.cpp.CharFunctions
 
-from FunctionCall fc, Expr arg
+from UseOfToOrIsChar useOfCharAPI, Expr arg
 where
-  not isExcluded(fc, Strings2Package::toCharacterHandlingFunctionsRepresentableAsUCharQuery()) and
-  // examine all impacted functions
-  fc.getTarget() instanceof CToOrIsCharFunction and
-  arg = fc.getArgument(0).getFullyConverted() and
-  // report on cases where either the explicit or implicit cast
-  // on the parameter type is not unsigned
-  not arg.(CStyleCast).getExpr().getType() instanceof UnsignedCharType
-select fc, "$@ to character-handling function may not be representable as an unsigned char.", arg,
-  "Argument"
+  not isExcluded(useOfCharAPI,
+    Strings2Package::toCharacterHandlingFunctionsRepresentableAsUCharQuery()) and
+  arg = useOfCharAPI.getConvertedArgument() and
+  not arg.getType() instanceof UnsignedCharType
+select useOfCharAPI,
+  "$@ to character-handling function may not be representable as an unsigned char.", arg, "Argument"
diff --git a/c/cert/test/rules/DCL41-C/test.c b/c/cert/test/rules/DCL41-C/test.c
index 2500c982f3..6b982c6d0b 100644
--- a/c/cert/test/rules/DCL41-C/test.c
+++ b/c/cert/test/rules/DCL41-C/test.c
@@ -23,6 +23,8 @@ void f1(int expr) {
 void f2(int expr) {
   switch (expr) {
   case 0:
+    0; // Note: required because a "case" is a label, and not permitted on a
+       // declaration, so we need a no-op statement
     int i = 4; // COMPLIANT
   case 1:
     i = 6; // COMPLIANT
diff --git a/c/cert/test/rules/STR34-C/CastCharBeforeConvertingToLargerSizes.expected.clang b/c/cert/test/rules/STR34-C/CastCharBeforeConvertingToLargerSizes.expected.clang
new file mode 100644
index 0000000000..1cf143a196
--- /dev/null
+++ b/c/cert/test/rules/STR34-C/CastCharBeforeConvertingToLargerSizes.expected.clang
@@ -0,0 +1,15 @@
+| test.c:7:7:7:14 | * ... | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:28:3:28:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:29:3:29:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:31:3:31:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:32:3:32:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:33:3:33:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:34:3:34:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:35:3:35:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:36:3:36:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:37:3:37:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:38:3:38:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:39:3:39:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:40:3:40:14 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:42:11:42:12 | * ... | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:43:11:43:12 | * ... | Expression not converted to `unsigned char` before converting to a larger integer type. |
diff --git a/c/cert/test/rules/STR34-C/CastCharBeforeConvertingToLargerSizes.expected.gcc b/c/cert/test/rules/STR34-C/CastCharBeforeConvertingToLargerSizes.expected.gcc
new file mode 100644
index 0000000000..1cf143a196
--- /dev/null
+++ b/c/cert/test/rules/STR34-C/CastCharBeforeConvertingToLargerSizes.expected.gcc
@@ -0,0 +1,15 @@
+| test.c:7:7:7:14 | * ... | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:28:3:28:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:29:3:29:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:31:3:31:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:32:3:32:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:33:3:33:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:34:3:34:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:35:3:35:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:36:3:36:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:37:3:37:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:38:3:38:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:39:3:39:13 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:40:3:40:14 | (...) | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:42:11:42:12 | * ... | Expression not converted to `unsigned char` before converting to a larger integer type. |
+| test.c:43:11:43:12 | * ... | Expression not converted to `unsigned char` before converting to a larger integer type. |
diff --git a/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected b/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected
index b655289f4e..43008e02d0 100644
--- a/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected
+++ b/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected
@@ -1,28 +1,28 @@
-| test.c:7:3:7:9 | call to isalnum | $@ to character-handling function may not be representable as an unsigned char. | test.c:7:11:7:12 | (int)... | Argument |
-| test.c:8:3:8:13 | call to isalpha | $@ to character-handling function may not be representable as an unsigned char. | test.c:8:11:8:12 | (int)... | Argument |
-| test.c:10:3:10:9 | call to isblank | $@ to character-handling function may not be representable as an unsigned char. | test.c:10:11:10:12 | (int)... | Argument |
-| test.c:11:3:11:9 | call to iscntrl | $@ to character-handling function may not be representable as an unsigned char. | test.c:11:11:11:12 | (int)... | Argument |
-| test.c:12:3:12:13 | call to isdigit | $@ to character-handling function may not be representable as an unsigned char. | test.c:12:11:12:12 | (int)... | Argument |
-| test.c:13:3:13:13 | call to isgraph | $@ to character-handling function may not be representable as an unsigned char. | test.c:13:11:13:12 | (int)... | Argument |
-| test.c:14:3:14:13 | call to islower | $@ to character-handling function may not be representable as an unsigned char. | test.c:14:11:14:12 | (int)... | Argument |
-| test.c:15:3:15:13 | call to isprint | $@ to character-handling function may not be representable as an unsigned char. | test.c:15:11:15:12 | (int)... | Argument |
-| test.c:16:3:16:9 | call to ispunct | $@ to character-handling function may not be representable as an unsigned char. | test.c:16:11:16:12 | (int)... | Argument |
-| test.c:17:3:17:13 | call to __isspace | $@ to character-handling function may not be representable as an unsigned char. | test.c:17:11:17:12 | (int)... | Argument |
-| test.c:18:3:18:13 | call to isupper | $@ to character-handling function may not be representable as an unsigned char. | test.c:18:11:18:12 | (int)... | Argument |
-| test.c:19:3:19:10 | call to isxdigit | $@ to character-handling function may not be representable as an unsigned char. | test.c:19:12:19:13 | (int)... | Argument |
-| test.c:21:3:21:9 | call to toupper | $@ to character-handling function may not be representable as an unsigned char. | test.c:21:11:21:12 | (int)... | Argument |
-| test.c:22:3:22:9 | call to tolower | $@ to character-handling function may not be representable as an unsigned char. | test.c:22:11:22:12 | (int)... | Argument |
+| test.c:7:3:7:9 | call to isalnum | $@ to character-handling function may not be representable as an unsigned char. | test.c:7:11:7:12 | * ... | Argument |
+| test.c:8:3:8:13 | isalpha(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:8:11:8:12 | * ... | Argument |
+| test.c:10:3:10:9 | call to isblank | $@ to character-handling function may not be representable as an unsigned char. | test.c:10:11:10:12 | * ... | Argument |
+| test.c:11:3:11:9 | call to iscntrl | $@ to character-handling function may not be representable as an unsigned char. | test.c:11:11:11:12 | * ... | Argument |
+| test.c:12:3:12:13 | isdigit(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:12:3:12:13 | (...) | Argument |
+| test.c:13:3:13:13 | isgraph(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:13:3:13:13 | (...) | Argument |
+| test.c:14:3:14:13 | islower(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:14:3:14:13 | (...) | Argument |
+| test.c:15:3:15:13 | isprint(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:15:3:15:13 | (...) | Argument |
+| test.c:16:3:16:9 | call to ispunct | $@ to character-handling function may not be representable as an unsigned char. | test.c:16:11:16:12 | * ... | Argument |
+| test.c:17:3:17:13 | call to __isspace | $@ to character-handling function may not be representable as an unsigned char. | test.c:17:11:17:12 | * ... | Argument |
+| test.c:18:3:18:13 | isupper(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:18:3:18:13 | (...) | Argument |
+| test.c:19:3:19:10 | call to isxdigit | $@ to character-handling function may not be representable as an unsigned char. | test.c:19:12:19:13 | * ... | Argument |
+| test.c:21:3:21:9 | call to toupper | $@ to character-handling function may not be representable as an unsigned char. | test.c:21:11:21:12 | * ... | Argument |
+| test.c:22:3:22:9 | call to tolower | $@ to character-handling function may not be representable as an unsigned char. | test.c:22:11:22:12 | * ... | Argument |
 | test.c:70:3:70:9 | call to isalnum | $@ to character-handling function may not be representable as an unsigned char. | test.c:70:11:70:11 | t | Argument |
-| test.c:71:3:71:12 | call to isalpha | $@ to character-handling function may not be representable as an unsigned char. | test.c:71:11:71:11 | t | Argument |
+| test.c:71:3:71:12 | isalpha(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:71:11:71:11 | t | Argument |
 | test.c:73:3:73:9 | call to isblank | $@ to character-handling function may not be representable as an unsigned char. | test.c:73:11:73:11 | t | Argument |
 | test.c:74:3:74:9 | call to iscntrl | $@ to character-handling function may not be representable as an unsigned char. | test.c:74:11:74:11 | t | Argument |
-| test.c:75:3:75:12 | call to isdigit | $@ to character-handling function may not be representable as an unsigned char. | test.c:75:11:75:11 | t | Argument |
-| test.c:76:3:76:12 | call to isgraph | $@ to character-handling function may not be representable as an unsigned char. | test.c:76:11:76:11 | t | Argument |
-| test.c:77:3:77:12 | call to islower | $@ to character-handling function may not be representable as an unsigned char. | test.c:77:11:77:11 | t | Argument |
-| test.c:78:3:78:12 | call to isprint | $@ to character-handling function may not be representable as an unsigned char. | test.c:78:11:78:11 | t | Argument |
+| test.c:75:3:75:12 | isdigit(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:75:3:75:12 | (...) | Argument |
+| test.c:76:3:76:12 | isgraph(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:76:3:76:12 | (...) | Argument |
+| test.c:77:3:77:12 | islower(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:77:3:77:12 | (...) | Argument |
+| test.c:78:3:78:12 | isprint(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:78:3:78:12 | (...) | Argument |
 | test.c:79:3:79:9 | call to ispunct | $@ to character-handling function may not be representable as an unsigned char. | test.c:79:11:79:11 | t | Argument |
 | test.c:80:3:80:12 | call to __isspace | $@ to character-handling function may not be representable as an unsigned char. | test.c:80:11:80:11 | t | Argument |
-| test.c:81:3:81:12 | call to isupper | $@ to character-handling function may not be representable as an unsigned char. | test.c:81:11:81:11 | t | Argument |
+| test.c:81:3:81:12 | isupper(a) | $@ to character-handling function may not be representable as an unsigned char. | test.c:81:3:81:12 | (...) | Argument |
 | test.c:82:3:82:10 | call to isxdigit | $@ to character-handling function may not be representable as an unsigned char. | test.c:82:12:82:12 | t | Argument |
 | test.c:84:3:84:9 | call to toupper | $@ to character-handling function may not be representable as an unsigned char. | test.c:84:11:84:11 | t | Argument |
 | test.c:85:3:85:9 | call to tolower | $@ to character-handling function may not be representable as an unsigned char. | test.c:85:11:85:11 | t | Argument |
diff --git a/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected.clang b/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected.clang
new file mode 100644
index 0000000000..3ad77b8c58
--- /dev/null
+++ b/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected.clang
@@ -0,0 +1,28 @@
+| test.c:7:3:7:13 | isalnum(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:7:3:7:13 | (...) | Argument |
+| test.c:8:3:8:13 | isalpha(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:8:3:8:13 | (...) | Argument |
+| test.c:10:3:10:13 | isblank(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:10:3:10:13 | (...) | Argument |
+| test.c:11:3:11:13 | iscntrl(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:11:3:11:13 | (...) | Argument |
+| test.c:12:3:12:13 | isdigit(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:12:3:12:13 | (...) | Argument |
+| test.c:13:3:13:13 | isgraph(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:13:3:13:13 | (...) | Argument |
+| test.c:14:3:14:13 | islower(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:14:3:14:13 | (...) | Argument |
+| test.c:15:3:15:13 | isprint(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:15:3:15:13 | (...) | Argument |
+| test.c:16:3:16:13 | ispunct(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:16:3:16:13 | (...) | Argument |
+| test.c:17:3:17:13 | isspace(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:17:3:17:13 | (...) | Argument |
+| test.c:18:3:18:13 | isupper(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:18:3:18:13 | (...) | Argument |
+| test.c:19:3:19:14 | isxdigit(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:19:3:19:14 | (...) | Argument |
+| test.c:21:3:21:9 | call to toupper | $@ to character-handling function may not be representable as an unsigned char. | test.c:21:11:21:12 | * ... | Argument |
+| test.c:22:3:22:9 | call to tolower | $@ to character-handling function may not be representable as an unsigned char. | test.c:22:11:22:12 | * ... | Argument |
+| test.c:70:3:70:12 | isalnum(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:70:3:70:12 | (...) | Argument |
+| test.c:71:3:71:12 | isalpha(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:71:3:71:12 | (...) | Argument |
+| test.c:73:3:73:12 | isblank(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:73:3:73:12 | (...) | Argument |
+| test.c:74:3:74:12 | iscntrl(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:74:3:74:12 | (...) | Argument |
+| test.c:75:3:75:12 | isdigit(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:75:3:75:12 | (...) | Argument |
+| test.c:76:3:76:12 | isgraph(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:76:3:76:12 | (...) | Argument |
+| test.c:77:3:77:12 | islower(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:77:3:77:12 | (...) | Argument |
+| test.c:78:3:78:12 | isprint(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:78:3:78:12 | (...) | Argument |
+| test.c:79:3:79:12 | ispunct(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:79:3:79:12 | (...) | Argument |
+| test.c:80:3:80:12 | isspace(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:80:3:80:12 | (...) | Argument |
+| test.c:81:3:81:12 | isupper(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:81:3:81:12 | (...) | Argument |
+| test.c:82:3:82:13 | isxdigit(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:82:3:82:13 | (...) | Argument |
+| test.c:84:3:84:9 | call to toupper | $@ to character-handling function may not be representable as an unsigned char. | test.c:84:11:84:11 | t | Argument |
+| test.c:85:3:85:9 | call to tolower | $@ to character-handling function may not be representable as an unsigned char. | test.c:85:11:85:11 | t | Argument |
diff --git a/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected.gcc b/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected.gcc
new file mode 100644
index 0000000000..3ad77b8c58
--- /dev/null
+++ b/c/cert/test/rules/STR37-C/ToCharacterHandlingFunctionsRepresentableAsUChar.expected.gcc
@@ -0,0 +1,28 @@
+| test.c:7:3:7:13 | isalnum(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:7:3:7:13 | (...) | Argument |
+| test.c:8:3:8:13 | isalpha(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:8:3:8:13 | (...) | Argument |
+| test.c:10:3:10:13 | isblank(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:10:3:10:13 | (...) | Argument |
+| test.c:11:3:11:13 | iscntrl(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:11:3:11:13 | (...) | Argument |
+| test.c:12:3:12:13 | isdigit(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:12:3:12:13 | (...) | Argument |
+| test.c:13:3:13:13 | isgraph(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:13:3:13:13 | (...) | Argument |
+| test.c:14:3:14:13 | islower(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:14:3:14:13 | (...) | Argument |
+| test.c:15:3:15:13 | isprint(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:15:3:15:13 | (...) | Argument |
+| test.c:16:3:16:13 | ispunct(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:16:3:16:13 | (...) | Argument |
+| test.c:17:3:17:13 | isspace(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:17:3:17:13 | (...) | Argument |
+| test.c:18:3:18:13 | isupper(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:18:3:18:13 | (...) | Argument |
+| test.c:19:3:19:14 | isxdigit(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:19:3:19:14 | (...) | Argument |
+| test.c:21:3:21:9 | call to toupper | $@ to character-handling function may not be representable as an unsigned char. | test.c:21:11:21:12 | * ... | Argument |
+| test.c:22:3:22:9 | call to tolower | $@ to character-handling function may not be representable as an unsigned char. | test.c:22:11:22:12 | * ... | Argument |
+| test.c:70:3:70:12 | isalnum(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:70:3:70:12 | (...) | Argument |
+| test.c:71:3:71:12 | isalpha(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:71:3:71:12 | (...) | Argument |
+| test.c:73:3:73:12 | isblank(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:73:3:73:12 | (...) | Argument |
+| test.c:74:3:74:12 | iscntrl(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:74:3:74:12 | (...) | Argument |
+| test.c:75:3:75:12 | isdigit(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:75:3:75:12 | (...) | Argument |
+| test.c:76:3:76:12 | isgraph(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:76:3:76:12 | (...) | Argument |
+| test.c:77:3:77:12 | islower(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:77:3:77:12 | (...) | Argument |
+| test.c:78:3:78:12 | isprint(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:78:3:78:12 | (...) | Argument |
+| test.c:79:3:79:12 | ispunct(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:79:3:79:12 | (...) | Argument |
+| test.c:80:3:80:12 | isspace(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:80:3:80:12 | (...) | Argument |
+| test.c:81:3:81:12 | isupper(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:81:3:81:12 | (...) | Argument |
+| test.c:82:3:82:13 | isxdigit(c) | $@ to character-handling function may not be representable as an unsigned char. | test.c:82:3:82:13 | (...) | Argument |
+| test.c:84:3:84:9 | call to toupper | $@ to character-handling function may not be representable as an unsigned char. | test.c:84:11:84:11 | t | Argument |
+| test.c:85:3:85:9 | call to tolower | $@ to character-handling function may not be representable as an unsigned char. | test.c:85:11:85:11 | t | Argument |
diff --git a/c/misra/src/rules/RULE-2-4/UnusedTagDeclaration.ql b/c/misra/src/rules/RULE-2-4/UnusedTagDeclaration.ql
index e3d0d74c31..565b9fb407 100644
--- a/c/misra/src/rules/RULE-2-4/UnusedTagDeclaration.ql
+++ b/c/misra/src/rules/RULE-2-4/UnusedTagDeclaration.ql
@@ -29,5 +29,7 @@ where
   // expansions of the same macro.
   // Note: due to a bug in the CodeQL CLI version 2.9.4, this will currently have no effect, because
   // `isInMacroExpansion` is broken for `UserType`s.
-  not s.isInMacroExpansion()
+  not s.isInMacroExpansion() and
+  // Exclude template parameters, in case this is run on C++ code.
+  not s instanceof TemplateParameter
 select s, "struct " + s.getName() + " has an unused tag."
diff --git a/c/misra/test/rules/RULE-2-4/test.c b/c/misra/test/rules/RULE-2-4/test.c
index ae73b17a6e..64d05a1cc2 100644
--- a/c/misra/test/rules/RULE-2-4/test.c
+++ b/c/misra/test/rules/RULE-2-4/test.c
@@ -29,10 +29,10 @@ void test() {
   int x = state1; // enum access on E1
   enum E2 e2;
   struct S7 { // NON_COMPLIANT
-    int x
+    int x;
   } s7;
   struct S8 { // COMPLIANT
-    int x
+    int x;
   } s8;
   struct S8 s8_2;
 
@@ -55,7 +55,7 @@ struct S10 { // NON_COMPLIANT
 
 struct S12 { // COMPLIANT
   int x;
-} foo(struct S12 s);
+} foo2(struct S12 s);
 
 #define STRUCT_MACRO                                                           \
   struct S13 {                                                                 \
@@ -77,4 +77,8 @@ void testMacroNameNotUsed() {
   { int x; }
 
 struct s14 PARTIAL; // NON_COMPLIANT - affected by macro, but not fully
-                    // generated, so fair to report as unused
\ No newline at end of file
+                    // generated, so fair to report as unused
+
+typedef struct {
+  int x;
+} S15; // COMPLIANT - not a tag
\ No newline at end of file
diff --git a/c/misra/test/rules/RULE-2-6/test.c b/c/misra/test/rules/RULE-2-6/test.c
index d3a643c1f1..e358cdcb07 100644
--- a/c/misra/test/rules/RULE-2-6/test.c
+++ b/c/misra/test/rules/RULE-2-6/test.c
@@ -1,12 +1,12 @@
 void test1(int p1) {
 dead_label_1: // NON_COMPLIANT
 live_label_1: // COMPLIANT
-  int x = 0;
+  p1 + 1;
 live_label_2: // COMPLIANT
 dead_label_2: // NON_COMPLIANT
-  int y = 0;
+  p1 + 2;
 dead_label_3: // NON_COMPLIANT
-  int z = 0;
+  p1 + 3;
 
   if (p1 > 1) {
     goto live_label_1;
diff --git a/c/misra/test/rules/RULE-21-1/DefineAndUndefUsedOnReservedIdentifierOrMacroName.expected b/c/misra/test/rules/RULE-21-1/DefineAndUndefUsedOnReservedIdentifierOrMacroName.expected
index 299626d6fc..ef9700d8d3 100644
--- a/c/misra/test/rules/RULE-21-1/DefineAndUndefUsedOnReservedIdentifierOrMacroName.expected
+++ b/c/misra/test/rules/RULE-21-1/DefineAndUndefUsedOnReservedIdentifierOrMacroName.expected
@@ -1,4 +1,3 @@
 | test.c:1:1:1:17 | #define _NOT_OKAY | Reserved identifier '_NOT_OKAY' has been undefined or redefined. |
 | test.c:2:1:2:16 | #undef _NOT_OKAY | Reserved identifier '_NOT_OKAY' has been undefined or redefined. |
-| test.c:4:1:4:15 | #define defined | Reserved identifier 'defined' has been undefined or redefined. |
 | test.c:5:1:5:13 | #define errno | Reserved identifier 'errno' has been undefined or redefined. |
diff --git a/c/misra/test/rules/RULE-21-1/test.c b/c/misra/test/rules/RULE-21-1/test.c
index 380679d84a..dc709ca220 100644
--- a/c/misra/test/rules/RULE-21-1/test.c
+++ b/c/misra/test/rules/RULE-21-1/test.c
@@ -1,7 +1,7 @@
 #define _NOT_OKAY // NON_COMPLIANT
 #undef _NOT_OKAY  // NON_COMPLIANT
 
-#define defined // NON_COMPLIANT
-#define errno   // NON_COMPLIANT
+// #define defined // NON_COMPILABLE
+#define errno // NON_COMPLIANT
 
 #define NDEBUG 1 // COMPLIANT
\ No newline at end of file
diff --git a/c/misra/test/rules/RULE-8-2/FunctionTypesNotInPrototypeForm.expected.clang b/c/misra/test/rules/RULE-8-2/FunctionTypesNotInPrototypeForm.expected.clang
new file mode 100644
index 0000000000..8d933c8b4d
--- /dev/null
+++ b/c/misra/test/rules/RULE-8-2/FunctionTypesNotInPrototypeForm.expected.clang
@@ -0,0 +1,3 @@
+| test.c:3:6:3:7 | f1 | Function f1 declares parameter that is unnamed. |
+| test.c:4:6:4:7 | f2 | Function f2 does not specifiy void for no parameters present. |
+| test.c:7:5:7:6 | f5 | Function f5 declares parameter in unsupported declaration list. |
diff --git a/c/misra/test/rules/RULE-8-2/test.c.clang b/c/misra/test/rules/RULE-8-2/test.c.clang
new file mode 100644
index 0000000000..f4cfb7da14
--- /dev/null
+++ b/c/misra/test/rules/RULE-8-2/test.c.clang
@@ -0,0 +1,9 @@
+void f(int x); // COMPLIANT
+void f0(void); // COMPLIANT
+void f1(int);  // NON_COMPLIANT
+void f2();     // NON_COMPLIANT
+// void f3(x);       // NON_COMPILABLE
+void f4(const x); // NON_COMPLIANT[FALSE_NEGATIVE]
+int f5(x)         // NON_COMPLIANT
+    int x;
+{ return 1; }
\ No newline at end of file
diff --git a/change_notes/2023-03-13-ctype-improvements.md b/change_notes/2023-03-13-ctype-improvements.md
new file mode 100644
index 0000000000..c6eb55c56d
--- /dev/null
+++ b/change_notes/2023-03-13-ctype-improvements.md
@@ -0,0 +1 @@
+ * `STR37-C` - reduce false negatives by improving detection when the `<ctype>` functions are implemented using macros.
\ No newline at end of file
diff --git a/cpp/common/src/codingstandards/cpp/CharFunctions.qll b/cpp/common/src/codingstandards/cpp/CharFunctions.qll
index 352f61858c..7f69c353e5 100644
--- a/cpp/common/src/codingstandards/cpp/CharFunctions.qll
+++ b/cpp/common/src/codingstandards/cpp/CharFunctions.qll
@@ -1,31 +1,107 @@
 import cpp
 
-/**
- * Models a class of functions that are either testers of characters
- * or standard library conversion functions.
- */
-class CToOrIsCharFunction extends Function {
-  CToOrIsCharFunction() {
-    this instanceof CIsCharFunction or
-    this instanceof CToCharFunction
-  }
+private string getCToOrIsName() {
+  result =
+    [
+      "isalnum", "isalpha", "isascii", "isblank", "iscntrl", "isdigit", "isgraph", "islower",
+      "isprint", "ispunct", "isspace", "isupper", "isxdigit", "__isspace", "toascii", "toupper",
+      "tolower"
+    ]
 }
 
 /**
- * Models a class of functions that test characters.
+ * A use of one of the APIs in the `<ctype.h>` header that test or convert characters.
+ *
+ * Note: these operations are commonly implemented as either function or a macro. This class
+ * abstracts away from those details, providing a `getConvertedArgument` predicate to get the
+ * argument after any conversions specified by the user, excluding any conversions induced by
+ * the structure of the macro, or
  */
-class CIsCharFunction extends Function {
-  CIsCharFunction() {
-    getName() in [
-        "isalnum", "isalpha", "isascii", "isblank", "iscntrl", "isdigit", "isgraph", "islower",
-        "isprint", "ispunct", "isspace", "isupper", "isxdigit", "__isspace"
-      ]
+abstract class UseOfToOrIsChar extends Element {
+  /** */
+  abstract Expr getConvertedArgument();
+}
+
+private class CToOrIsCharFunctionCall extends FunctionCall, UseOfToOrIsChar {
+  CToOrIsCharFunctionCall() {
+    getTarget().getName() = getCToOrIsName() and
+    // Some library implementations, such as musl, include a "dead" call to the same function
+    // that has also been implemented as a macro, in order to retain the right types. We exclude
+    // this call because it does not appear in the control flow or data flow graph. However,
+    // isspace directly calls __isspace, which is allowed
+    (
+      getTarget().getName() = "__isspace" or
+      not any(CToOrIsCharMacroInvocation mi).getAnExpandedElement() = this
+    )
   }
+
+  override Expr getConvertedArgument() { result = getArgument(0).getExplicitlyConverted() }
 }
 
-/**
- * Models a class of functions convert characters.
- */
-class CToCharFunction extends Function {
-  CToCharFunction() { getName() in ["toascii", "toupper", "tolower"] }
+private class CToOrIsCharMacroInvocation extends MacroInvocation, UseOfToOrIsChar {
+  CToOrIsCharMacroInvocation() { getMacroName() = getCToOrIsName() }
+
+  override Expr getConvertedArgument() {
+    /*
+     * There is no common approach to how the macros are defined, so we handle
+     * each compiler/library case individually. Fortunately, there's no conflict
+     * between different compilers.
+     */
+
+    // For the "is" APIs, if clang and gcc use a macro, then it expands to an
+    // array access on the left hand side of an &
+    exists(ArrayExpr ae | ae = getExpr().(BitwiseAndExpr).getLeftOperand() |
+      // Casted to an explicit (int), so we want unwind only a single conversion
+      result = ae.getArrayOffset().getFullyConverted().(Conversion).getExpr()
+    )
+    or
+    // For the tolower/toupper cases, a secondary macro is expanded
+    exists(MacroInvocation mi |
+      mi.getParentInvocation() = this and
+      mi.getMacroName() = "__tobody"
+    |
+      /*
+       * tolower and toupper can be defined by macros which:
+       * - if the size of the type is greater than 1
+       *   - then check if it's a compile time constant
+       *     - then use c < -128 || c > 255 ? c : (a)[c]
+       *     - else call the function
+       *   - else (a)[c]
+       */
+
+      exists(ArrayExpr ae |
+        ae = mi.getAnExpandedElement() and
+        result = ae.getArrayOffset() and
+        // There are two array access, but only one should be reachable
+        result.getBasicBlock().isReachable()
+      )
+      or
+      exists(ConditionalExpr ce |
+        ce = mi.getAnExpandedElement() and
+        result = ce.getThen() and
+        result.getBasicBlock().isReachable()
+      )
+    )
+    or
+    // musl uses a conditional expression as the expansion
+    exists(ConditionalExpr ce | ce = getExpr() |
+      // for most macro expansions, the else is a subtraction inside a `<`
+      exists(SubExpr s |
+        not getMacroName() = "isalpha" and
+        s = ce.getElse().(LTExpr).getLeftOperand() and
+        // Casted to an explicit (int), so we want unwind only a single conversion
+        result = s.getLeftOperand().getFullyConverted().(Conversion).getExpr()
+      )
+      or
+      // for isalpha, the else is a bitwise or inside a subtraction inside a `<`
+      exists(BitwiseOrExpr bo |
+        // Casted to an explicit (unsigned)
+        getMacroName() = "isalpha" and
+        bo = ce.getElse().(LTExpr).getLeftOperand().(SubExpr).getLeftOperand() and
+        // Casted to an explicit (int), so we want unwind only a single conversion
+        result =
+          bo.getLeftOperand().getFullyConverted().(Conversion).getExpr().(ParenthesisExpr).getExpr()
+      )
+    )
+  }
 }