diff --git a/cpp/ql/src/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.ql b/cpp/ql/src/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.ql
new file mode 100644
index 00000000000..0a75d83f805
--- /dev/null
+++ b/cpp/ql/src/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.ql	
@@ -0,0 +1,158 @@
+/**
+ * @name Implicit function declaration
+ * @description An implicitly declared function is assumed to take no
+ * arguments and return an integer. If this assumption does not hold, it
+ * may lead to unpredictable behavior.
+ * @kind problem
+ * @problem.severity warning
+ * @precision medium
+ * @id cpp/implicit-function-declaration
+ * @tags correctness
+ *       maintainability
+ */
+
+import cpp
+
+pragma[inline]
+predicate arithTypesMatch(Type arg, Type parm) {
+  arg = parm
+  or
+  arg.getSize() = parm.getSize() and
+  (
+    arg instanceof IntegralOrEnumType and
+    parm instanceof IntegralOrEnumType
+    or
+    arg instanceof FloatingPointType and
+    parm instanceof FloatingPointType
+  )
+}
+
+pragma[inline]
+predicate nestedPointerArgTypeMayBeUsed(Type arg, Type parm) {
+  // arithmetic types
+  arithTypesMatch(arg, parm)
+  or
+  // conversion to/from pointers to void is allowed
+  arg instanceof VoidType
+  or
+  parm instanceof VoidType
+}
+
+pragma[inline]
+predicate pointerArgTypeMayBeUsed(Type arg, Type parm) {
+  nestedPointerArgTypeMayBeUsed(arg, parm)
+  or
+  // nested pointers
+  nestedPointerArgTypeMayBeUsed(arg.(PointerType).getBaseType().getUnspecifiedType(),
+    parm.(PointerType).getBaseType().getUnspecifiedType())
+  or
+  nestedPointerArgTypeMayBeUsed(arg.(ArrayType).getBaseType().getUnspecifiedType(),
+    parm.(PointerType).getBaseType().getUnspecifiedType())
+}
+
+pragma[inline]
+predicate argTypeMayBeUsed(Type arg, Type parm) {
+  // arithmetic types
+  arithTypesMatch(arg, parm)
+  or
+  // pointers to compatible types
+  pointerArgTypeMayBeUsed(arg.(PointerType).getBaseType().getUnspecifiedType(),
+    parm.(PointerType).getBaseType().getUnspecifiedType())
+  or
+  pointerArgTypeMayBeUsed(arg.(ArrayType).getBaseType().getUnspecifiedType(),
+    parm.(PointerType).getBaseType().getUnspecifiedType())
+  or
+  // C11 arrays
+  pointerArgTypeMayBeUsed(arg.(PointerType).getBaseType().getUnspecifiedType(),
+    parm.(ArrayType).getBaseType().getUnspecifiedType())
+  or
+  pointerArgTypeMayBeUsed(arg.(ArrayType).getBaseType().getUnspecifiedType(),
+    parm.(ArrayType).getBaseType().getUnspecifiedType())
+}
+
+// This predicate holds whenever expression `arg` may be used to initialize
+// function parameter `parm` without need for run-time conversion.
+pragma[inline]
+predicate argMayBeUsed(Expr arg, ParameterDeclarationEntry pde) {
+  argTypeMayBeUsed(arg.getFullyConverted().getUnspecifiedType(), pde.getUnspecifiedType())
+}
+
+// True if this file (or header) was compiled as a C file
+predicate isCompiledAsC(File f) {
+  f.compiledAsC()
+  or
+  exists(File src | isCompiledAsC(src) | src.getAnIncludedFile() = f)
+}
+
+// True if function was ()-declared, but not (void)-declared or K&R-defined
+// or implicitly declared (i.e., lacking a prototype)
+predicate hasZeroParamDeclTooMany(Function f) {
+  exists(FunctionDeclarationEntry fde | fde = f.getADeclarationEntry() |
+    not fde.isImplicit() and
+    not fde.hasVoidParamList() and
+    fde.getNumberOfParameters() = 0 and
+    not fde.isDefinition()
+  )
+}
+
+// True if function was ()-declared, but not (void)-declared or K&R-defined
+predicate hasZeroParamDecl(Function f) {
+  exists(FunctionDeclarationEntry fde | fde = f.getADeclarationEntry() |
+    not fde.hasVoidParamList() and fde.getNumberOfParameters() = 0 and not fde.isDefinition()
+  )
+}
+
+bindingset[name]
+predicate notAlreadyReported(string name, FunctionCall fc) {
+  forall(FunctionDeclarationEntry fdeEx |
+    fdeEx.getName() = name and
+    not fdeEx.isImplicit()
+  |
+    // only report if not reported by cpp/futile-params
+    (
+      fdeEx.getNumberOfParameters() >= fc.getNumberOfArguments()
+      or
+      exists(Function f |
+        f = fc.getTarget() and
+        not exists(f.getBlock())
+        or
+        not hasZeroParamDeclTooMany(f)
+      )
+    ) and
+    // only report if not reported by cpp/too-few-arguments
+    (
+      fdeEx.getNumberOfParameters() <= fc.getNumberOfArguments()
+      or
+      exists(Function f |
+        f.isVarargs() or
+        f instanceof BuiltInFunction or
+        not hasZeroParamDecl(f)
+      )
+    ) and
+    // only report if not reported by cpp/mistyped-function-arguments
+    (
+      not hasZeroParamDecl(fc.getTarget())
+      or
+      forall(ParameterDeclarationEntry pde |
+        pde = fdeEx.getAParameterDeclarationEntry() and
+        pde.getIndex() < fc.getNumberOfArguments()
+      |
+        argMayBeUsed(fc.getArgument(pde.getIndex()), pde)
+      )
+    )
+  )
+}
+
+predicate sameLocation(Location loc1, Location loc2) {
+  loc1.getFile() = loc2.getFile() and
+  loc1.getStartLine() = loc2.getStartLine() and
+  loc1.getStartColumn() = loc2.getStartColumn()
+}
+
+from FunctionDeclarationEntry fdeIm, FunctionCall fc
+where
+  isCompiledAsC(fdeIm.getFile()) and
+  fdeIm.isImplicit() and
+  sameLocation(fdeIm.getLocation(), fc.getLocation()) and
+  notAlreadyReported(fdeIm.getName(), fc)
+select fc, "Function call implicitly declares $@", fc, fc.toString()
diff --git a/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.expected b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.expected
new file mode 100644
index 00000000000..50985b54176
--- /dev/null
+++ b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.expected	
@@ -0,0 +1,9 @@
+| test.c:28:3:28:12 | call to undeclared | Function call implicitly declares $@ | test.c:28:3:28:12 | call to undeclared | call to undeclared |
+| test.c:31:3:31:19 | call to not_yet_declared1 | Function call implicitly declares $@ | test.c:31:3:31:19 | call to not_yet_declared1 | call to not_yet_declared1 |
+| test.c:32:3:32:19 | call to not_yet_declared2 | Function call implicitly declares $@ | test.c:32:3:32:19 | call to not_yet_declared2 | call to not_yet_declared2 |
+| test.c:43:3:43:27 | call to not_declared_defined_with | Function call implicitly declares $@ | test.c:43:3:43:27 | call to not_declared_defined_with | call to not_declared_defined_with |
+| test.c:54:3:54:21 | call to defined_with_double | Function call implicitly declares $@ | test.c:54:3:54:21 | call to defined_with_double | call to defined_with_double |
+| test.c:66:3:66:22 | call to defined_with_ptr_ptr | Function call implicitly declares $@ | test.c:66:3:66:22 | call to defined_with_ptr_ptr | call to defined_with_ptr_ptr |
+| test.c:68:3:68:22 | call to defined_with_ptr_arr | Function call implicitly declares $@ | test.c:68:3:68:22 | call to defined_with_ptr_arr | call to defined_with_ptr_arr |
+| test.c:132:3:132:22 | call to implicit_declaration | Function call implicitly declares $@ | test.c:132:3:132:22 | call to implicit_declaration | call to implicit_declaration |
+| test.c:133:3:133:30 | call to implicit_declaration_k_and_r | Function call implicitly declares $@ | test.c:133:3:133:30 | call to implicit_declaration_k_and_r | call to implicit_declaration_k_and_r |
diff --git a/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.qlref b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.qlref
new file mode 100644
index 00000000000..38492f2a203
--- /dev/null
+++ b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.qlref	
@@ -0,0 +1 @@
+Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.ql
\ No newline at end of file
diff --git a/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/MistypedFunctionArguments.expected b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/MistypedFunctionArguments.expected
index 14dedbdebed..2fb422155d2 100644
--- a/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/MistypedFunctionArguments.expected	
+++ b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/MistypedFunctionArguments.expected	
@@ -18,3 +18,4 @@
 | test.c:58:3:58:24 | call to defined_with_long_long | Calling $@: argument $@ of type $@ is incompatible with parameter $@. | test.c:103:11:103:32 | defined_with_long_long | defined_with_long_long | test.c:58:26:58:26 | 3 | 3 | file://:0:0:0:0 | int | int | test.c:103:44:103:45 | ll | long long ll |
 | test.c:60:3:60:21 | call to defined_with_double | Calling $@: argument $@ of type $@ is incompatible with parameter $@. | test.c:99:8:99:26 | defined_with_double | defined_with_double | test.c:60:23:60:25 | 2 | 2 | file://:0:0:0:0 | long long | long long | test.c:99:35:99:35 | d | double d |
 | test.c:61:3:61:24 | call to defined_with_long_long | Calling $@: argument $@ of type $@ is incompatible with parameter $@. | test.c:103:11:103:32 | defined_with_long_long | defined_with_long_long | test.c:61:26:61:31 | 3500000000000000.0 | 3500000000000000.0 | file://:0:0:0:0 | double | double | test.c:103:44:103:45 | ll | long long ll |
+| test.c:137:3:137:27 | call to implicit_declaration_good | Calling $@: argument $@ of type $@ is incompatible with parameter $@. | test2.c:8:6:8:30 | implicit_declaration_good | implicit_declaration_good | test.c:137:35:137:38 | 2.0 | 2.0 | file://:0:0:0:0 | float | float | test2.c:8:52:8:52 | f | float f |
diff --git a/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/test.c b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/test.c
index 38059f48f9a..5b54e519b70 100644
--- a/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/test.c	
+++ b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/test.c	
@@ -25,11 +25,11 @@ void test(int *argv[]) {
   declared_void(); // GOOD
   declared_with(1); // GOOD
   
-  undeclared();  // GOOD
+  undeclared();  // BAD (GOOD for everything except cpp/implicit-function-declaration)
   undeclared(1); // GOOD
   
-  not_yet_declared1(1); // GOOD
-  not_yet_declared2(1); // GOOD
+  not_yet_declared1(1); // BAD (GOOD for everything except for cpp/implicit-function-declaration)
+  not_yet_declared2(1); // BAD (GOOD for everything except for cpp/implicit-function-declaration)
   not_yet_declared2(ca); // BAD
   not_yet_declared2(); // BAD
 
@@ -40,7 +40,7 @@ void test(int *argv[]) {
   declared_empty_defined_with(&x); // BAD
   declared_empty_defined_with(3, &x); // BAD
 
-  not_declared_defined_with(-1, 0, 2U); // GOOD
+  not_declared_defined_with(-1, 0, 2U); // BAD (GOOD for everything except for cpp/implicit-function-declaration)
   not_declared_defined_with(4LL, 0, 2.5e9f); // BAD
 
   declared_with_pointers(pv, ca); // GOOD
@@ -51,7 +51,7 @@ void test(int *argv[]) {
   defined_with_float(2.f);  // BAD
   defined_with_float(2.0);  // BAD
   
-  defined_with_double(2.f); // GOOD
+  defined_with_double(2.f); // BAD (GOOD for everything except for cpp/implicit-function-declaration)
   defined_with_double('c');  // BAD
   
   defined_with_long_long('c');  // BAD
@@ -63,9 +63,9 @@ void test(int *argv[]) {
   k_and_r_func(2.5, &s);  // GOOD
   
   int (*parameterName)[2];
-  defined_with_ptr_ptr(parameterName); // GOOD
+  defined_with_ptr_ptr(parameterName); // // BAD (GOOD for everything except for cpp/implicit-function-declaration)
   defined_with_ptr_ptr(argv);   // GOOD
-  defined_with_ptr_arr(parameterName); // GOOD
+  defined_with_ptr_arr(parameterName); // // BAD (GOOD for everything except for cpp/implicit-function-declaration)
   defined_with_ptr_arr(argv);   // GOOD
 
   declared_and_defined_empty(); // GOOD
@@ -124,3 +124,17 @@ int call_k_and_r(int i) {
 int will_be_k_and_r(val)
   int val;
 { return val + 1; }
+
+extern int extern_definition(double, double*);
+
+void test_implicit_function_declaration(int x, double d) {
+  int y;
+  implicit_declaration(1, 2); // BAD
+  implicit_declaration_k_and_r(1, 2); // BAD
+
+  implicit_declaration(1, 2); // GOOD (no longer an implicit declaration)
+
+  implicit_declaration_good(1, x, 2.0f); // BAD
+
+  y = extern_definition(3.0f, &d); // GOOD
+}
\ No newline at end of file
diff --git a/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/test2.c b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/test2.c
new file mode 100644
index 00000000000..80dbc1db4ce
--- /dev/null
+++ b/cpp/ql/test/query-tests/Likely Bugs/Underspecified Functions/test2.c	
@@ -0,0 +1,10 @@
+void implicit_declaration(int x) {}
+
+int implicit_declaration_k_and_r(x) int x;
+{
+    return x;
+}
+
+void implicit_declaration_good(int x, int y, float f)
+{
+}
\ No newline at end of file