From 8be363eae3aafcd8f4aa9d12ff673cb959fc4d3c Mon Sep 17 00:00:00 2001
From: "romin.tomasetti" <romin.tomasetti@gmail.com>
Date: Sat, 22 Jan 2022 15:05:23 +0000
Subject: [PATCH] parser(C++11) : explicit template function support, see
 https://github.com/swig/swig/issues/2131

---
 Examples/test-suite/cpp11_template_explicit.i | 25 +++++++++++++++++++
 .../python/cpp11_template_explicit_runme.py   | 11 ++++++++
 Source/CParse/parser.y                        | 17 ++++++++++---
 3 files changed, 50 insertions(+), 3 deletions(-)
 create mode 100644 Examples/test-suite/python/cpp11_template_explicit_runme.py

diff --git a/Examples/test-suite/cpp11_template_explicit.i b/Examples/test-suite/cpp11_template_explicit.i
index 71752f822..0a76b93d8 100644
--- a/Examples/test-suite/cpp11_template_explicit.i
+++ b/Examples/test-suite/cpp11_template_explicit.i
@@ -4,6 +4,7 @@
 */
 %module cpp11_template_explicit
 
+/* Explicitely silence SWIG warning related to explicit templates */
 #pragma SWIG nowarn=SWIGWARN_PARSE_EXPLICIT_TEMPLATE
 
 %inline %{
@@ -33,6 +34,30 @@ extern template class Temper<B*>;
 
 template class Temper<int>;
 extern template class Temper<short>;
+
+/* Templated function to check support for extern template functions */
+template <typename T>
+T my_templated_function(int a, double b)
+{
+  return T();
+}
+
+/* Explicit extern function template instantiation with simple type */
+extern template int my_templated_function<int>(int, double);
+template int my_templated_function<int>(int, double);
+
+/* Explicit extern function template instantiation with more complex types */
+extern template A my_templated_function<A>(int, double);
+template A my_templated_function<A>(int, double);
+
+extern template Temper<int> my_templated_function<Temper<int>>(int, double);
+template Temper<int> my_templated_function<Temper<int>>(int, double);
+
 %}
 
 %template(TemperInt) Temper<int>;
+
+/* Enable several versions of the templated function */
+%template(my_templated_function_int      ) my_templated_function<int>;
+%template(my_templated_function_A        ) my_templated_function<A>;
+%template(my_templated_function_TemperInt) my_templated_function<Temper<int>>;
diff --git a/Examples/test-suite/python/cpp11_template_explicit_runme.py b/Examples/test-suite/python/cpp11_template_explicit_runme.py
new file mode 100644
index 000000000..e713ad0d9
--- /dev/null
+++ b/Examples/test-suite/python/cpp11_template_explicit_runme.py
@@ -0,0 +1,11 @@
+import cpp11_template_explicit
+
+# Call variants of the same templated function
+t1 = cpp11_template_explicit.my_templated_function_int      (1,1.0)
+t2 = cpp11_template_explicit.my_templated_function_A        (2,2.0)
+t3 = cpp11_template_explicit.my_templated_function_TemperInt(3,3.0)
+
+# Check return types
+assert isinstance(t1,int)
+assert isinstance(t2,cpp11_template_explicit.A)
+assert isinstance(t3,cpp11_template_explicit.TemperInt)
diff --git a/Source/CParse/parser.y b/Source/CParse/parser.y
index c48d3a6f2..a21d029e0 100644
--- a/Source/CParse/parser.y
+++ b/Source/CParse/parser.y
@@ -4361,12 +4361,23 @@ cpp_template_decl : TEMPLATE LESSTHAN template_parms GREATERTHAN {
                   $$ = 0; 
 		}
 
-		/* Explicit template instantiation without the translation unit */
+		/* Explicit function template instantiation */
+		| TEMPLATE cpp_alternate_rettype idcolon LPAREN parms RPAREN {
+			Swig_warning(WARN_PARSE_EXPLICIT_TEMPLATE, cparse_file, cparse_line, "Explicit function template instantiation ignored.\n");
+                  $$ = 0; 
+				}
+
+		/* Explicit class template instantiation without the translation unit */
 		| EXTERN TEMPLATE cpptype idcolon {
-		  Swig_warning(WARN_PARSE_EXPLICIT_TEMPLATE, cparse_file, cparse_line, "Explicit template instantiation ignored.\n");
+		  Swig_warning(WARN_PARSE_EXPLICIT_TEMPLATE, cparse_file, cparse_line, "Explicit (extern) class template instantiation ignored.\n");
                   $$ = 0; 
                 }
-                ;
+		/* Explicit function template instantiation without the translation unit */
+		| EXTERN TEMPLATE cpp_alternate_rettype idcolon LPAREN parms RPAREN {
+			Swig_warning(WARN_PARSE_EXPLICIT_TEMPLATE, cparse_file, cparse_line, "Explicit (extern) function template instantiation ignored.\n");
+                  $$ = 0; 
+				}
+				;
 
 cpp_template_possible:  c_decl {
 		  $$ = $1;