GDScript: Fix missing conversion for default argument values

modules/gdscript/tests/scripts/runtime/errors/bad_conversion_for_default_parameter.gd

@@ -0,0 +1,8 @@
+var weakling = 'not float'
+func weak(x: float = weakling):
+	print(x)
+	print('typeof x is', typeof(x))
+
+func test():
+	print(typeof(weak()))
+	print('not ok')

modules/gdscript/tests/scripts/runtime/errors/bad_conversion_for_default_parameter.out

@@ -0,0 +1,8 @@
+GDTEST_RUNTIME_ERROR
+>> SCRIPT ERROR
+>> on function: weak()
+>> runtime/errors/bad_conversion_for_default_parameter.gd
+>> 2
+>> Trying to assign value of type 'String' to a variable of type 'float'.
+0
+not ok

modules/gdscript/tests/scripts/runtime/features/conversion_for_default_parameter.gd

@@ -0,0 +1,19 @@
+func literal(x: float = 1):
+	print('x is ', x)
+	print('typeof x is ', typeof(x))
+
+var inferring := 2
+func inferred(x: float = inferring):
+	print('x is ', x)
+	print('typeof x is ', typeof(x))
+
+var weakling = 3
+func weak(x: float = weakling):
+	print('x is ', x)
+	print('typeof x is ', typeof(x))
+
+func test():
+	literal()
+	inferred()
+	weak()
+	print('ok')

modules/gdscript/tests/scripts/runtime/features/conversion_for_default_parameter.out

@@ -0,0 +1,8 @@
+GDTEST_OK
+x is 1
+typeof x is 3
+x is 2
+typeof x is 3
+x is 3
+typeof x is 3
+ok